Incidence Of Influenza Virus Infection Research Articles

Specific and Nonspecific Effects of Influenza Vaccines.

With the introduction of the influenza vaccine in the official immunization schedule of most countries, several data regarding the efficacy, tolerability, and safety of influenza immunization were collected worldwide. Interestingly, together with the confirmation that influenza vaccines are effective in reducing the incidence of influenza virus infection and the incidence and severity of influenza disease, epidemiological data have indicated that influenza immunization could be useful for controlling antimicrobial resistance (AMR) development. Knowledge of the reliability of these findings seems essential for precise quantification of the clinical relevance of influenza immunization. If definitively confirmed, these findings can have a relevant impact on influenza vaccine development and use. Moreover, they can be used to convince even the most recalcitrant health authorities of the need to extend influenza immunization to the entire population. In this narrative review, present knowledge regarding these particular aspects of influenza immunization is discussed. Literature analysis showed that the specific effects of influenza immunization are great enough per se to recommend systematic annual immunization of younger children, old people, and all individuals with severe chronic underlying diseases. Moreover, influenza immunization can significantly contribute to limiting the emergence of antimicrobial resistance. The problem of the possible nonspecific effects of influenza vaccines remains unsolved. The definition of their role as inducers of trained immunity seems essential not only to evaluate how much they play a role in the prevention of infectious diseases but also to evaluate whether they can be used to prevent and treat clinical conditions in which chronic inflammation and autoimmunity play a fundamental pathogenetic role.

Impact of COVID-19 on the incidence of respiratory viral infections and clinical characteristics of associated febrile seizures.

Viral infections of the upper respiratory tract are one of the most common causes of febrile seizures (FSs). During the coronavirus disease-2019 (COVID-19) pandemic, mitigation measures have contributed to changes in the incidence of respiratory viral infections. Therefore, we aimed to evaluate the impact of the COVID-19 pandemic on the incidence of respiratory viral infections and clinical characteristics of FSs. We retrospectively reviewed the medical records of 988 episodes of FS (865 before the pandemic and 123 during the pandemic) between March 2016 and February 2022. Seizure characteristics and their outcomes, along with the distribution of identified respiratory viruses were compared before and during the pandemic. The occurrence of FSs decreased during the COVID-19 pandemic compared to that before the pandemic. A substantial reduction in the incidence of influenza virus infections was observed (P<0.001) during the pandemic, while the incidence of rhinovirus infection was not significantly changed (P=0.811). Interestingly, a significantly high incidence of parainfluenza virus (P=0.001) infections was observed during the pandemic. No statistically significant between-group differences were observed in the clinical presentation and outcomes of FSs before and during the pandemic. Despite epidemiological changes in respiratory viral infections, the clinical characteristics and outcomes of FSs before and during the COVID-19 pandemic were comparable.

Serum 25-Hydroxyvitamin D Was Not Associated with Influenza Virus Infection in Children and Adults in Hong Kong, 2009–201

Background: Some studies have hypothesized that vitamin D may have a role to play in protection against influenza virus infections and illnesses, and that seasonal fluctuation in serum 25-hydroxyvitamin D [25(OH)D] may affect seasonal patterns of influenza virus infections.Objective: We aimed to investigate whether serum 25(OH)D concentrations were associated with the incidence of influenza virus infections and illnesses in children and adults in Hong Kong.Methods: In 2009–2010, 3030 children and adults of all ages from 796 households in Hong Kong were followed up to identify acute respiratory illnesses. Sera from 2694 participants were collected at baseline and after ∼1 mo, 6 mo, and 12 mo. Influenza virus infections were confirmed by reverse transcriptase–polymerase chain reaction performed on nasal and throat swab samples collected during illness episodes. Serologic evidence of influenza virus infection was measured by hemagglutination inhibition assays in unvaccinated participants. The serum 25(OH)D concentrations were measured after collection of all specimens. Each individual’s baseline serum 25(OH)D concentration on 1 January 2010 was predicted by a random-effects linear regression model.Results: We found that, in children and adults who had not received a seasonal influenza vaccine, baseline serum 25(OH)D concentrations (<50 nmol/L compared with ≥50 nmol/L) were not statistically significantly associated with serologic evidence of influenza A(H1N1)pdm09 (RR, 1.18; 95% CI: 0.85, 1.65) or seasonal influenza virus infections [including A(H3N2) and B virus] (RR, 1.13; 95% CI: 0.86, 1.49). In all participants, baseline serum 25(OH)D concentrations were not statistically significantly associated with polymerase chain reaction–confirmed influenza virus infection (RR, 1.15; 95% CI: 0.73, 1.83) and influenza-like illness (RR, 1.18; 95% CI: 0.98, 1.43).Conclusions: These findings indicate that lower serum vitamin D concentrations may not contribute to the seasonality of influenza and are not associated with an increased risk of influenza virus infections in persons of all ages in Hong Kong.

