New research shows that influenza vaccines provide strong protection in children, but highlights the need to improve efficacy against specific subtypes like H3N2.
Rapid Communication: Influenza Vaccine Efficacy in Children 6 to 59 Months of Age: A Negative Test Case-Control Study at Primary Care and Hospital Level, Spain 2023/24. Image credit: Billion Photos / Shutterstock
A Spanish study examining the effectiveness of influenza vaccination in children found that the vaccine was highly effective in preventing influenza infection and related hospitalizations.
The study was published in the journal Eurosurveillance.
background
Global estimates indicate that influenza infections affect 109 million infants aged 0 to 59 months each year. Both young and older people are much more likely to develop a serious influenza infection.
In Spain, children under 5 years of age and elderly people over 65 years of age have the highest influenza-related hospitalization rates. When it comes to viral transmission, evidence shows that children play an important role in the community circulation of viruses and the spread of influenza epidemics.
For the first time at the national level, influenza vaccination is recommended for all children aged 6 to 59 months during the 2023-2024 season.
In this study, scientists estimated the effectiveness of influenza vaccines against acute respiratory infections in children attending primary care or severe acute respiratory infections in hospitalized children.
research design
This study analyzed data collected from the Spanish Surveillance System for Acute Respiratory Infections (SiVIRA), which monitors acute respiratory infections at the primary care level and severe acute respiratory infections at the hospital level.
This test-negative case-control study analyzed SiVIRA data from 12 regions and 27 hospitals in Spain. Children who tested positive for influenza virus were considered cases, and children who tested negative served as controls.
important findings
The study population included a total of 244 cases and 1,120 controls in primary care and 48 cases and 254 controls in hospitals.
Influenza vaccination rates were significantly higher in the control group compared to primary care cases.
In both primary care and hospitals, influenza A (H1N1) pdm09 was the predominant subtype, followed by influenza A (H3N2) subtype. The prevalence of influenza B was very low, reducing the ability to estimate the effectiveness of vaccines against this virus.
Effectiveness of influenza vaccine
Influenza vaccination effectiveness was estimated by virus type, subtype, and clade. Analyzes adjusted for potential confounders such as gender, age, epidemiological week, presence of chronic disease, region and hospital of acute or severe respiratory infection.
Influenza vaccination is estimated to be 70% effective against influenza infection in children with acute respiratory infections receiving primary care.
Vaccine efficacy against influenza A H1N1 pdm09 subtype and clade 5a.2a was 77% and 96%, respectively. However, no significant protective effect was observed against influenza A H3N2 subtype or other clades. This is likely due to the limited number of cases and very wide confidence intervals (range of estimates for a particular parameter).
Influenza vaccination is estimated to be 77% effective against influenza infection in children hospitalized with severe acute respiratory infections.
This study was unable to accurately estimate vaccine efficacy by subtype or clade in hospitalized children due to insufficient sample size and imprecise estimates, particularly for H3N2.
Sensitivity analyzes including positive controls for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or respiratory syncytial virus (RSV) were performed. This changed vaccine efficacy in primary care to 68% against any influenza infection, 76% against influenza A H1N1 pdm09 subtype, and 2% against influenza A H3N2 subtype.
In hospitals, vaccine efficacy against influenza infections, influenza A H1N1 pdm09 subtype, and influenza A H3N2 subtype changed to 76%, 78%, and 19%, respectively.
In this analysis, efficacy against H3N2 remained low and the small sample size further highlighted the challenges in drawing firm conclusions about this subtype.
Another set of sensitivity analyzes was performed by classifying unknown vaccination status as unvaccinated. This changed the efficacy of the vaccine in hospitals to 86% and 76% against any influenza infection and influenza A H1N1 pdm09 subtype, respectively. Vaccine efficacy against influenza A H3N2 subtype came back negative (-7%).
This negative result highlights the difficulty in interpreting vaccine efficacy when sample sizes are small and certain subtypes, such as H3N2, have low amounts in circulation.
In a second sensitivity analysis, no change in vaccine efficacy was observed in primary care settings.
Significance of research
This study found that influenza vaccination was highly effective in preventing influenza infection and related hospitalizations in children aged 6 to 59 months.
However, the study acknowledged limitations in the accuracy of its results, particularly for influenza A H3N2 subtypes and influenza B viruses, which were much less prevalent. This study found relatively low vaccine efficacy against clade 5a.2a.1. Existing evidence indicates that mutations in the 2023-2024 clade 5a.2a.1 vaccine high-growth reassortant may be responsible for reduced vaccine efficacy.
This mutation was found to only affect highly proliferative reassortants used in egg-based vaccines. This indicates that more information may be obtained by comparing vaccine efficacy between different vaccine brands for the same clade.
Considering the study’s observations, scientists emphasize the need for continued efforts to increase influenza vaccination coverage in future seasons among children belonging to this age group.
