Influenza Vaccine Effectiveness Research Articles

Adapting COVID-19 research infrastructure to capture influenza and respiratory syncytial virus alongside SARS-CoV-2 in UK healthcare workers winter 2022/23 and beyond: protocol for a pragmatic sub-study

Introduction During the COVID-19 pandemic, extensive research was conducted on SARS-CoV-2; however, important questions about other respiratory pathogens remain unanswered. A severe influenza season in 2022–2023 with simultaneous circulation of SARS-CoV2 and respiratory syncytial virus is anticipated. This sub-study aims to determine the incidence and impact of these respiratory viruses on healthcare workers, the symptoms they experienced, the effectiveness of both COVID-19 and influenza vaccination and the burden of these infections on the National Health Service (NHS) workforce. Methods and analysis This is a longitudinal prospective cohort sub-study, utilising the population and infrastructure of the SARS-CoV-2 Immunity & Reinfection Evaluation (SIREN) study, which focuses on hospital staff in the UK. Participants undergo fortnightly nucleic acid amplification testing on a multiplex assay including SARS-CoV-2, influenza A and B and RSV, regardless of symptoms. Questionnaires are completed every two weeks, capturing symptoms, sick days, exposures, and vaccination records. Serum samples are collected monthly or quarterly from participants associated with a SIREN site. This sub-study commenced on 28/11/22 to align with the predicted influenza season and participants’ influenza vaccine status. The SIREN Participant Involvement Panel shaped the aims and methods for the study, highlighting its acceptability. UK devolved administrations were supported to develop local protocols. Analysis plans include incidence of asymptomatic and symptomatic infection, comparisons of vaccination coverage, assessment of sick day burden, and effectiveness of seasonal influenza against infection and time off work. Data are also integrated into UKHSA nosocomial modelling. Ethics and dissemination The protocol was approved by the Berkshire Research Ethics Committee (IRAS ID 284460, REC Reference 20SC0230) on 14/11/2022. Participants were informed in advance. As the frequency and method of sampling remained the same, implied consent processes were approved by the committee. Participants returning to the study give informed consent. Regular reports to advisory groups and peer-reviewed publications are planned to disseminate findings and inform decision making. Clinical trial registration number: ISRCTN11041050; registration date: 12 January 2021. Sub study included in protocol version: v8.0, and amended in v9.0

A retrospective cohort study on the protective effect of influenza vaccination during pregnancy in Gansu Province

ABSTRACT To evaluate the Vaccine Effectiveness (VE) of the Influenza Vaccine (InfV) during pregnancy in Gansu province. A retrospective cohort of pregnant women vaccinated with InfV was constructed using the electronic case system and immunization planning system of Gansu province. Outcome information of Influenza-Like Illness (ILI) in pregnant women after vaccination with InfV during the influenza epidemic seasons from 2019 to 2021 was collected. The log-binomial model was used to estimate the adjusted Risk Ratio (RR), VE, and their Confidence Intervals (CI). The incidence rates of the InfV vaccination group and the non-vaccination group during pregnancy in Gansu province were 4.70% (54/1,150) and 6.91% (16,990/245,754), respectively. The adjusted RR was 0.78 (95%CI 0.43–0.75), and the adjusted VE was 22.20 (95%CI 0.79–40.30). Vaccination with InfV during pregnancy in Gansu province from 2019 to 2021 has a protective effect against ILI in pregnant women, and there is no difference in vaccination at different stages of pregnancy.

Challenges in Influenza Control and Surveillance in the Republic of Kazakhstan

The COVID-19 pandemic has significantly disrupted the circulation of influenza viruses in Kazakhstan, highlighting the vulnerabilities in the country’s public health infrastructure. This review critically examines the challenges faced in infiltrating and controlling influenza in Kazakhstan, particularly in light of the shifting epidemiological landscape post-pandemic. Key issues include the decline in influenza cases during the pandemic, which complicates the assessment of influenza epidemiology, vaccine effectiveness, and planning of vaccination campaigns. Although part of the Global Influenza Hospital Surveillance Network (GIHSN), Kazakhstan's surveillance systems face data collection, coordination, and public awareness gaps. The review discusses the prevalence of various influenza strains, the impact of zoonotic infections, and the necessity for improved monitoring frameworks. Additionally, the historical context of infectious disease control in Kazakhstan is explored, emphasising the need for enhanced international collaboration and targeted public health strategies. The findings underscore the importance of vaccination and robust surveillance to mitigate the risks of seasonal and pandemic influenza, advocating for a comprehensive approach to safeguard public health in Kazakhstan.

