Flu vaccination during 2021-2022 season significantly reduced outpatient visits
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During the 2021-2022 influenza season, vaccination significantly reduced outpatient visits, consistent with prior seasons during which A(H3N2) viruses were predominant, especially among younger patients, recent study data showed.
“CDC estimates influenza vaccine effectiveness annually and has published estimates most seasons since 2004-05,” Jessie R. Chung, MPH, CDC epidemiologist in the Influenza Division, told Healio. “Estimates were not available in the 2020-21 influenza season due to historically low levels of influenza circulation.”
To assess estimates from the most recent completed influenza season running from October 2021 and April 2022, staff at seven sites enrolled patients aged 6 months and older seeking outpatient care for acute respiratory illness with cough and used a test negative design to assess vaccine efficacy (VE) against influenza A(H3N2).
According to the study, among 7,031 participants with acute respiratory infections at the seven sites, 2,657 were excluded due to influenza vaccination less than 2 weeks before illness onset, missing testing or vaccination data or codetection of influenza and COVID-19.
Among influenza-positive participants, 440 (94%) were subtyped as A(H3N2), with 206 A(H3N2) viruses belonging to genetic group 3C.2a1b subclade 2a.2 — which the researchers noted has antigenic differences from the A(H3N2) influenza vaccine component that belongs to clade 3C.2a1b subclade 2a.1.
After excluding patients who tested positive for SARS-CoV-2 who were tested for influenza, 4,312 patients were included in analyses of influenza VE, although only 2,463 (57%) were vaccinated against influenza. For all ages combined, VE against influenza A was 36% (95% CI, 21%-48%) and 36% (95% CI, 20%-49%) for A(H3N2), specifically.
According to the study, VE against A(H3N2) varied by age from 51% (95% CI, 19%-70%) among patients aged 6 months to 8 years and 32% (95% CI, 3%-52%) among adults aged 18 to 49 years. For adults aged 50 years and older, the researchers determined VE was 10% (95% CI, –60% to 49%), noting that they were “unable to detect statistically significant protection.”
“In general, detection of statistically significant VE below 30% with high vaccine coverage requires a larger sample size than we were able to enroll this season,” they explained.
Also of note was a sensitivity analysis that included participants who tested positive for SARS-CoV-2 at enrollment. The analysis showed that VE was slightly lower at 30% (95% CI, 14%-43%) for the full enrollment period.
The researchers added that there was no statistically significant difference between VE among those enrolled on or before Jan. 15, 2022, (VE = 29%; 95% CI, 5% to 53%) compared with those enrolled after Jan. 15, 2022 (VE = 37%; 95% CI, 19%-51%).
Based on these results, Chung concluded that “influenza vaccine is an effective tool against mild to moderate influenza that requires a medical encounter in the outpatient setting.”
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