A new CDC analysis of 2021-22 data reveals that flu vaccine effectiveness (VE) against the H3N2 (influenza A) strain for all ages was 36%. The study was published today in Clinical Infectious Diseases.
The study was based on outpatients seen at seven clinic sites across the United States from October 2021 to April 2022. Research staff across the sites enrolled patients aged 6 months or older seeking outpatient care for acute respiratory illness with cough.
Patients who tested positive for SARS-CoV-2 were excluded from VE estimations.
In total, out of 6,260 participants, 468 (7%) tested positive for influenza only, including 440 (94%) for H3N2; after removing patients with COVID-19, 4,312 patients were included in analyses of influenza VE, with 2,463 (57%) vaccinated against influenza.
The effectiveness against H3N2 was 36% (95% confidence interval, 20% to 49%) overall in patients aged 50 years or younger.
"We were unable to detect statistically significant protection against laboratory-confirmed influenza among adults aged 50 years and older," the authors said. "Detection of statistically significant VE below 30% with high vaccine coverage requires a larger sample size than we were able to enroll this season."