Incidence of Influenza Virus Infection among Wroclaw's Healthcare Workers in Pre-COVID-19 2019-2020 Influenza Season Using Novel Flu SensDx Device.

Background: Healthcare workers (HCWs) are more exposed to influenza infection, and the influenza vaccination is recommended each year, to reduce the risk of influenza infection and prevent influenza transmission. This study is a cross-sectional study and the objectives were to determine the rate of influenza virus infection among HCWs in the 2019–2020 influenza season. Methods: Between January and March 2020, a survey was carried out in 2 hospitals and 15 primary health-care settings (PHCS) in Wroclaw (Poland). The novel point-of-care testing Flu SensDx device was used, which detects the M1 protein of the influenza virus using electrochemical impedance spectroscopy from biological material (throat/nasal swabs). Results: A total of 150 samples were collected. The majority of participating HCWs by profession were 83 physicians (55.3%) and half (51.3%) of the participating HCWs worked in PHCS. Influenza vaccination coverage was 61.3% in 2019–2020 and 46.0% in the 2018–2019 season for all participants. Of the participating HCWs, 44.0% were positive tested by the Flu SensDx device. There were no statistically significant differences among the positive tested HCWs, their influenza immunization history, and the presence of symptoms of influenza-like illness (p > 0.05). Conclusion: Although the results of the present study suggest that influenza vaccination does not reduce the frequency of influenza virus detection by Flu SensDx testing in the HCWs participants, larger studies are needed to estimate the incidence of influenza virus infection among HCWs to understand the underlying mechanism and fine-tune policies aimed at reducing nosocomial infections.

Incidence of influenza virus infections confirmed by serology in children and adult in a suburb community, northern China, 2018-2019 influenza season.

BackgroundIn mainland China, seasonal influenza disease burden at community level is unknown. The incidence rate of influenza virus infections in the community is difficult to determine due to the lack of well‐defined catchment populations of influenza‐like illness surveillance sentinel hospitals.ObjectivesWe established a community‐based cohort to estimate incidence of seasonal influenza infections indicated by serology and protection conferred by antibody titers against influenza infections during 2018‐2019 influenza season in northern China.MethodsWe recruited participants in November 2018 and conducted follow‐up in May 2019 with collection of sera every survey. Seasonal influenza infections were indicated by a 4‐fold or greater increase of hemagglutination inhibition (HI) antibody between paired sera.ResultsTwo hundred and three children 5‐17 years of age and 413 adults 18‐59 years of age were followed up and provided paired sera. The overall incidence of seasonal influenza infection and incidence of A(H3N2) infection in children (31% and 17%, respectively) were significantly higher than those in adults (21% and 10%, respectively). The incidences of A(H1N1)pdm09 infection in children and adults were both about 10%, while the incidences of B/Victoria and/Yamagata infection in children and adults were from 2% to 4%. HI titers of 1:40 against A(H1N1)pdm09 and A(H3N2) viruses were associated with 63% and 75% protection against infections with the two subtypes, respectively.ConclusionsIn the community, we identified considerable incidence of seasonal influenza infections. A HI titer of 1:40 could be sufficient to provide 50% protection against influenza A virus infections indicated by serology.

