Influenza Vaccine Effectiveness Estimates Research Articles

Effectiveness of inactivated influenza vaccine against laboratory-confirmed influenza among Chinese elderly: a test-negative design

BackgroundEvidence on the effectiveness of influenza vaccination in the elderly is limited, and results are controversial. There are also few reports from China.MethodsWe conducted a test-negative case-control study design to estimate influenza vaccine effectiveness (VE) against laboratory-confirmed influenza-associated visits among elderly (aged ≥ 60 years) across four influenza seasons in Ningbo, China, from 2018 to 19 to 2021-22. Influenza-positive cases and negative controls were randomly matched in a 1:1 ratio according to age, sex, hospital, and date of influenza testing. We used logistic regression models to compare vaccination odds ratios (ORs) in cases to controls. We calculated the VE as [100% × (1-adjusted OR)] and calculated the 95% confidence interval (CI) around the estimate.ResultsA total of 30,630 elderly patients tested for influenza with virus nucleic acid or antigen during the study period. After exclusions, we included 1 825 influenza-positive cases and 1 825 influenza-negative controls. Overall, the adjusted VE for influenza-related visits was 63.5% (95% CI, 56.3–69.5%), but varied by season. Influenza VE was 59.8% (95% CI, 51.5–66.7%) for influenza A and 89.6% (95% CI, 77.1–95.3%) for influenza B. The VE for ages 60–69 and 70–79 was 65.2% (95% CI, 55.4–72.9%) and 69.8% (95% CI, 58.7–77.9%), respectively, but only 45.4% (95% CI, 6.2–68.2%) for ages 80 and over.ConclusionsStandard-dose inactivated influenza vaccine has shown good protection in the elderly in China. However, protection may not be satisfactory in people aged 80 years and older.

Estimates of Seasonal Influenza Burden That Could Be Averted by Improved Influenza Vaccines in the Australian Population Aged Under 65 Years, 2015-2019.

The interpretation of relative vaccine effectiveness (rVE) of improved influenza vaccines is complex. Estimation of burden averted is useful to contextualise their potential impact across different seasons. For the population aged under 65 years in Australia, this study estimated the additional morbidity and mortality that could be averted using improved influenza vaccines. We used observed, season-specific (2015-2019) influenza notification and influenza-coded hospitalisation frequencies and published modelled estimates of influenza-associated hospitalisations and deaths that occurred under the prevailing influenza vaccination coverage scenario. After back-calculating to the estimated burden in the population without vaccination, we applied published standard influenza vaccine effectiveness and coverage estimates to calculate the burden potentially averted by standard and improved influenza vaccines. A plausible range of rVE values were used, assuming 50% coverage. The percentage point difference in absolute vaccine effectiveness (VE) of an improved vaccine compared to a standard vaccine is directly proportional to its rVE and inversely proportional to the effectiveness of the standard vaccine. The incremental burden averted by an improved vaccine is a function of both its difference in absolute VE and the severity of the influenza season. Assuming an rVE of 15% with 50% coverage, the improved vaccine was estimated to additionally avert 1517 to 12,641 influenza notifications, 287 to 1311 influenza-coded hospitalisations and 9 to 33 modelled all-cause influenza deaths per year compared to the standard vaccine. Improved vaccines can have substantial clinical and population impact, particularly when the effectiveness of standard vaccines is low, and burden is high.

Mid-Term Estimates of Influenza Vaccine Effectiveness against the A(H1N1)pdm09 Prevalent Circulating Subtype in the 2023/24 Season: Data from the Sicilian RespiVirNet Surveillance System.

