2019–2020 herd immunity to seasonal influenza viruses prior to epidemic season and rate of severe disease cases

Abstract

The aim was to analyze heard immunity against influenza viruses as well as severe course of influenza infection prior to the 2019–2020 epidemic season. Methods. Blood sera samples were collected prior to and after conducting population-wide influenza vaccination campaign at the sanitary and epidemiological centers in different regions of the Russian Federation as well as at the Siberian Federal District, respectively. Sera samples were tested by using hemagglutination inhibition (HI) assay with vaccine strains A/Brisbane/02/2018 (H1N1)pdm09, A/Kansas/14/2017 (H3N2), B/Colorado/06/2017 (Victoria lineage). Baseline clinical and autopsy materials in case of influenza infection in vaccinated patients or severe and fatal influenza cases were collected to be tested by RT-PCR at the sanitary and epidemiological centers, Rospotrebnadzor. All influenza-virus positive samples were further sent to the SRC VB “Vector”. Results. A total of 7,896 and 600 blood serum samples were collected from subjects at Siberian Federal District prior to and after the populationwide influenza vaccination campaign, respectively. Prior to the epidemic season, the proportion of individuals seropositive for the influenza A virus subtypes A/(H1N1)pdm09 and A/H3N2 exceeded 50% in most of the regions, whereas frequency of those seropositive for the influenza B virus was profoundly lower ranging from 12 to 46% in the Northwestern Federal District and Volga Federal District, respectively. After influenza vaccination, the percentage of seropositive subjects in the Siberian Federal District increased as follows: for influenza subtype A/(H1N1)pdm09 — from 66 up to 79%, influenza subtype A/H3N2 — from 68 up to 78%, and for influenza B/Victoria — from 32 up to 47%. In 2019–2020, influenza B virus more frequently caused severe infection that agrees with the herd immunity data prior to the epidemic season. However, the vast majority of the influenza cases with fatal outcome was associated with influenza virus A A/H1N1pdm09 subtype. Conclusion. Quality of influenza vaccine, especially that one intended to vaccinate risk group subjects remains a crucial issue for contemporary scientific community. The study was conducted within the framework of the State Assignments no. 1/16 and 2/18.