Features of influenza virus hemagglutinin genes and their recoding possibilities

The world has already entered the stage of increasing odds for a new pandemic, which prompts to seek out for new flu vaccines, because existing vaccines demonstrate only suboptimal effectiveness. With the Covid-19 pandemic, the possibility of using mRNA vaccines has been opened up, and a prospect of finding hemagglutinin (HA) gene mRNA-based new influenza vaccines seems very attractive. As a rule, the mRNA vaccine is a product of recoding, which ensures the mRNA stability. However, the results of mRNA recoding can be ambiguous. The purpose of this report is to analyze the features of genes and proteins and to consider opportunities and limitations in their recoding. Primary structures of NA proteins and relevant genes were retrieved from Internet publicly available databases. The amino acid composition and frequency of dipeptides, nucleotide and dinucleotide compositions, %GC, translational code and compositions of neighboring di- and tricodones, distribution along primary structure for explicit and synonymous mutations were determined. H1N1 and H3N2 subtypes have both specific and general features (limitations) in their genes, differing not only in the number of protein substitutions, but also in the number and distribution of gene synonymous codons, which do not manifest in the protein primary structure, but appear, apparently, as a hidden factor, which causes the low effectiveness of classical influenza vaccines. The identification of several limitations in gene structure suggests that its any modification (in any gene) must not contradict each of the restrictions established by nature. The frequency of CpG dinucleotides in all studied strains is low, but a potential for optimizing it in H1N1 strains due to the prohibition of the quartet in the gene for arginine-encoding codons is especially limited and can be implemented through synonymous codons of other amino acids (alanine, proline, threonine or serine). Compared to the H1N1 subtype, the H3N2 subtype can be expected to have more possibilities in constructing stable NA gene mRNA.

Protective Impact of Influenza Vaccination on Healthcare Workers

Background: Influenza vaccine uptake among healthcare workers is crucial for preventing influenza infections, yet its effectiveness needs further investigation. Objectives: This prospective observational study aimed to assess the protective effect of influenza vaccination among healthcare workers in Shenzhen. Methods: We enrolled 100 participants, with 50 receiving the 2023–2024 quadrivalent influenza vaccine (QIV) and 50 serving as unvaccinated controls. Epidemiological data were collected when the participants presented influenza-like illness. Serum samples were collected at three time points (pre-vaccination and 28 and 180 days after vaccination). Hemagglutination inhibition (HI) assay was performed against the strains included in the 2023–2024 QIV (H1N1, H3N2, BV and BY strains) to assess antibody protection levels. Demographics comparisons revealed no significant differences between the vaccinated and control groups (p > 0.05), ensuring group comparability. Results: The incidence of influenza-like illness was significantly lower in the vaccinated (18%) compared to the control group (36%; p = 0.046; OR = 0.39; 95% CI: 0.15 to 0.98). The vaccinated group also exhibited a higher rate of consecutive two-year vaccinations (48% vs. 24% in the control group, p < 0.05). Additionally, the vaccinated healthcare workers were more inclined to recommend vaccination to their families (80% vs. 48%, p < 0.05). HI titers against H1N1 (p < 0.01), H3N2 (p < 0.01), BV (p < 0.001) and BY (p < 0.01) significantly increased in the vaccinated group at 28 days post-vaccination. Moreover, a marked and sustained increase in HI titers against the H3N2 strain (p < 0.001) was observed at 180 days post-vaccination, highlighting the vaccine’s enduring impact on the immune response. The fold change in the HI titers, indicative of the magnitude of the immune response, was significantly higher for H1N1 (p < 0.01), H3N2 (p < 0.001), BV (p < 0.01) and BY (p < 0.05) among the vaccinated individuals compared to the control group, underscoring the vaccine’s efficacy in eliciting a robust and sustained antibody response. Conclusion: Influenza vaccination significantly reduces the incidence of influenza-like illness among healthcare workers and promotes a sustained immune response. The study supports the importance of annual vaccination for this group to enhance personal and public health.

Influenza vaccine effectiveness in Ireland during two influenza seasons, 2022/23 and 2023/24