The current influenza season started in Italy in October 2023, approaching the epidemic peak in late December (52nd week of the year). We aimed to explore the mid-term virologic surveillance data of the 2023/2024 influenza season (from 16 October 2023 to 7 January 2024) in Sicily, the fourth most populous Italian region. A test-negative design was used to estimate the effectiveness of seasonal influenza vaccine (VE) against A(H1N1)pdm09 virus, the predominant subtype in Sicily (96.2% of laboratory-confirmed influenza cases). Overall, 29.2% (n = 359/1230) of oropharyngeal swabs collected from patients with influenza-like illness (ILI) were positive for influenza. Among the laboratory-confirmed influenza cases, 12.5% (n = 45/359) were vaccinated against influenza, with higher prevalence of laboratory-confirmed diagnosis of influenza A among subjects vaccinated with quadrivalent inactivated standard dose (29.4%), live attenuated intranasal (25.1%), and quadrivalent inactivated high-dose (23.8%) influenza vaccines. Comparing influenza vaccination status for the 2023/2024 season among laboratory-confirmed influenza-positive and -negative samples, higher vaccination rates in influenza-negative samples (vs. positive) were observed in all age groups, except for 45-64 years old, regardless of sex and comorbidities. The overall adjusted VE (adj-VE) was 41.4% [95%CI: 10.5-61.6%], whereas the adj-VE was 37.9% [95%CI: -0.7-61.7%] among children 7 months-14 years old and 52.7% [95%CI: -38.0-83.8%] among the elderly (≥65 years old).

Interim Influenza Vaccine Effectiveness Against Laboratory-Confirmed Influenza - California, October 2023-January 2024.

Surveillance data can provide rapid, within-season influenza vaccine effectiveness (VE) estimates to guide public health recommendations. Mandatory reporting of influenza vaccine administration to California's immunization information registry began January 1, 2023, and mandatory reporting of all influenza laboratory test results, including negative results, was instituted in California on June 15, 2023. These data, collected by the California Department of Public Health during October 1, 2023-January 31, 2024, were used to calculate interim influenza VE against laboratory-confirmed influenza by comparing the odds of vaccination among case-patients (persons who received a positive influenza laboratory test result) and control patients (those who received a negative influenza laboratory test result). VE was calculated as 1 - adjusted odds ratio using mixed-effects logistic regression, with age, race, and ethnicity as fixed effects and specimen collection week and county as random effects. Overall, during October 1, 2023-January 31, 2024, estimated VE was 45% among persons aged ≥6 months, 56% among children and adolescents aged 6 months-17 years, 48% among adults aged 18-49 years, 36% among those aged 50-64 years, and 30% among those aged ≥65 years. Consistent with some previous influenza seasons, influenza vaccination provided moderate protection against laboratory-confirmed influenza among infants, children, adolescents, and adults. All persons aged ≥6 months without a contraindication to vaccination should receive annual influenza vaccination to reduce influenza illness, severe influenza, and strain on health care resources. Influenza vaccination remains the best way to prevent influenza.

Interim 2023/24 influenza A vaccine effectiveness: VEBIS European primary care and hospital multicentre studies, September 2023 to January 2024.

Influenza A viruses circulated in Europe from September 2023 to January 2024, with influenza A(H1N1)pdm09 predominance. We provide interim 2023/24 influenza vaccine effectiveness (IVE) estimates from two European studies, covering 10 countries across primary care (EU-PC) and hospital (EU-H) settings. Interim IVE was higher against A(H1N1)pdm09 than A(H3N2): EU-PC influenza A(H1N1)pdm09 IVE was 53% (95% CI: 41 to 63) and 30% (95% CI: -3 to 54) against influenza A(H3N2). For EU-H, these were 44% (95% CI: 30 to 55) and 14% (95% CI: -32 to 43), respectively.

Early influenza vaccine effectiveness estimates using routinely collected data, Alberta, Canada, 2023/24 season.

Timely and precise influenza vaccine effectiveness (VE) estimates are needed to guide public health messaging and impact vaccine uptake immediately. Using routinely collected laboratory, vaccination and health administrative data from Alberta, Canada, we estimated influenza VE against infection for the 2023/24 season on a near real-time basis, to late December, at 61% (95% CI: 58-64) against influenza A(H1N1), 49% (95% CI: 28-63) against influenza A(H3N2) and 75% (95% CI: 58-85) against influenza B.

