Examining efficacy of flu shot in preventing severe flu

The influenza vaccine is recommended every year as a way to reduce hospitalizations and emergency department visits linked to influenza. A report in Pediatrics offers a look at how well the 2018-2019 influenza vaccine worked in reducing both.¹ During the season, the influenza A(H1N1)pdm09 peaked in the initial part of the season and the antigenically drifted influenza A(H3N2) had a peak later in the season.

The investigators used molecular assays to test children aged 6 months to 17 years who had acute respiratory illness for influenza at 7 pediatric hospitals. Parental report, provider records, and state immunization information systems were used to determine the vaccination status for inpatient children. Parental report was used for the vaccination status of children in the emergency department.

There were 1792 inpatient children with 226 patients testing positive for influenza: 47% for influenza A(H3N2), 36% for A(H1N1)pdm09, 9% for A (not subtyped), and 7% for B viruses. In the 1944 children in the emergency department, 420 children tested positive for influenza: 48% for A(H3N2), 35% for A(H1N1)pdm09, 11% for A (not subtyped), and 5% for B viruses. The researchers found that vaccine effectiveness against any influenza-related hospitalization was 41% (95% CI, 20% to 56%). When looking at the 2 influenza types, the effectiveness was for 41% (95% CI, 11% to 61%) for A(H3N2), and 47% (95% CI, 16% to 67%) for A(H1N1)pdm09. Overall vaccine effectiveness against influenza-related emergency department visit was 51% (95% CI, 38% to 62%) and when broken across the 2 influenza types, 39% (95% CI, 15% to 56%) against A(H3N2), and 61% (95% CI, 44% to 73%) against A(H1N1)pdm09.

The investigators concluded that their research provides timely, necessary evidence of the benefit of vaccinating children for influenza to reduce the hospitalizations and emergency department visits. They noted that the data offers key information on vaccine efficacy in preventing severe influenza. They also noted that the vaccine was highly effective in reducing hospitalizations and visits by 40% to 60%, even in a season when the virus had experienced antigenic shift.

Reference

1. Campbell A, Ogokeh C, Lively J, et al. Vaccine effectiveness against pediatric influenza hospitalizations and emergency visits. Pediatrics. October 5, 2020. Epub ahead of print. doi:10.1542/peds.2020-1368