Abstract Background Live Attenuated Influenza Vaccine (LAIV) is recommended in Ireland for all children aged 2-17 years. Quadrivalent influenza vaccine is recommended for all others eligible for vaccination, including those aged ≥18 years with underlying medical conditions and all aged ≥65 years. We aimed to estimate influenza vaccine effectiveness (IVE) against Acute Respiratory Infection (ARI) presentations to primary care due to influenza over two influenza seasons in Ireland, to inform vaccination recommendations and communication campaigns. Methods We undertook a test-negative design case control study within the national sentinel surveillance General Practice network, as part of the European Vaccine Effectiveness Burden and Impact Studies (VEBIS) network. We compared influenza vaccination status among influenza Polymerase Chain Reaction (PCR) positive cases with influenza PCR negative controls, amongst patients consulting with ARI. We verified vaccination status using the national influenza vaccination database. We estimated end of season IVE for 2022/23, and mid-season IVE (up to 7th April 2024) for 2023/24, using multivariable logistic regression and adjusting for age, onset time, medical condition, and sex. Results In 2022/23, there were 288 cases and 765 controls. In 2023/24 there were 515 cases and 1,482 controls. Overall vaccination coverage was 23% in both seasons. Coverage in 2-17 year-olds was 10% in 2022/23 and 12% in 2023/24. In 2022/23, overall IVE was 42% (95%CI 9-54) and 50% (95%CI -30-83) in 2-17 year-olds. Overall IVE in 2023/24 was 37% (95%CI 16-54) and was 67% (95%CI 28-87) in 2-17 year-olds. Conclusions Influenza vaccination reduced medical attendance to primary care with ARI due to influenza, demonstrating benefits of vaccination, particularly among children. Influenza vaccination should continue to be offered and promoted to recommended groups, including children. Key messages • This study suggests that influenza vaccination protected population health and the health service from medical attendance to primary care with acute respiratory infection due to influenza. • Higher influenza vaccine effectiveness was observed in children aged 2-17 years. These findings can inform influenza vaccination programmes with LAIV for children in Ireland and internationally.

Relative effectiveness of high-dose versus standard-dose quadrivalent influenza vaccine against hospitalizations and mortality according to frailty score: a post-hoc analysis of the DANFLU-1 trial

Abstract Background Frailty is a major risk factor for adverse cardiovascular events. Influenza vaccination has been shown to protect against cardiovascular fatal and nonfatal events. However, limited data exist on the added benefit of high-dose versus standard-dose influenza vaccination according to frailty status. Purpose We sought to assess the relative effectiveness of high-dose (QIV-HD) versus standard-dose (QIV-SD) quadrivalent influenza vaccination according to frailty status in elderly individuals. Methods This is a post-hoc analysis of the randomized controlled feasibility trial of QIV-HD versus QIV-SD conducted during the 2021–2022 influenza season in adults aged 65–79 years. We assessed the following prespecified outcomes: hospitalizations for pneumonia or influenza, cardio-respiratory hospitalizations, cardiovascular hospitalizations, all-cause hospitalizations and all-cause mortality. Level of frailty was defined according to the Hospital Frailty Risk Score based on ICD-10 codes for relevant comorbidities any time prior to vaccination. We divided the participants into two categories according to Frailty Score (FS): low frailty (0-4 points) and intermediate or high frailty (≥5 points). We analyzed outcomes as time to first event using Cox proportional hazards regression. Results Among 12,473 randomly assigned participants to QIV-HD versus QIV-SD (mean age 71.7 years, 47.1% female), 10,711 (85.9%) were categorized as having low frailty and 1,762 (14.1%) had intermediate or high frailty. In total, 38 individuals were hospitalized for pneumonia or influenza during follow-up, 219 were hospitalized for cardiorespiratory illness, 1055 were hospitalized due to any cause and 62 died during follow-up. FS significantly modified the effect of QIV-HD versus QIV-SD for all-cause mortality (P for interaction 0.02). In participants with low frailty, QIV-HD was associated with a lower risk of all-cause mortality (Figure 1: HR 0.26, 95% CI 0.13 to 0.55), whereas there was no evidence of effect among those with intermediate or high frailty (HR 1.68, 95% CI 0.66 to 4.26). QIV-HD versus QIV-SD was associated with a lower incidence of hospitalizations for pneumonia and influenza, this effect was consistent regardless of frailty status (P for interaction 0.88). QIV-HD was not associated with a significantly lower incidence of hospitalizations due to cardiovascular, cardiorespiratory or respiratory disease or any cause regardless of frailty status. Conclusions In this post-hoc analysis of a large-scale pragmatic trial, QIV-HD was similarly associated with a lower incidence of hospitalizations for pneumonia and influenza among older adults irrespective of frailty status. The potential benefit of QIV-HD versus QIV-SD may therefore be applicable regardless of frailty status. However, our results suggest that QIV-HD is associated with a lower risk of all-cause mortality among adults with low frailty only.Figure 1.

