Interim US Flu Vaccine Effectiveness (VE) Data for 2021-2022

Limited flu circulation this season made it challenging to evaluate vaccine effectiveness, but these preliminary, interim estimates indicate that vaccination did not reduce the risk of mild or moderate illness from flu A(H3N2) viruses, which circulated most commonly during the 2021-22 flu season. Final vaccine effectiveness estimates will include information on how well 2021-22 flu vaccine protected against severe flu illness and will be  published later this year.

Description of patients enrolled in the US Flu VE Network, 2021-2022

Total respondents: 3,636*

*Note: At the time of this report, there were not enough people enrolled in the study to be able to estimate how well vaccines worked by age group or for different types of influenza vaccines.

Age Number % Total
6 months-8 years 386 11
9-17 years 454 13
18-49 years 1786 49
50-64 years 672 19
65+ years 338 9
Table: Interim flu vaccine effectiveness (VE) among outpatients with acute respiratory infection (ARI) for all vaccine types, against flu A viruses overall and flu A(H3N2) viruses—US Flu VE Network, October 4, 2021-February 12, 2022
Flu type Flu-positive Flu-negative Adjusted Interim VE*
Total Vaccinated no.
Total Vaccinated no.
% Confidence Interval
Flu A
Overall 194 79 (41) 3,442 1,738 (50) 14 (−17–37)
Flu A (H3N2)
Overall 177 69 (39) 3,174 1,564 (49) 16 (−16–39)

Abbreviations: OR = odds ratio; VE = vaccine effectiveness.
*VE was estimated using the test-negative design as 100% x (1 − OR [ratio of odds of being vaccinated among outpatients with influenza-positive test results to odds of being vaccinated among outpatients with influenza-negative test results]); ORs were estimated using logistic regression.

 Adjusted for study site, age group, number of days from illness onset to enrollment, and month of illness using logistic regression