In a recent study published in the Eurosurveillance Journal, researchers provided interim results on 2022-2023 influenza vaccine effectiveness (VE) estimates from six European studies, including primary care facilities, emergency departments, and hospitals across 16 nations.
Study: Interim 2022/23 influenza vaccine effectiveness: six European studies, October 2022 to January 2023. Image Credit: LeighPrather/Shutterstock.com
Background
Different types (A or B) or subtypes (e.g., A(H3N2), A(H1N1)pdm09) of influenza viruses exist. In the European continent, between 2022 and 2023, several influenza virus subtypes have been circulating.
Research is required to improve our understanding of the immune protection conferred by the influenza vaccine during this period, considering the influence of individual ages, settings, and viral subtypes.
About the study
In the present study, researchers presented interim influenza VE estimates for the 2022-2023 influenza season from six studies conducted in outpatient (primary healthcare facilities), inpatient (hospitals), and emergency care settings in Europe to inform measures of influenza control and prevention for the remaining season.
The included studies used a similar (test-negative) study design but differed in characteristics such as patient selection, data sources, age groups, and case definitions.
Logistic regression modeling was performed to estimate influenza subtype-specific and overall vaccine effectiveness for every study, adjusting for potential confounding.
Data were obtained through electronic database linkage and prospective patient recruitment. In primary care facilities, nasopharyngeal or salivary specimens were obtained from individuals with severe acute respiratory infection (SARI) or influenza-like illness (ILI) symptoms.
The patients were admitted to the emergency care departments if they had undergone influenza swab tests from two weeks before one week after the emergency department visit for influenza infections and associated complications.
Swabs were obtained from SARI patients in hospitals. A reverse transcription-polymerase chain reaction (RT-PCR) was performed to detect the influenza virus. Individuals receiving the 2022-2023 influenza vaccine ≥two weeks before symptom onset were vaccinated.
The team excluded individuals vaccinated before two weeks of symptom onset or with unknown vaccination dates.
Influenza virus-positive samples were selected for genome sequencing and haemagglutinin genome segment analysis. Subsequently, phylogenetic analysis was performed for genetic clade determination. VE (age-specific, study setting-specific, and overall) was computed based on the adjusted odds ratio (OR) values for vaccination, determined by logistic regression modeling.
In addition, the team performed a sensitivity analysis using penalized logistic regression (PLR) to analyze data from studies including less than 10 cases or controls per parameter.
Results
In total, 20,477 individuals with influenza were enrolled, 81% of whom (n=16,589) were infected with the influenza A virus, and 19% (3,913) were infected with the influenza B virus.
Among the participants, 44 were coinfected with influenza A and B, and 15 were coinfected with influenza A(H1N1)pdm09 and A(H3N2). VE estimates against influenza A ranged between 27% and 44%, irrespective of age and setting.
For all age groups and study settings, VE point estimates ranged between 28.0% and 46.0% for preventing A(H1N1)pdm09 infection and were higher for children at 49.0% to 77.0%.
The overall VE of the influenza vaccine in preventing A(H3N2) infection ranged between 2.0% and 44.0% and was higher for children (between 62.0% and 70.0%). The overall and age-specific estimated VE in preventing influenza B/Victoria infection were ≥50.0% (between 87% and 95% for children).
Among the 175 A(H1N1)pdm09 viruses sequenced, all but one 99% (n=174) belonged to genetic clade 6B.1A.5a.2, the same as the vaccine virus. The viruses had undergone the K54Q, A186T, Q189E, E224A, R259K, and K308R mutations compared with the vaccine virus A/Victoria/2570/2019.
Of the 570 influenza A(H3N2) viruses sequenced, all belonged to the same clade as the vaccine strain (3C.2a1b.2a.2). Of the influenza B/Victoria viruses sequenced (n=82), all belonged to the V1A.3a.2 clade, represented by B/Austria/1359417/2021, which is also the vaccine virus.
Results with small sample sizes were subject to sensitivity analyses, most of which gave similar results.
Conclusions
The study findings showed a ≥27% decrease in influenza A infections and a ≥50% decrease in influenza B infections across ages due to influenza vaccines, with a higher VE among children.
The findings indicated that the vaccine conferred some immune protection against the influenza virus and its subtypes in primary care facilities, emergency departments, and hospitals.
The VE estimates were higher for influenza B and among children. Although the study provides interim results, the findings underpin the continuation of influenza vaccination as the influenza season unfolds, following national recommendations.
Future studies must include the genetic characterization of circulating influenza viruses and the determination of VE at the season’s end to enhance our knowledge of the differences in VE results by influenza subtype and inform policymaking and strategy development to improve preparedness and reduce the disease burden in Europe.