Effect of high-dose vs. standard-dose quadrivalent influenza vaccine on death and hospitalizations in older adults with cardiovascular disease: a prespecified analysis of the DANFLU-1 study

Abstract Background The results of the DANFLU-1 trial indicated that high-dose quadrivalent influenza vaccines (HDQIV) as compared to standard-dose quadrivalent influenza vaccines (SDQIV) may reduce the incidence of death and hospitalization with pneumonia or influenza in older adults. Whether this effect differs by the presence of cardiovascular disease is unknown. Purpose To conduct a prespecified analysis aimed at assessing whether the presence of cardiovascular disease at baseline modified the effect of HDQIV vs SDQIV on clinical outcomes in the DANFLU-1 trial. Methods DANFLU-1 was a pragmatic, open-label trial randomizing 12,477 adults aged 65-79 years 1:1 to HDQIV vs SDQIV during the 2021/2022 influenza season. The trial used the Danish nationwide registers to collect both baseline and outcome data. Participants were included in October and November of 2021 and follow-up ended on May 31, 2022. In this prespecified analysis, we assessed the effectiveness of HDQIV vs SDQIV on clinical endpoints according to the presence of cardiovascular disease at baseline and across different cardiovascular subgroups including ischemic heart disease, heart failure and atrial fibrillation. Effectiveness was assessed with Cox proportional hazards regression. In addition, hospitalizations were also assessed with recurrent event analysis using negative binomial regression. Results The mean age of the 12,477 participants in the DANFLU-1 trial was 71.7 years (SD 3.9) and 6600 (53%) were male. During a median follow-up of 237 days (interquartile range 232-239 days), 62 participants died (0.5%), 38 (0.3%) were hospitalized with pneumonia or influenza, 220 (1.8%) were hospitalized with a cardiorespiratory diagnosis, and 192 (1.5%) were hospitalized with a cardiovascular diagnosis. HDQIV was associated with a lower incidence of death during follow-up as compared with SDQIV (HR 0.51 [95% CI 0.30-0.86], p=0.012), and this effect was consistent in participants with CVD and without CVD and across cardiovascular subgroups (Figure 1). HDQIV was also associated with a lower incidence of hospitalization with pneumonia or influenza (IRR 0.30 (95% CI 0.14-0.64, p=0.002), and this effect was consistent irrespective of CVD (subgroup estimates were inestimable due to low power) (Figure 2). There was no difference in the incidence of cardiorespiratory or cardiovascular hospitalization between the HDQIV and SDQIV group. No significant effect modification by the presence of CVD or cardiovascular subgroups was observed (Figure 1 and 2). Conclusions In this prespecified analysis of the DANFLU-1 randomized trial, high-dose influenza vaccine was associated with a reduced rate of death and hospitalization with influenza or pneumonia and this effect was consistent in participants with CVD and across cardiovascular subgroups.Figure 1:Time to event analysisFigure 2:Recurrent event analysis

Relative effectiveness of high-dose vs. standard-dose quadrivalent influenza vaccine according to time of day of vaccination: a post-hoc analysis of the DANFLU-1 randomised clinical trial

Abstract Background Circadian variations have been shown to affect antibody response following vaccination. We sought to evaluate whether time of day of vaccination (ToV) modified the relative effectiveness of high-dose (QIV-HD) vs. standard-dose (QIV-SD) quadrivalent influenza vaccination, and further whether ToV was independently associated with hospitalisations and mortality. Methods This is a post-hoc analysis of the DANFLU-1 trial which was a randomized trial of QIV-HD vs. QIV-SD in adults aged 65-79 years conducted during the 2021-2022 influenza season. Vaccination took place between 7:05 AM and 8:08 PM. For this study, we stratified participants into early and late vaccination groups according to ToV before or after 11 AM, respectively. Prespecified outcomes included hospitalisations for pneumonia or influenza, respiratory disease, cardiovascular disease, and cardio-respiratory disease as well as all-cause hospitalisations and mortality. Hospitalisations were analysed as recurrent events using negative binomial regression, while mortality was analysed using Cox proportional hazard models, both of which were also used to test for interaction. For multivariable analysis, models were adjusted for baseline risk factors including sex, age, and multiple lung diseases. ToV was furthermore assessed as a continuous variable with 2-hour increments. Results In the final study population, the early group consisted of 5,371 (43%) participants (mean age 71.6±3.9 years, male sex 53.6%, QIV-SD 50.1%), while the late group consisted of 7,106 (57%) (mean age 71.8±4.0 years, male sex 52.4%, QIV-HD 49.9%). During follow-up, we observed 43 hospitalisations for pneumonia or influenza, 76 respiratory hospitalisations, and 62 mortalities. In recurrent event analysis, QIV-HD was associated with lower incidence of hospitalisation for pneumonia or influenza compared with QIV-SD irrespective of whether administered early (IRR 0.17 (95% CI 0.03-0.86), p=0.032) or late (IRR 0.37 (95% CI 0.16-0.88), p=0.024) (p-value for interaction = 0.69) (Figure 1). Effect estimates were consistent for all outcomes excluding cardiovascular hospitalisations regardless of early or late vaccination, and no effect modification was found (Figure 1). Interestingly, late vaccination was associated with higher incidence rate of respiratory hospitalisation (IRR 3.00 (95% CI 1.13-7.98), p=0.024, Figure 2), irrespective of vaccine type. The same association was found for ToV as a continuous variable (IRR 1.28 per 2-hour increment (95% CI 1.02-1.60), p=0.035). All above associations remained significant in multivariable analysis. Conclusion QIV-HD was associated with lower incidence of hospitalisation for pneumonia or influenza regardless of time of day for vaccination. In addition, later vaccination was associated with higher incidence of respiratory hospitalisation independent of vaccine type and known risk factors. Due to low power, however, this finding should be regarded as exploratory.Figure 1:Forest plotFigure 2:Incidence rates