997. Protection of Unvaccinated Individuals by Influenza Vaccine Coverage of Household Contacts

BackgroundThe evidence that influenza vaccination programs provide some protection to unvaccinated members of a community (i.e., indirect effects) is lacking. We sought to determine the indirect effects of influenza vaccine in prospective cohort study of households with children.MethodsWe used longitudinal data over six influenza seasons from the Household Influenza Vaccine Evaluation (HIVE) study. We categorized households each season based on the proportion of members who received a seasonal influenza vaccination: unvaccinated (0%), low coverage (1–50%), moderate coverage (50–99%), and fully vaccinated (100%). We used mixed-effect Poisson regression models adjusted for age group and sex with random effects to account for household clustering and repeated measures. We estimated the association between the proportion of vaccinated household members and the incidence of influenza virus infection in the entire cohort. We then estimated the indirect effects of influenza vaccination by comparing unvaccinated members of households with low levels of vaccination to unvaccinated members of households with higher levels of vaccination.ResultsDuring 7,286 person-seasons of follow-up, we detected 578 cases (8 per 100/season) of influenza virus infection. The seasonal incidence rate was highest in completely unvaccinated households (10 per 100/seasons) and lower at all other levels of vaccine coverage (Figure 1). Individuals in fully vaccinated households had a 29% lower seasonal incidence rate of influenza infection compare to those in unvaccinated households (IRR 0.72, 95% CI 0.56–0.93). The estimated indirect VE comparing unvaccinated individuals in completely unvaccinated households to those in households with moderate vaccine coverage was 40 % (95% CI −4 to 65).ConclusionWe demonstrate that vaccination of close contacts can reduce incidence of influenza in unvaccinated members of a community. Despite insufficient evidence, guidance from public health authorities currently suggests that vaccination protects close contacts. Our findings quantifying the protective effects of seasonal influenza vaccination of household contacts in unvaccinated individuals can provide clearer evidence for global vaccine recommendations. Disclosures All authors: No reported disclosures.

Investigating Viral Interference Between Influenza A Virus and Human Respiratory Syncytial Virus in a Ferret Model of Infection.

Epidemiological studies have observed that the seasonal peak incidence of influenza virus infection is sometimes separate from the peak incidence of human respiratory syncytial virus (hRSV) infection, with the peak incidence of hRSV infection delayed. This is proposed to be due to viral interference, whereby infection with one virus prevents or delays infection with a different virus. We investigated viral interference between hRSV and 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09) in the ferret model. Infection with A(H1N1)pdm09 prevented subsequent infection with hRSV. Infection with hRSV reduced morbidity attributed to infection with A(H1N1)pdm09 but not infection, even when an increased inoculum dose of hRSV was used. Notably, infection with A(H1N1)pdm09 induced higher levels of proinflammatory cytokines, chemokines, and immune mediators in the ferret than hRSV. Minimal cross-reactive serological responses or interferon γ-expressing cells were induced by either virus ≥14 days after infection. These data indicate that antigen-independent mechanisms may drive viral interference between unrelated respiratory viruses that can limit subsequent infection or disease.

Influenza-associated mortality in Yancheng, China, 2011-15.

IntroductionThe Yangtze river delta in eastern China, centered on Shanghai, is one of the most populated regions of the world with more than 100 million residents. We examined the impact of influenza on excess mortality in Yancheng, a prefecture‐level city with 8.2 million population located 250 km north of Shanghai, during 2011‐2015.MethodsWe obtained individual data on deaths by date, age, sex, and cause in Yancheng from the Chinese Centers for Disease Control and Prevention, and used these to derive weekly rates of mortality from respiratory causes, respiratory and cardiovascular causes combined, and all causes. We used data on influenza‐like illnesses and laboratory detections of influenza to construct a proxy measure of the weekly incidence of influenza virus infections in the community. We used regression models to estimate the association of influenza activity with mortality and excess mortality by age, cause, and influenza type/subtype.ResultsWe estimated that an annual average of 4.59 (95% confidence interval: 3.94, 7.41) excess respiratory deaths per 100 000 persons were associated with influenza, which was 4.6% of all respiratory deaths in the years studied. Almost all influenza‐associated excess deaths occurred in persons ≥65 years. Influenza A(H3N2) had the greatest impact on mortality and was associated with around 50% of the influenza‐associated respiratory deaths in the 5 years studied.ConclusionsInfluenza has a substantial impact on respiratory mortality in Yancheng, mainly in older adults. Influenza vaccination has the potential to reduce disease burden, and cost‐effectiveness analysis could be used to compare policy options.

