Effects of Influenza Vaccination in the United States During the 2018–2019 Influenza Season

Abstract

Background Multivalent influenza vaccine products provide protection against influenza A(H1N1)pdm09, A(H3N2), and B lineage viruses. The 2018–2019 influenza season in the United States included prolonged circulation of A(H1N1)pdm09 viruses well-matched to the vaccine strain and A(H3N2) viruses, the majority of which were mismatched to the vaccine. We estimated the number of vaccine-prevented influenza-associated illnesses, medical visits, hospitalizations, and deaths for the season. Methods We used a mathematical model and Monte Carlo algorithm to estimate numbers and 95% uncertainty intervals (UIs) of influenza-associated outcomes prevented by vaccination in the United States. 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. Results 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 3500 (95%UI, 1000–13 000) 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 children aged 6 months–4 years. Conclusions Influenza vaccination averted substantial influenza-associated disease including hospitalizations and deaths in the United States, primarily due to effectiveness against A(H1N1)pdm09. Our findings underscore the value of influenza vaccination, highlighting that vaccines measurably decrease illness and associated healthcare utilization even in a season in which a vaccine component does not match to a circulating virus.

Interim Estimates of 2019–20 Seasonal Influenza Vaccine Effectiveness — United States, February 2020

“…Vaccination can protect against infection with influenza viruses that are currently circulating and those that might circulate later in the season. During the 2018-19 influenza season, in which influenza A(H3N2) and A(H1N1)pdm09 viruses cocirculated, interim VE was estimated to be 29% against illnesses associated with any influenza virus (8) and vaccination was estimated to prevent 4.4 million illnesses, 2.3 million medical visits, 58,000 hospitalizations, and 3,500 deaths (9).…”

Clinical and Economic Outcomes Associated with Cell-Based Quadrivalent Influenza Vaccine vs. Standard-Dose Egg-Based Quadrivalent Influenza Vaccines during the 2018–19 Influenza Season in the United States

“…Although not significant, trends suggested a benefit with QIVc with positive rVEs. It is also relevant to note that the 2018–19 influenza season was characterized by A(H3N2) vaccine-circulating strain mismatch [ 1 , 2 , 3 , 4 ]. Results from the current study, along with the Izurieta study [ 16 ], and studies conducted for the prior influenza season [ 8 , 9 ], potentially suggest that QIVc may offer significantly more protective effect particularly among the adult and high-risk populations during a high intensity season, where there is a better match between the circulating virus and the vaccine virus.…”

“…Annual influenza exerts a significant clinical and economic burden on the health care system in the United States (U.S.). According to estimates from the Centers for Disease Control and Prevention (CDC), seasonal influenza in the U.S. has caused 9 to 45 million symptomatic illnesses, 4 to 21 million medical visits, 140,000 to 810,000 hospitalizations, and 12,000 to 61,000 deaths annually during the last 10 influenza seasons (2010–2011 to 2019–20) [ 1 , 2 , 3 , 4 ]. Vaccination remains the most cost-effective measure to prevent influenza and related untoward outcomes.…”

Human Influenza Epidemiology

“…Vaccines have moderate effectiveness in preventing laboratory-confirmed influenza with effectiveness varying from year to year for a variety of reasons (Belongia et al 2016). Nevertheless, even with moderate effectiveness, influenza vaccines prevent a considerable number of illnesses, hospitalizations, and deaths each year (Chung et al 2020). There is currently a major international research effort to develop improved influenza vaccines and ultimately "universal" influenza vaccines, which provide broad and long-lasting protection against a wide range of strains (Erbelding et al 2018;Paules et al 2018).…”