Influenza incidence, lineages, and vaccine effectiveness estimates in Lima, Peru, 2023

Influenza incidence, lineages, and vaccine effectiveness estimates in Lima, Peru, 2023

Schätzung der Wirksamkeit der Grippeimpfung anhand von Sekundärdaten: Eine Kohortenstudie und Propensity-Score-Matching-Analyse von Leistungsdaten aus Baden-Württemberg

Ziel war es die Wirksamkeit der Influenza-Impfung (VE) für die Grippesaison 2014/2015 auf Grundlage von Routinedaten aus Krankenkassendatensatz zu schätzen und zu replizieren. Zusätzlich sollten methodische Aspekte untersucht werden. Es wurden Abrechnungsdaten von 2,64 Millionen Versicherten der AOK Baden-Württemberg mit dortigem Wohnsitz ab 15 Jahren analysiert. Basierend auf Abrechnungsdaten für die Influenza-Impfung 2014, wurden die Teilnehmer als ungeimpft oder geimpft klassifiziert. Kovariablen, die den Zusammenhang zwischen Impfung und Influenzainfektion beeinträchtigen könnten, wurden berücksichtigt. Hierzu gehörten Alter, Geschlecht, Wohnort sowie Kovariablen, die auf den Gesundheitszustand und die Inanspruchnahme von Gesundheitsdienstleistungen hinweisen. Der primäre Endpunkt war ein Krankenhausaufenthalt wegen Influenza während der Grippesaison 2015. Zu den sekundären Endpunkten gehörten unter anderem Krankenhausaufenthalte wegen Lungenentzündung und die Gesamtmortalität. Um eine vergleichbare Gruppe von geimpften und ungeimpften Teilnehmern zu ermitteln, wurde ein Propensity-Score-Matching (PSM) durchgeführt. Es wurde eine Bias-Analyse durchgeführt, bei der die VE vor und nach der Grippesaison geschätzt wurde, also zu Zeitpunkten, in denen angenommen wurde, dass die Influenza nicht in der Bevölkerung zirkulierte und die Impfung nicht wirken konnte. Insgesamt konnten 839.706 Teilnehmer 1:1 gematcht werden. Die geschätzte VE (basierend auf Influenza bedingten Krankenhausaufenthalten) betrug 27% [95%Konfidenzintervall (KI): 17%; 36%], was der Schätzung des RKI für dieselbe Saison (27% [95%KI: -1%; 47%]) entspricht. Die Bias-Analyse zeigte, dass das Ergebnis teilweise durch residuale Konfundierung erklärt werden kann, was zu einer potenziellen Überschätzung des zugrunde liegenden Effekts führt. Die Ergebnisse der sekundären Endpunkte zeigten ähnliche Ergebnisse, obwohl sie wahrscheinlich in höherem Maße durch residuale Konfundierung bedingt sind. Zusammenfassend zeigt sich, dass (1) sekundäre Daten der deutschen Krankenkassen verwendet werden können, um plausible VE-Schätzungen abzuleiten, und dass (2) das PSM eine nützliche und transparente Methode zur Ableitung dieser Schätzungen ist. Darüber hinaus ist (3) residuale Konfundierung ein relevantes Problem in Beobachtungsstudien zu VE und (4) Bias-Analysen vor- und nach der Grippesaison sind eine wesentliche Ergänzung für die Interpretation der Ergebnisse.

1789. Comparison of influenza vaccine effectiveness estimates from the US Influenza Vaccine Effectiveness Network and Electronic Health Record Source Population Data, 2021-2022

Abstract Background The COVID-19 pandemic led to increased respiratory viral testing for patients presenting with acute respiratory infection (ARI). Electronic health record (EHR) data from these visits can be used to assess influenza and COVID-19 vaccine effectiveness (VE). We compared VE data from participating healthcare institutions with VE against medically attended influenza among systematically tested patients actively enrolled in the US Flu VE Network. Methods During the 2021-2022 influenza season, seven sites in the US Flu VE Network identified medically attended episodes of ARI in ambulatory settings based on ICD-10 diagnostic codes and clinical influenza and SARS-CoV-2 testing. Influenza status was determined from the clinical test results. Vaccination status was extracted from the EHR. During the same period, patients with ARI presenting at ambulatory clinics were actively enrolled and tested for influenza. VE and 95% confidence intervals (CI) were estimated comparing the odds of vaccination among influenza-positive versus influenza-negative patients using logistic regression models controlling for age, site, and calendar time. Results During the 2021-2022 season, 220,162 medical visits for ARI with clinical testing were identified; 2% of tests were influenza positive, 26% were SARS-CoV-2 positive and 73% were negative for both. Overall, 34% of influenza positive and 42% of influenza negative patients had received influenza vaccines; VE using EHR data was 34% [95% CI 29-38]. Among 6,260 enrolled patients, 7% tested positive for influenza, 34% were SARS-CoV-2 positive and 62% were negative for both. Overall, 42% of influenza positive and 59% of influenza negative patients had received influenza vaccines. VE using enrolled patient data was 36% [95% CI 20-49]. VE using EHR data was 35%, 34%, and 6% for patients aged 6 months-17 years, 18-49, and 50+ years versus 40%, 32%, and 10% for enrolled patients. Conclusion EHR based influenza VE estimates were similar to estimates from the US Flu VE study. Through continued widespread clinical testing, EHR-based platforms may present opportunities to assess VE in sub-groups with insufficient sample size in active surveillance. Disclosures MaryPatricia Nowalk, PhD, RDN, Merck & Co.: Grant/Research Support|Merck & Co.: Honoraria|Sanofi: Grant/Research Support Emily T. Martin, PhD, MPH, Merck: Grant/Research Support Edward Belongia, MD, Seqirus: Grant/Research Support Carlos G. Grijalva, MD, MPH, AHRQ: Grant/Research Support|CDC: Grant/Research Support|FDA: Grant/Research Support|Merck: Advisor/Consultant|NIH: Grant/Research Support|Syneos Health: Grant/Research Support