Incidence of influenza virus infection among pregnant women: a systematic review

BackgroundThe World Health Organization (WHO) considers pregnant women to be a risk group for severe influenza disease. We conducted a systematic review to evaluate influenza disease incidence in pregnant women in order to inform estimates of influenza vaccine impact for low-resource countries.MethodsWe performed electronic literature searches, targeting studies on the following outcomes in pregnant women: attack rate, hospitalization rate, intensive care unit admission rate, mortality rate, and disability-adjusted life years lost. Only original studies published in peer-reviewed journals that had laboratory confirmation for influenza virus infection and included population-based incidence rates with denominator data were included. We summarized study characteristics in descriptive tables and outcome-specific Forest plots. We generated summary incidence rates using random effects models and assessed statistical heterogeneity by visual examination of Forest plots, and by χ2 and I2 tests.ResultsWe identified 1543 articles, of which nine articles met the study inclusion criteria. Five were case series, three were cohort studies, and one was a randomized controlled trial. Eight studies were from high-income countries, and one was from an upper middle-income country. Six studies reported results for pandemic influenza, and three reported seasonal influenza. Statistical heterogeneity was high for all outcomes, and methodologies and duration of surveillance varied considerably among studies; therefore, we did not perform meta-analyses.ConclusionsStudy quality was very low according to GRADE criteria. More data on influenza disease incidence in pregnant women, particularly in low- and middle-income countries and for seasonal influenza disease, are needed to inform public health decision-making.

Serum 25-Hydroxyvitamin D Was Not Associated with Influenza Virus Infection in Children and Adults in Hong Kong, 2009-2010.

Some studies have hypothesized that vitamin D may have a role to play in protection against influenza virus infections and illnesses, and that seasonal fluctuation in serum 25-hydroxyvitamin D [25(OH)D] may affect seasonal patterns of influenza virus infections. We aimed to investigate whether serum 25(OH)D concentrations were associated with the incidence of influenza virus infections and illnesses in children and adults in Hong Kong. In 2009-2010, 3030 children and adults of all ages from 796 households in Hong Kong were followed up to identify acute respiratory illnesses. Sera from 2694 participants were collected at baseline and after ∼1 mo, 6 mo, and 12 mo. Influenza virus infections were confirmed by reverse transcriptase-polymerase chain reaction performed on nasal and throat swab samples collected during illness episodes. Serologic evidence of influenza virus infection was measured by hemagglutination inhibition assays in unvaccinated participants. The serum 25(OH)D concentrations were measured after collection of all specimens. Each individual's baseline serum 25(OH)D concentration on 1 January 2010 was predicted by a random-effects linear regression model. We found that, in children and adults who had not received a seasonal influenza vaccine, baseline serum 25(OH)D concentrations (<50 nmol/L compared with ≥50 nmol/L) were not statistically significantly associated with serologic evidence of influenza A(H1N1)pdm09 (RR, 1.18; 95% CI: 0.85, 1.65) or seasonal influenza virus infections [including A(H3N2) and B virus] (RR, 1.13; 95% CI: 0.86, 1.49). In all participants, baseline serum 25(OH)D concentrations were not statistically significantly associated with polymerase chain reaction-confirmed influenza virus infection (RR, 1.15; 95% CI: 0.73, 1.83) and influenza-like illness (RR, 1.18; 95% CI: 0.98, 1.43). These findings indicate that lower serum vitamin D concentrations may not contribute to the seasonality of influenza and are not associated with an increased risk of influenza virus infections in persons of all ages in Hong Kong.

Interpreting Seroepidemiologic Studies of Influenza in a Context of Nonbracketing Sera.