Evaluation of the preventive effectiveness of russian influenza vaccines in the epidemic seasons of 2018-2021

Monitoring the effectiveness of influenza vaccines annually enables the development of measures aimed at decreasing influenza incidence, hospitalization, and mortality rates. The epidemiological effectiveness of several Russian domestic influenza vaccines was evaluated in St. Petersburg in the 2018-2021 epidemic seasons. Male and female participants (N = 6912) aged 18-23 were monitored from 2018 to 2021. Sovigripp, Grippol Plus, Ultrix, and Ultrix Quadri inactivated influenza vaccines were used for immunization. In 2018-2019, when the vaccine strains fully matched the circulating influenza viruses, prophylactic vaccination resulted in a 2.7 to 7.1-fold reduction in influenza cases. The protective effectiveness of the vaccines against influenza and acute respiratory infections (ARIs) totalled 52.4%, reaching 73.0% effectiveness against influenza after laboratory confirmation. In the epidemic season 2019-2020, when the circulating viruses and vaccine strains did not fully match, the incidence of influenza and ARIs reduced twice due to vaccination; the total vaccine effectiveness against influenza and ARIs amounted to 50.0%. The Grippol Plus, Ultrix, and Ultrix Quadri vaccines proved to be the most effective, demonstrating a total effectiveness of 70.6%-75.0% and a 65.5%-83.5% effectiveness against influenza A and B. In the 2020-2021 epidemic season, no data on the preventive effectiveness of the influenza vaccines could be obtained due to the outbreak of COVID-19 and the absence of detectable influenza virus shedding in the study participants. The results obtained are consistent with WHO experts’ estimates which indicate that modern influenza vaccines reduce the incidence of influenza in adults by 70-90% if the vaccine strains match the circulating virus strains.

Influenza Vaccine Effectiveness Against Illness and Asymptomatic Infection in 2022-2023: A Prospective Cohort Study.

Previous estimates of vaccine effectiveness (VE) against asymptomatic influenza virus infection based on seroconversion have varied widely and may be biased. We estimated 2022-2023 influenza VE against illness and asymptomatic infection in a prospective cohort. In the HEROES-RECOVER cohort, adults at increased occupational risk of influenza exposure across 7 US sites provided weekly symptom reports and nasal swabs for reverse transcription-polymerase chain reaction (RT-PCR) influenza testing. Laboratory-confirmed influenza virus infections were classified as symptomatic (≥1 symptom) or asymptomatic during the week of testing. Participants reported demographic information and vaccination through surveys; most sites verified vaccination through medical record and immunization registry review. Person-time was calculated as days from the site-specific influenza season start (September-October 2022) through date of infection, study withdrawal, or season end (May 2023). We compared influenza incidence among vaccinated versus unvaccinated participants overall, by symptom status, and by influenza A subtype, using Cox proportional hazards regression adjusted for site and occupation. We estimated VE as (1 - adjusted hazard ratio) × 100%. In total, 269 of 3785 (7.1%) participants had laboratory-confirmed influenza, including 263 (98%) influenza A virus infections and 201 (75%) symptomatic illnesses. Incidence of laboratory-confirmed influenza illness among vaccinated versus unvaccinated participants was 23.7 and 33.2 episodes per 100 000 person-days, respectively (VE: 38%; 95% CI: 15%-55%). Incidence of asymptomatic influenza virus infection was 8.0 versus 11.6 per 100 000 (VE: 13%; 95% CI: -47%, 49%). Vaccination reduced incidence of symptomatic but not asymptomatic influenza virus infection, suggesting that influenza vaccination attenuates progression from infection to illness.