Influenza Vaccine Effectiveness Against Influenza-Associated Hospitalization in Hong Kong Children Aged 9 Months to 17 Years, March-June 2023.

In March-June 2023, we conducted a test-negative study in 1671 children who were hospitalized with acute respiratory illness in Hong Kong. Two hundred and eighty-six children (17.2%) were tested positive for influenza virus including 188 with A(H1N1). We estimated influenza vaccine effectiveness against influenza-associated hospitalization as 69.6% (95% confidence interval: 49.3%, 81.7%).

Influenza Vaccine Effectiveness in Preventing Laboratory-Confirmed Influenza Cases and Hospitalizations in Navarre, Spain, 2022-2023.

We estimated influenza vaccine effectiveness (IVE) in preventing outpatient and hospitalized cases in the 2022-2023 season. A test-negative design included a representative sample of outpatients and all hospitalized patients with influenza-like illness (ILI) from October 2022 to May 2023 in Navarre, Spain. ILI patients were tested by PCR for influenza virus. Influenza vaccination status was compared between confirmed influenza cases and test-negative controls. Among 3321 ILI patients tested, IVE to prevent influenza cases was 34% (95% confidence interval (CI): 16 to 48) overall, 85% (95%CI: 63 to 94) against influenza B, and 28% (95%CI: 3 to 46) against A(H3N2). Among 558 outpatients, 222 (40%) were confirmed for influenza: 55% A(H3N2), 11% A(H1N1), and 31% B. Overall, IVE to prevent outpatient cases was 48% (95%CI: 8 to 70), 88% (95%CI: 3 to 98) against influenza B, and 50% (95%CI: -4 to 76) against A(H3N2). Of 2763 hospitalized patients, 349 (13%) were positive for influenza: 64% A(H3N2), 17% A(H1N1), and 8% B. IVE to prevent hospitalization was 24% (95%CI: -1 to 42) overall, 82% (95%CI: 49 to 93) against influenza B, and 16% (95%CI: -17 to 40) against A(H3N2). No IVE was observed in preventing influenza A(H1N1). IVE was high to prevent influenza B, moderate against A(H3N2) and null against A(H1N1). A lower proportion of influenza B cases may explain the smaller IVE in hospitalized patients than in outpatients. The null IVE against A(H1N1) was consistent with the observed antigenic drift and supports the new composition of the 2023-2024 influenza vaccine.

Influenza Vaccine Effectiveness against Hospitalization, Season 2021/22: A Test-Negative Design Study in Barcelona.