In influenza epidemiology, analysis of paired sera collected from people before and after influenza seasons has been used for decades to study the cumulative incidence of influenza virus infections in populations. However, interpretation becomes challenging when sera are collected after the start or before the end of an epidemic, and do not neatly bracket the epidemic. Serum samples were collected longitudinally in a community-based study. Most participants provided their first serum after the start of circulation of influenza A(H1N1)pdm09 virus in 2009. We developed a Bayesian hierarchical model to correct for nonbracketing sera and estimate the cumulative incidence of infection from the serological data and surveillance data in Hong Kong. We analyzed 4,843 sera from 2,097 unvaccinated participants in the study, collected from April 2009 to December 2010. After accounting for nonbracketing, we estimated that the cumulative incidence of H1N1pdm09 virus infection was 45% (95% credible interval [CI] = 40%, 49%), 17% (95% CI = 13%, 20%), and 11% (95% CI = 6%, 18%) for children ages 0-18 years, adults 19-50 years, and older adults >50 years, respectively. Including all available data substantially increased precision compared with a simpler analysis based only on sera collected at 6-month intervals in a subset of participants. We developed a framework for the analysis of antibody titers that accounted for the timing of sera collection with respect to influenza activity and permitted robust estimation of the cumulative incidence of infection during an epidemic.

Respiratory Virus Infection and Risk of Invasive Meningococcal Disease in Central Ontario, Canada

BackgroundIn temperate climates, invasive meningococcal disease (IMD) incidence tends to coincide with or closely follow peak incidence of influenza virus infection; at a seasonal level, increased influenza activity frequently correlates with increased seasonal risk of IMD.MethodsWe evaluated 240 cases of IMD reported in central Ontario, Canada, from 2000 to 2006. Associations between environmental and virological (influenza A, influenza B and respiratory syncytial virus (RSV)) exposures and IMD incidence were evaluated using negative binomial regression models controlling for seasonal oscillation. Acute effects of weekly respiratory virus activity on IMD risk were evaluated using a matched-period case-crossover design with random directionality of control selection. Effects were estimated using conditional logistic regression.ResultsMultivariable negative binomial regression identified elevated IMD risk with increasing influenza A activity (per 100 case increase, incidence rate ratio = 1.18, 95% confidence interval (CI): 1.06, 1.31). In case-crossover models, increasing weekly influenza A activity was associated with an acute increase in the risk of IMD (per 100 case increase, odds ratio (OR) = 2.03, 95% CI: 1.28 to 3.23). Increasing weekly RSV activity was associated with increased risk of IMD after adjusting for RSV activity in the previous 3 weeks (per 100 case increase, OR = 4.31, 95% CI: 1.14, 16.32). No change in disease risk was seen with increasing influenza B activity.ConclusionsWe have identified an acute effect of influenza A and RSV activity on IMD risk. If confirmed, these finding suggest that influenza vaccination may have the indirect benefit of reducing IMD risk.

Population dynamics of swine influenza virus in farrow-to-finish and specialised finishing herds in the Netherlands

Influenza virus infections with subtypes H1N1, H3N2 and H1N2 are very common in domestic pigs in Europe. Data on possible differences of population dynamics in finishing pigs in farrow-to-finish herds and in specialised finishing herds are, however, scarce. The presence of sows and weaned piglets on the same premises may, however, affect the exposure of finishing pigs to influenza viruses. In a longitudinal study on 14 farrow-to-finish herds and 15 finishing herds, groups of pigs were followed by repeatedly testing the same animals for antibodies against all three influenza virus subtypes (H1N1, H3N2 and H1N2). At the end of the finishing period, the seroprevalences in farrow-to-finish and specialised finishing herds were 44.3% and 62.0%, respectively for H1N1, 6.6% and 19.3%, respectively for H3N2, and 57.2% and 25.6%, respectively for H1N2. For all three subtypes, the incidence of influenza virus infections was highest at the beginning of the finishing period in farrow-to-finish herds, while the incidence of influenza virus infections was highest at the end of the finishing period in finishing herds. Respiratory disease, probably related to the influenza infections, was observed in five of these herds only, but also occurred at the beginning of the finishing period in farrow-to-finish herds and at the end of the finishing period in finishing herds. The observed differences of population dynamics of influenza virus may affect choice and timing of intervention measures.