Contribution of immunoglobulin products in influencing seasonal influenza infection and severity in antibody immune deficiency patients receiving immunoglobulin replacement therapy.

Seasonal and pandemic influenza infection present a potential threat to patients with antibody deficiency. The acceptance and effect of the current recommendation for annual vaccination against influenza for patients with antibody deficiency is not well investigated and due to antigenic drift or shift the protective capacity of regular IgG replacement therapy (IgRT) is considered low. This narrative review considers the effect of influenza vaccination in immunodeficient patients and discusses available information on the effect of immunoglobulin products on seasonal influenza infectivity and severity in antibody deficiency patients receiving IgRT. The humoral immune response to seasonal influenza vaccination is reduced in patients with antibody immune deficiency. However, there is no evidence that the proportion of patients with primary antibody deficiency who develop influenza illness, and the severity of such illness, is increased when compared with the general population. The IgRT that patients receive has been shown to contain neutralizing antibodies as a consequence of past flu infections against both the hemagglutinin and neuraminidase surface proteins and other viral internal proteins of different influenza A virus strains. Studies have demonstrated not only significant levels of specific but also cross-reactive antibodies against seasonal influenza virus strains. Thus, despite the yearly changes in influenza viral antigenicity that occur, IgRT could potentially contribute to the protection of patients against seasonal influenza. Currently, only limited clinical data are available confirming a preventative effect of IgRT with respect to seasonal influenza infection. In conclusion, there is some evidence that IgRT could contribute to protection against seasonal influenza in patients with antibody-related immunodeficiency. However, additional clinical data are needed to confirm the extent and relevance of this protection and identify the main responsible virus targets of that protection.

The effect of influenza vaccination on the rate of dementia amongst older adults.

Previous studies have reported conflicting results regarding the association between influenza vaccination and dementia. This association was investigated in a nationwide register-based cohort study. Using nationwide registries, dementia-free adults aged ≥65 years in Denmark from 2002 to 2018 without previous influenza vaccinations were included. Poisson regression facilitated confounder-adjusted comparisons of dementia rates for ever versus never vaccinated, number of vaccinationsand within/after 5 years from first vaccination. Sensitivity analyses included stratification on age and sex. Vaccination during follow-up was associated with a slightly higher rate of dementia when adjusted for sociodemographic factors and comorbidities, both within and after the first 5 years from first vaccination (incidence rate ratio [IRR] 1.04; 95% confidence interval [CI] 1.03-1.05). The rate of dementia decreased with increasing number of vaccinations. The highest rate was amongst those with only one vaccination (IRR 1.14; 95% CI 1.12-1.17) and the rate of dementia was only decreased amongst those with six or more vaccinations (IRR 0.95; 95% CI 0.93-0.97). Applying the same models to control outcomes of hip fracture and cancer resulted in higher rates amongst vaccinated people of 6% and 7%, respectively. Vaccinated people also had a 10% higher mortality rate. Our results do not support the case for a preventive effect of influenza vaccination on the risk of dementia in the general population, as reported by some previous studies. However, the higher dementia rate amongst vaccinated people found in this study is probably due to residual confounding, indicated by a higher rate for control outcomes and mortality.

Interim Effectiveness Estimates of 2024 Southern Hemisphere Influenza Vaccines in Preventing Influenza-Associated Hospitalization - REVELAC-i Network, Five South American Countries, March-July 2024.

To reduce influenza-associated morbidity and mortality, countries in South America recommend annual influenza vaccination for persons at high risk for severe influenza illness, including young children, persons with preexisting health conditions, and older adults. Interim estimates of influenza vaccine effectiveness (VE) from Southern Hemisphere countries can provide early information about the protective effects of vaccination and help guide Northern Hemisphere countries in advance of their season. Using data from a multicountry network, investigators estimated interim VE against influenza-associated severe acute respiratory illness (SARI) hospitalization using a test-negative case-control design. During March 13-July 19, 2024, Argentina, Brazil, Chile, Paraguay, and Uruguay identified 11,751 influenza-associated SARI cases; on average, 21.3% of patients were vaccinated against influenza, and the adjusted VE against hospitalization was 34.5%. The adjusted VE against the predominating subtype A(H3N2) was 36.5% and against A(H1N1)pdm09 was 37.1%. These interim VE estimates suggest that although the proportion of hospitalized patients who were vaccinated was modest, vaccination with the Southern Hemisphere influenza vaccine significantly lowered the risk for hospitalization. Northern Hemisphere countries should, therefore, anticipate the need for robust influenza vaccination campaigns and early antiviral treatment to achieve optimal protection against influenza-associated complications.

Effectiveness of influenza vaccines in children aged 6 to 59 months: a test-negative case-control study at primary care and hospital level, Spain 2023/24.

