Effects of Influenza Vaccination in the United States during the 2018-2019 Influenza Season.


Current multivalent influenza vaccine products provide protection against influenza A(H1N1)pdm09, A(H3N2), and B lineage viruses. The 2018-2019 influenza season in the US included prolonged circulation of both A(H1N1)pdm09 viruses well-matched to the vaccine strain, and A(H3N2) viruses the majority of which were mismatched to the vaccine. We estimate the number of vaccine-prevented influenza-associated illnesses, medical visits, hospitalizations, and deaths for the season.

Influenza vaccination prevented an estimated 4.4 million (95% UI: 3.4 million-7.1 million) illnesses, 2.3 million (95% UI: 1.8 million-3.8 million) medical visits, 58,000 (95% UI: 30,000-156,000) hospitalizations, and 3,500 (95% UI: 1,000-13,000) deaths due to influenza viruses during the US 2018-2019 influenza season. Vaccination prevented 14% of projected hospitalizations associated with A(H1N1)pdm09 overall and 43% among young children aged 6 months-4 years.

We used a mathematical model and Monte Carlo algorithm to estimate numbers and 95% uncertainty intervals (UI) of influenza-associated outcomes prevented by vaccination in the US. The model incorporated age-specific estimates of national 2018-2019 influenza vaccine coverage, influenza virus-specific vaccine effectiveness from the US Influenza Vaccine Effectiveness Network, and disease burden estimated from population-based rates of influenza-associated hospitalizations through the Influenza Hospitalization Surveillance Network.

Influenza vaccination averted substantial influenza-associated disease including hospitalizations and deaths in the US, primarily due to effectiveness against A(H1N1)pdm09. Our findings underscore the value of influenza vaccination, highlighting that vaccines measurably decrease illness and associated health care utilization even in a season in which a vaccine component does not match to a circulating virus.

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