Vaccination is considered the most effective measure for preventing influenza and its complications. The influenza vaccine effectiveness (IVE) varies annually due to the evolution of influenza viruses and the update of vaccine composition. Assessing the IVE is crucial to facilitate decision making in public health policies. to estimate the IVE against hospitalization and its determinants in the 2021/22 season in a Spanish tertiary hospital. We conducted a prospective observational test-negative design study within the Development of Robust and Innovative Vaccine Effectiveness (DRIVE) project. Hospitalized patients with severe acute respiratory infection (SARI) and an available influenza reverse transcription polymerase chain reaction (RT-PCR) were selected and classified as cases (positive influenza RT-PCR) or controls (negative influenza RT-PCR). Vaccine information was obtained from electronic clinical records shared by public healthcare providers. Information about potential confounders was obtained from hospital clinical registries. The IVE was calculated by subtracting the ratio of the odds of vaccination in cases and controls from one, as a percentage (IVE = (1 - odds ratio (OR)) × 100). Multivariate IVE estimates were calculated using logistic regression. In total, 260 severe acute respiratory infections (SARI) were identified, of which 34 were positive for influenza, and all were subtype A(H3N2). Fifty-three percent were vaccinated. Adjusted IVE against hospitalization was 26.4% (95% CI -69% to 112%). IVE determinants could not be explored due to sample size limitations. Our data revealed non-significant moderate vaccine effectiveness against hospitalization for the 2021/2022 season.

Brand-specific estimates of influenza vaccine effectiveness for the 2021-2022 season in Europe: results from the DRIVE multi-stakeholder study platform.

Development of Robust and Innovative Vaccine Effectiveness (DRIVE) was a European public-private partnership (PPP) that aimed to provide annual, brand-specific estimates of influenza vaccine effectiveness (IVE) for regulatory and public health purposes. DRIVE was launched in 2017 under the umbrella of the Innovative Medicines Initiative (IMI) and conducted IVE studies from its pilot season in 2017-2018 to its final season in 2021-2022. In 2021-2022, DRIVE conducted four primary care-based test-negative design (TND) studies (Austria, Italy, Iceland, and England; involving >1,000 general practitioners), nine hospital-based TND studies (France, Iceland, Italy, Romania, and Spain, for a total of 21 hospitals), and one population-based cohort study in Finland. In the TND studies, patients with influenza-like illness (primary care) or severe acute respiratory infection (hospital) were enrolled, and laboratory tested for influenza using RT-PCR. Study contributor-specific IVE was calculated using logistic regression, adjusting for age, sex, and calendar time, and pooled by meta-analysis. In 2021-2022, pooled confounder-adjusted influenza vaccine effectiveness (IVE) estimates against laboratory-confirmed influenza (LCI) overall and per type and subtype/lineage was produced, albeit with wide confidence intervals (CI). The limited circulation of influenza in Europe did not allow the network to reach the optimal sample size to produce precise IVE estimates for all the brands included. The most significant IVE estimates were 76% (95% CI 23%-93%) for any vaccine and 81% (22%-95%) for Vaxigrip Tetra in adults ≥65 years old and 64% (25%-83%) for Fluenz Tetra in children (TND primary care setting), 85% (12%-97%) for any vaccine in adults 18-64 years (TND hospital setting), and 38% (1%-62%) in children 6 months-6 years (population-based cohort, mixed setting). Over five seasons, DRIVE collected data on >35,000 patients, more than 60 variables, and 13 influenza vaccines. DRIVE demonstrated that estimating brand-specific IVE across Europe is possible, but achieving sufficient sample size to obtain precise estimates for all relevant stratifications remains a challenge. Finally, DRIVE's network of study contributors and lessons learned have greatly contributed to the development of the COVID-19 vaccine effectiveness platform COVIDRIVE.

Interim 2022/23 influenza vaccine effectiveness: six European studies, October 2022 to January 2023.

BackgroundBetween October 2022 and January 2023, influenza A(H1N1)pdm09, A(H3N2) and B/Victoria viruses circulated in Europe with different influenza (sub)types dominating in different areas.AimTo provide interim 2022/23 influenza vaccine effectiveness (VE) estimates from six European studies, covering 16 countries in primary care, emergency care and hospital inpatient settings.MethodsAll studies used the test-negative design, but with differences in other study characteristics, such as data sources, patient selection, case definitions and included age groups. Overall and influenza (sub)type-specific VE was estimated for each study using logistic regression adjusted for potential confounders.ResultsThere were 20,477 influenza cases recruited across the six studies, of which 16,589 (81%) were influenza A. Among all ages and settings, VE against influenza A ranged from 27 to 44%. Against A(H1N1)pdm09 (all ages and settings), VE point estimates ranged from 28% to 46%, higher among children (< 18 years) at 49-77%. Against A(H3N2), overall VE ranged from 2% to 44%, also higher among children (62-70%). Against influenza B/Victoria, overall and age-specific VE were ≥ 50% (87-95% among children < 18 years).ConclusionsInterim results from six European studies during the 2022/23 influenza season indicate a ≥ 27% and ≥ 50% reduction in disease occurrence among all-age influenza vaccine recipients for influenza A and B, respectively, with higher reductions among children. Genetic virus characterisation results and end-of-season VE estimates will contribute to greater understanding of differences in influenza (sub)type-specific results across studies.