During 2023/24, all children aged 6 to 59 months were targeted for seasonal influenza vaccination in Spain nationally. Using a test-negative case-control design with sentinel surveillance data, we estimated adjusted influenza vaccine effectiveness (IVE) against any influenza type to be 70% (95% confidence interval (CI): 51 to 81%) for primary care patients with acute respiratory illness (ARI) and 77% (95% CI: 21 to 93%) for hospitalised patients with severe ARI. In primary care, where most subtyped viruses (61%; 145/237) were A(H1N1), adjusted IVE was 77% (95% CI: 56 to 88%) against A(H1N1)pdm09.

Influenza vaccine effectiveness against influenza-associated hospitalizations in children, Hong Kong, November 2023 to June 2024

We conducted a test negative study from November 2023 to June 2024, enrolling 4,367 children hospitalized with acute respiratory illness in Hong Kong. Among the children who tested negative for influenza virus and SARS-CoV-2, 56.8 % had received influenza vaccination. Between November 2023 and March 2024, influenza A(H3N2) predominated and the VE against influenza A(H3N2) was estimated as 55 % (95 % CI: 29.6 %, 71.8 %). VE point estimates were higher for younger children than older children. In February to June 2024 influenza A(H1N1) predominated and VE against influenza A(H1N1) was 54 % (95 % CI: 33 %, 69 %) during this period. Influenza B/Victoria circulated at low intensity throughout the 2023/24 season and VE against influenza B was 66 % (95 % CI: 42 %, 80 %). Since its introduction in 2018/19 the school-based influenza vaccination program has substantially increased vaccine uptake in children in Hong Kong and prevented influenza-associated hospitalizations.

Randomized Immunogenicity Trial Comparing 2019-2020 Recombinant and Egg-Based Influenza Vaccines among Frequently Vaccinated Healthcare Personnel in Israel

Trivalent inactivated influenza vaccine effectiveness was low in a prospective cohort of healthcare personnel (HCP) in Israel from 2016-2019. We conducted a randomized immunogenicity trial of quadrivalent recombinant influenza vaccine (RIV4) and standard-dose inactivated influenza vaccine (IIV4) among frequently and infrequently vaccinated previous cohort participants. From October 2019 to January 2020, we enrolled and randomly allocated HCP from two Israeli hospitals to receive IIV4 or RIV4. Hemagglutination inhibition (HAI) antibody titers against 2019-2020 vaccine reference influenza viruses were compared between vaccine groups using geometric mean titer (GMT) ratios from sera collected one-month post-vaccination and by frequency of vaccination in the past 5 years (>2 versus ≤2). Among 415 HCP, the GMT ratio comparing RIV4 to IIV4 was 2.0 (95% confidence interval [CI] 1.7-2.7) for A(H1N1)pdm09, 1.6 (95% CI: 1.3-1.9) for A(H3N2), 1.8 (95% CI: 1.4-2.2) for B(Yamagata), and 1.1 (95% CI: 0.9-1.4) for B(Victoria). Similarly, RIV4 elicited higher HAI titers than IIV4 against all 2019-2020 vaccine reference viruses except B(Victoria) among infrequently and frequently vaccinated HCP (lower bound of GMT ratio 95% CIs ≥1.0). RIV4 had improved immunogenicity for influenza vaccine strains among both infrequent and frequent vaccinees compared to standard-dose IIV4.

Influenza Vaccine Effectiveness against Influenza A-Associated Outpatient and Emergency-Department-Attended Influenza-like Illness during the Delayed 2022-2023 Season in Beijing, China.

During the 2022-2023 influenza season, the influenza activities in most regions of China were postponed, including Beijing. The unusually delayed influenza epidemic posed a challenge to the effectiveness of the influenza vaccine. Using the test-negative design, we evaluated influenza vaccine effectiveness (VE) during the 2022-2023 influenza season against influenza A-associated outpatient and emergency-department-attended influenza-like illness (ILI) in Beijing, China, from 9 January to 30 April 2023. The analysis included 8301 medically attended ILI patients, of which 1342 (46.2%) had influenza A(H1N1)pdm09, 1554 (53.4%) had influenza A(H3N2), and 11 (0.4%) had co-infection of the two viruses. VE against influenza A-associated ILI patients was 23.2% (95% CI: -6.5% to 44.6%) overall, and 23.1%, 9.9%, and 33.8% among children aged 6 months to 17 years, adults aged 18-59 years, and adults aged ≥60 years, respectively. VE against influenza A(H1N1)pdm09 and against influenza A(H3N2) were 36.2% (95% CI: -1.9% to 60.1%) and 9.5% (95% CI: -34.1% to 39.0%), respectively. VE of the group with vaccination intervals of 14-90 days (70.1%, 95% CI: -145.4 to 96.4) was higher than that of the groups with a vaccination interval of 90-149 days (18.7%, 95% CI: -42.4% to 53.6%) and ≥150 days (21.2%, 95% CI: -18.8% to 47.7%). A moderate VE against influenza A(H1N1)pdm09 and a low VE against influenza A(H3N2) were observed in Beijing during the 2022-2023 influenza season, a season characterized with a delayed and high-intensity influenza epidemic. VE appears to be better within three months after vaccination. Our findings indicate a potential need for the optimization of vaccination policies and underscore the importance of continuous monitoring of influenza to enhance vaccines and optimizing vaccination timing.

