Study confirms flu vaccine effectiveness against A H1N1 in young children

European Centre for Disease Prevention and ControlOct 4 2024

A study published on Eurosurveillance has demonstrated that Spain's influenza vaccination campaign for children aged 6-59 months during the 2023/24 season was effective in preventing acute respiratory infections (ARI) and hospitalization, as vaccination was recommended for this age group at the national level for the first time.

In the context of Child Health Day 2024, this research emphasizes that continued efforts should be made to increase vaccination coverage among children for future seasons.

Context and methods

Influenza A was dominant in the 2023/2024 season, with influenza B rarely being detected. The influenza vaccination campaign for children started in the autumn of 2023, with a national vaccination coverage of 31.6%.

The test-negative case–control study analyzed surveillance data from 12 of 19 regions in Spain and 27 hospitals, and included pediatric patients aged 6-59 months that had been tested for influenza. The study comprised 1,364 patients presenting at primary care with acute respiratory infections, 244 of which tested positive for influenza (i.e. cases), and 302 patients hospitalized with severe acute respiratory infections (SARI), 48 of which tested positive for influenza (i.e. cases).

Influenza vaccination data for patients was then collected from regional vaccination records. Vaccine effectiveness was determined by comparing the vaccination rate of those who tested positive for influenza to the rate of those who did not, with higher vaccination rates for those who tested negative (i.e. controls) being indicative of effectiveness. Vaccine effectiveness was then adjusted for potential confounders including sex, age in months, epidemiological week, presence of chronic conditions, and region or hospital for ARI or SARI models.

Findings

The study estimated the influenza vaccine to be 70% effective against any influenza type (95% confidence interval (CI): 51 to 81%) in primary care patients with ARI, and 77% effective against influenza (95% CI: 21 to 93%) in hospitalized patients with SARI.

In primary care, influenza vaccine was found to be 77% effective (95% CI: 56 to 88%) against A(H1N1)pdm09.

Where influenza A viruses were subtyped, A(H1N1)pdm09 was the most frequent influenza virus subtype with 61.2% and 43.8% in primary care and hospitals, respectively, followed by A(H3N2), with 19% and 18.8%, respectively, and unsubtyped influenza A virus with 18.6%, and 35.4%, respectively. Circulation of influenza B virus was low.

Public health implications

It is estimated that 109 million influenza virus infections occur among children between 0–59 months worldwide, and infections can lead to severe disease and outcomes. In Spain, children under 5 years of age have the second highest rate of hospitalization for influenza, just after the age group of over 65 year-olds. The results of this study are consistent with previous research supporting the effectiveness of influenza vaccines to protect children's health.