Burden of medically attended influenza infection and cases averted by vaccination - United States, 2016/17 through 2018/19 influenza seasons

Abstract

Background: Epidemics of seasonal influenza vary in intensity annually, and influenza vaccine effectiveness (VE) fluctuates based in part on antigenic match to circulating viruses. We estimated the incidence of influenza and influenza cases averted by vaccination in four ambulatory care sites in the United States, during seasons when overall influenza VE ranged from 29% to 40%.

Methods: We conducted active surveillance for influenza at ambulatory care settings at four sites within the United States Influenza Vaccine Effectiveness Network. We extrapolated the total number of influenza cases in the source populations served by these organizations based on incidence of medically attended acute respiratory illness in the source population and influenza test results in those actively tested for influenza. We estimated the number of medically attended influenza cases averted based on incidence, vaccine coverage, and VE.

Results: From 2016/17 through 2018/19, incidence of ambulatory visits for laboratory-confirmed influenza ranged from 31 to 51 per 1,000 population. Incidence was highest in children aged 9-17 years (range, 56 to 81 per 1,000) and lowest in adults aged 18-49 years (range, 23-32 per 1,000). Medically attended cases averted by vaccination ranged from a high of 46.6 (95 % CI, 12.1- 91.9) per 1,000 vaccinees in children aged 6 months to 8 years, to a low of 6.9 (95 % CI, -5.1- 27.3) per 1,000 vaccinees in adults aged ≥ 65 years.

Discussion: Even in seasons with low vaccine effectiveness for a particular virus subtype, influenza vaccines can still lead to clinically meaningful reductions in ambulatory care visits for influenza.

Figure 1:

Influenza incidence by season. A) Distribution of influenza subtype/lineage relative to all cases by week among United States Influenza Vaccine Effectiveness enrollees; B) Cumulative incidence of ambulatory care visits for influenza by age group; C) Cumulative incidence of ambulatory care visits for influenza by study site Panel A: Red = A(H3N2), Blue = A(H1N1)pdm09, Light Green = B/Yamagata, Dark Green = B/Victoria Panel B: <9 includes persons aged 6 months – 8 years. Panel C: KP, Kaiser Permanente Washington, Seattle, WA; MC, Marshfield Clinic Health System, Marshfield, WI; UM, University of Michigan and the Henry Ford healthcare systems, Ann Arbor and Detroit, MI; SW, Baylor Scott & White Health, Temple, TX

Figure 2:

Vaccine impact by season (2016/17, 2017/18, 2018/19). A) Influenza vaccine effectiveness by age group; B) Influenza vaccine coverage by age group; C) Ambulatory care visits for influenza averted per 1,000 vaccinees by age group; D) Percent of influenza visits averted by age group Panels A–D: <9 includes persons aged 6 months – 8 years.