Interim Estimates of 2022–23 Seasonal Influenza Vaccine Effectiveness — Wisconsin, October 2022–February 2023

In the United States, 2022-23 influenza activity began earlier than usual, increasing in October 2022, and has been associated with high rates of hospitalizations among children* (1). Influenza A(H3N2) represented most influenza viruses detected and subtyped during this period, but A(H1N1)pdm09 viruses cocirculated as well. Most viruses characterized were in the same genetic subclade as and antigenically similar to the viruses included in the 2022-23 Northern Hemisphere influenza vaccine (1,2). Effectiveness of influenza vaccine varies by season, influenza virus subtype, and antigenic match with circulating viruses. This interim report used data from two concurrent studies conducted at Marshfield Clinic Health System (MCHS) in Wisconsin during October 23, 2022-February 10, 2023, to estimate influenza vaccine effectiveness (VE). Overall, VE was 54% against medically attended outpatient acute respiratory illness (ARI) associated with laboratory-confirmed influenza A among patients aged 6 months-64 years. In a community cohort of children and adolescents aged <18 years, VE was 71% against symptomatic laboratory-confirmed influenza A virus infection. These interim analyses indicate that influenza vaccination substantially reduced the risk for medically attended influenza among persons aged <65 years and for symptomatic influenza in children and adolescents. Annual influenza vaccination is the best strategy for preventing influenza and its complications. CDC recommends that health care providers continue to administer annual influenza vaccine to persons aged ≥6 months as long as influenza viruses are circulating (2).

Investigating confounding in network-based test-negative design influenza vaccine effectiveness studies-Experience from the DRIVE project.

Establishing a large study network to conduct influenza vaccine effectiveness (IVE) studies while collecting appropriate variables to account for potential bias is important; the most relevant variables should be prioritized. We explored the impact of potential confounders on IVE in the DRIVE multi-country network of sites conducting test-negative design (TND) studies. We constructed a directed acyclic graph (DAG) to map the relationship between influenza vaccination, medically attended influenza infection, confounders, and other variables. Additionally, we used the Development of Robust and Innovative Vaccines Effectiveness (DRIVE) data from the 2018/2019 and 2019/2020 seasons to explore the effect of covariate adjustment on IVE estimates. The reference model was adjusted for age, sex, calendar time, and season. The covariates studied were presence of at least one, two, or three chronic diseases; presence of six specific chronic diseases; and prior healthcare use. Analyses were conducted by site and subsequently pooled. The following variables were included in the DAG: age, sex, time within influenza season and year, health status and comorbidities, study site, health-care-seeking behavior, contact patterns and social precautionary behavior, socioeconomic status, and pre-existing immunity. Across all age groups and settings, only adjustment for lung disease in older adults in the primary care setting resulted in a relative change of the IVE point estimate >10%. Our study supports a parsimonious approach to confounder adjustment in TND studies, limited to adjusting for age, sex, and calendar time. Practical implications are that necessitating fewer variables lowers the threshold for enrollment of sites in IVE studies and simplifies the pooling of data from different IVE studies or study networks.

Influenza Vaccine Effectiveness Against Influenza A(H3N2)-Related Illness in the United States During the 2021-2022 Influenza Season.