Quantifying and Adjusting for Confounding From Health-Seeking Behavior and Health Care Access in Observational Research.

Health-seeking behavior and health care access (HSB/HCA) are recognized confounders in many observational studies but are not directly measurable in electronic health records. We used proxy markers of HSB/HCA to quantify and adjust for confounding in observational studies of influenza and COVID-19 vaccine effectiveness (VE). This cohort study used primary care data prelinked to secondary care and death data in England. We included individuals aged ≥66 years on 1 September 2019 and assessed influenza VE in the 2019-2020 season and early COVID-19 VE (December 2020-March 2021). VE was estimated with sequential adjustment for demographics, comorbidities, and 14 markers of HSB/HCA. Influenza vaccination in the 2019-2020 season was also considered a negative control exposure against COVID-19 before COVID-19 vaccine rollout. We included 1 991 284, 1 796 667, and 1 946 943 individuals in the influenza, COVID-19, and negative control exposure populations, respectively. Markers of HSB/HCA were positively correlated with influenza and COVID-19 vaccine uptake. For influenza, adjusting for HSB/HCA markers in addition to demographics and comorbidities increased VE against influenza-like illness from -1.5% (95% CI, -3.2% to .1%) to 7.1% (95% CI, 5.4%-8.7%) with a less apparent trend for more severe outcomes. For COVID-19, adjusting for HSB/HCA markers did not change VE estimates against infection or severe disease (eg, 2 doses of BNT162b2 against infection: 82.8% [95% CI, 78.4%-86.3%] to 83.1% [95% CI, 78.7%-86.5%]). Adjusting for HSB/HCA markers removed bias in the negative control exposure analysis (-7.5% [95% CI, -10.6% to -4.5%] vs -2.1% [95% CI, -6.0% to 1.7%] before vs after adjusting for HSB/HCA markers). Markers of HSB/HCA can be used to quantify and account for confounding in observational vaccine studies.

Influenza vaccine effectiveness in immunocompromised patients with cancer: A Danish nationwide register‐based cohort study

AbstractBackgroundInfluenza vaccination is free of charge for Danish citizens with acquired immunodeficiency but recommendations do not specifically target patients with cancer. This study investigated whether influenza vaccination reduces the main outcome of overall mortality and the secondary outcomes of influenza requiring treatment, pneumonia, myocardial infarction, stroke, heart failure, and venous thromboembolism in patients with cancer.MethodsThis was a register‐based nationwide cohort study. Adjusted hazard ratios (aHRs) and 95% confidence intervals (95% CIs) for overall mortality and secondary outcomes were estimated using Cox proportional hazards models. Analyses were conducted separately for four subgroups: patients aged <65 years with solid tumors, patients aged ≥65 years with solid tumors, patients aged <65 years with hematological cancer, and patients aged ≥65 years with hematological cancer.ResultsA total of 53,249 adult patients with solid tumors who received chemotherapy and 22,182 adult patients with hematological cancer were followed for up to five influenza seasons in the study period of 2007–2018. In the main analysis covering December–March, influenza vaccination was associated with reduced overall mortality in all four subgroups. The reduction was most pronounced in patients with hematological cancer aged <65 years (aHR, 0.66; 95% CI, 0.51–0.87) and smallest in patients with solid tumors aged <65 years (aHR, 0.91; 95% CI, 0.84–0.99). In sensitivity analyses covering January–March, the aHR was 0.87 (95% CI, 0.65–1.16) in patients with hematological cancer aged <65 years and 1.01 (95% CI, 0.92–1.10) in patients with solid tumors aged <65 years. Results for the secondary outcomes were inconclusive.ConclusionsThe results of this study cannot reject that influenza vaccination reduces overall mortality in immunocompromised patients with cancer. The results must be interpreted with caution because of potential unmeasured confounding, which can result in the overestimation of influenza vaccine effectiveness.