Influenza Vaccine Effectiveness for Prevention of Severe Influenza-Associated Illness among Adults in the United States, 2019-2020: A test-negative study
Pulmonary/Critical Care Medicine; Medicine
Background: Influenza vaccine effectiveness (VE) against a spectrum of severe disease, including critical illness and death, remains poorly characterized.
Methods: We conducted a test-negative study in an intensive care unit (ICU) network at 10 United States hospitals to evaluate VE for preventing influenza-associated severe acute respiratory infection (SARI) during the 2019-2020 season, which was characterized by circulation of drifted A/H1N1 and B-lineage viruses. Cases were adults hospitalized in the ICU and a targeted number outside the ICU (to capture a spectrum of severity) with laboratory-confirmed influenza-associated SARI. Test-negative controls were frequency-matched based on hospital, timing of admission, and care location (ICU vs non-ICU). Estimates were adjusted for age, comorbidities, and other confounders.
Results: Among 638 patients, the median (interquartile) age was 57 (44-68) years; 286 (44.8%) patients were treated in the ICU and 42 (6.6%) died during hospitalization. Forty-five percent of cases and 61% of controls were vaccinated, which resulted in an overall VE of 32% (95% CI: 2 to 53%), including 28% (-9% to 52%) against influenza A, and 52% (13% to 74%) against influenza B. VE was higher in adults 18-49 years old (62%; 95% CI: 27% to 81%) than those 50-64 years old (20%, -48% to 57%) and ≥65 years old (-3%; 95% CI: -97% to 46%) (p=0.0789 for interaction). VE was significantly higher against influenza-associated death (80%, 95% CI: 4% to 96%) than non-fatal influenza illness.
Conclusions: During a season with drifted viruses, vaccination reduced severe influenza-associated illness among adults by 32%. VE was high among young adults.
Keywords: critical illness; immunization; influenza; vaccination; vaccine effectiveness.
Grijalva CG, Feldstein LR, Talbot HK, Aboodi M, Baughman AH, Brown SM, Casey JD, Erickson HL, Exline MC, Files DC, Gibbs KW, Ginde AA, Gong MN, Halasa N, Khan A, Lindsell CJ, Nwosu SK, Peltan ID, Prekker ME, Rice TW, Shapiro NI, Steingrub J, Stubblefield WB, Tenforde MW, Patel M, Self WH; Influenza and Other Viruses in the Acutely Ill (IVY) Network. Influenza Vaccine Effectiveness for Prevention of Severe Influenza-Associated Illness among Adults in the United States, 2019-2020: A test-negative study. Clin Infect Dis. 2021 May 20:ciab462. doi: 10.1093/cid/ciab462. Epub ahead of print.