Influenza Vaccine Effectiveness Among Children for the 2017-2018 Season

The 2017-2018 influenza season was of high severity. Circulating influenza strains change periodically, making it important to determine vaccine effectiveness on an annual basis, especially for susceptible populations. The primary aim of our study was to estimate the effectiveness of the influenza vaccine among children. Secondary aims were to assess the effect of previous season vaccination and intraseasonal waning of immunity.

Children 6 months to 17 years of age tested for influenza during the 2017-2018 season were included. Clinical charts were reviewed, and immunization status was confirmed via the Louisiana Immunization Registry. Influenza vaccine effectiveness (IVE) was estimated in a test-negative design by comparing vaccination status of influenza-positive vs influenza-negative cases.

A total of 3595 children were included, 26% of whom tested positive for influenza, mostly type A (79%); 15% had received an influenza vaccine prior to illness: 8% among the influenza-positive and 17% among influenza-negative cases (P <.0001). IVE for the 2017-2018 influenza season was 52% overall (95% confidence interval, 38%-62%), 49% for influenza A, and 60% for influenza B. While receiving current year (2017-2018) vaccine had the most effect, receiving the previous year (2016-2017) vaccine had a small benefit and no interference. We found no evidence of waning immunity of the vaccine for the 2017-2018 season.

IVE was moderate for children. Previous year vaccination had a small but significant benefit and there was no evidence of waning immunity in our cohort. Ongoing national and local surveillance is important to understand the benefit of influenza vaccination.