In the United States, influenza activity during the 2021-2022 season was modest and sufficient enough to estimate influenza vaccine effectiveness (VE) for the first time since the beginning of the coronavirus disease 2019 pandemic. We estimated influenza VE against laboratory-confirmed outpatient acute illness caused by predominant A(H3N2) viruses. Between October 2021 and April 2022, research staff across 7 sites enrolled patients aged ≥6 months seeking outpatient care for acute respiratory illness with cough. Using a test-negative design, we assessed VE against influenza A(H3N2). Due to strong correlation between influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination, participants who tested positive for SARS-CoV-2 were excluded from VE estimations. Estimates were adjusted for site, age, month of illness, race/ethnicity, and general health status. Among 6260 participants, 468 (7%) tested positive for influenza only, including 440 (94%) for A(H3N2). All 206 sequenced A(H3N2) viruses were characterized as belonging to genetic group 3C.2a1b subclade 2a.2, which has antigenic differences from the 2021-2022 season A(H3N2) vaccine component that belongs to clade 3C.2a1b subclade 2a.1. After excluding 1948 SARS-CoV-2-positive patients, 4312 patients were included in analyses of influenza VE; 2463 (57%) were vaccinated against influenza. Effectiveness against A(H3N2) for all ages was 36% (95% confidence interval, 20%-49%) overall. Influenza vaccination in 2021-2022 provided protection against influenza A(H3N2)-related outpatient visits among young persons.

Effectiveness of influenza vaccination in reducing influenza-like illness and related antibiotic prescriptions in adults from a primary care-based case-control study

BackgroundEvidence on the effectiveness of influenza vaccine in preventing antibiotic prescriptions for influenza-like illness (ILI) in adults is limited. MethodsA primary care-based case-control study was conducted to estimate influenza vaccine effectiveness (VE) against influenza-like illness (ILI) and antibiotic prescribing for ILI in adults aged ≥40 years. Cases were patients diagnosed with ILI from 1st June to 30th September in each year, 2015-2018; a subset of those with ILI prescribed antibiotics was also defined. Controls were patients attending a practice who did not receive an ILI diagnosis. Generalised estimating equations were used to calculate adjusted VE overall, by age (<65 versus ≥65 years) and comorbidity status. ResultsThe number of ILI cases varied from 558 in 2018 to 2901 in 2017 and controls from 86618 in 2015 to 136763 in 2017. Over 4 years the pooled estimate of VE was 24% (95%CI, 11% to 34%) against ILI and 15% (95%CI, -3% to 29%) against antibiotic prescription for ILI. Influenza vaccine was effective in reducing ILI with an associated antibiotic prescriptions in patients aged <65 years (VE=23%, 95%CI, 3% to 38%) and if no comorbidities were recorded (VE=22%, 95%CI, 1% to 39%) but not in other subgroups. ConclusionsInfluenza vaccine reduced the likelihood of antibiotic prescriptions for ILI in low-risk adults (40-64 years and those without comorbidities).

Repeat vaccination and influenza vaccine effectiveness

Influenza vaccine effectiveness estimates are estimates of the reduction in risk of influenza conferred by influenza vaccination and inform individuals and public health agencies about the benefits of vaccination. In many vaccine effectiveness studies, estimates are presented of the reduction in risk of medically-attended influenza virus infection among people who have been vaccinated versus those who have not in that season. The systematic review and meta-analysis by Elenor Jones-Gray and colleagues1 examines the important question of whether vaccination history can modify the vaccine effectiveness in the current year.

Influenza vaccination reduced myocardial infarctions in United Kingdom older adults: a prior event rate ratio study

ObjectivesWe aimed to estimate the real-world effectiveness of the influenza vaccine against myocardial infarction (MI) and influenza in the decade since adults aged ≥ 65 years were first recommended the vaccine. Study Design and SettingWe identified annual cohorts, 1997 to 2011, of adults aged ≥ 65 years, without previous influenza vaccination, from UK general practices, registered with the Clinical Practice Research Datalink. Using a quasi-experimental study design to control for confounding bias, we estimated influenza vaccine effectiveness on hospitalization for MI, influenza, and antibiotic prescriptions for lower respiratory tract infections. ResultsVaccination was moderately effective against influenza, the prior event rate ratio–adjusted hazard ratios ranging from 0.70 in 1999 to 0.99 in 2001. Prior event rate ratio–adjusted hazard ratios demonstrated a protective effect against MIs, varying between 0.40 in 2010 and 0.89 in 2001. Aggregated across the cohorts, influenza vaccination reduced the risk of MIs by 39% (95% confidence interval: 34%, 44%). ConclusionEffectiveness of the flu vaccine in preventing MIs in older UK adults is consistent with the limited evidence from clinical trials. Similar trends in effectiveness against influenza and against MIs suggest the risk of influenza mediates the effectiveness against MIs, although divergence in some years implies the mechanism may be complex.