Evaluation of the preventive effectiveness of russian influenza vaccines in the epidemic seasons of 2018-2021

Monitoring the effectiveness of influenza vaccines annually enables the development of measures aimed at decreasing influenza incidence, hospitalization, and mortality rates. The epidemiological effectiveness of several Russian domestic influenza vaccines was evaluated in St. Petersburg in the 2018-2021 epidemic seasons. Male and female participants (N = 6912) aged 18-23 were monitored from 2018 to 2021. Sovigripp, Grippol Plus, Ultrix, and Ultrix Quadri inactivated influenza vaccines were used for immunization. In 2018-2019, when the vaccine strains fully matched the circulating influenza viruses, prophylactic vaccination resulted in a 2.7 to 7.1-fold reduction in influenza cases. The protective effectiveness of the vaccines against influenza and acute respiratory infections (ARIs) totalled 52.4%, reaching 73.0% effectiveness against influenza after laboratory confirmation. In the epidemic season 2019-2020, when the circulating viruses and vaccine strains did not fully match, the incidence of influenza and ARIs reduced twice due to vaccination; the total vaccine effectiveness against influenza and ARIs amounted to 50.0%. The Grippol Plus, Ultrix, and Ultrix Quadri vaccines proved to be the most effective, demonstrating a total effectiveness of 70.6%-75.0% and a 65.5%-83.5% effectiveness against influenza A and B. In the 2020-2021 epidemic season, no data on the preventive effectiveness of the influenza vaccines could be obtained due to the outbreak of COVID-19 and the absence of detectable influenza virus shedding in the study participants. The results obtained are consistent with WHO experts’ estimates which indicate that modern influenza vaccines reduce the incidence of influenza in adults by 70-90% if the vaccine strains match the circulating virus strains.