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The 2022–23 U.S. flu season started earlier than usual, rising in October 2022, with high rates of hospitalization among children. Most viruses were influenza A(H3N2), with some A(H1N1)pdm09, and most were genetically and antigenically similar to the viruses in that season's vaccine. How well a flu vaccine works varies by season, subtype and match — so researchers at Marshfield Clinic Health System in Wisconsin produced early estimates.
Two studies
| Test-negative study | Community cohort | |
|---|---|---|
| Who | 545 patients aged 6 months–64 years seeking outpatient care for acute respiratory illness (cough ≤7 days, no flu antivirals) | 241 children and teens aged 1–17 in central Wisconsin |
| Period | Dec 2, 2022–Feb 10, 2023 | Oct 23, 2022–Feb 10, 2023 |
| Method | Compared vaccination among those positive vs. negative for flu (people positive for SARS-CoV-2 excluded) | Weekly symptom reports and home nasal swabs; compared infection rates in vaccinated and unvaccinated time |
| Vaccinated | 34% (84% of them got the cell culture–based vaccine) | 39% (84% cell culture–based) |
Vaccination counted from 14 days after the shot, per ACIP recommendations.
Results
Test-negative study
- 116 (21%) tested positive for influenza A; none for influenza B. Of 115 subtyped, 75% were H3N2 and 25% H1N1.
- All 43 viruses characterized were genetically similar to the vaccine strains.
- 22% of flu-positive patients were vaccinated, versus 37% of those who tested negative.
- Vaccine effectiveness: 54% against medically attended influenza A, and 60% against H3N2.
Community cohort (children)
- 34 influenza A infections were analyzed (29 H3N2, 1 H1N1, 4 unknown subtype); six were in vaccinated children.
- Infection rates: 1.79 per 1,000 unvaccinated person-days versus 0.82 per 1,000 vaccinated person-days.
- Vaccine effectiveness: 71% against symptomatic influenza A.
What it means
The season's vaccine was giving substantial protection, consistent with estimates from the Southern Hemisphere's 2022 season and Canada's current season. But U.S. vaccination coverage was lower than before the COVID-19 pandemic — especially among children, pregnant women and rural residents — so more vaccination was needed to get the full benefit.
These are early-season estimates and could change. National flu activity was low by early February 2023, but many past seasons have had two waves, and both influenza A and B can circulate late. Annual vaccination is the best way to prevent flu and its complications, and CDC recommends vaccinating everyone 6 months and older as long as flu viruses are circulating.
Limitations
- Both studies were in one region (central Wisconsin), though its viruses resembled those nationwide.
- Adults 65 and older weren't in the test-negative study; effectiveness against H3N2 is generally lower in older adults.
- Small samples limited precision and ruled out estimates for H1N1 or by age.
- Observational studies can be biased, though both designs gave similar results.
Sources
Based on McLean HQ, Petrie JG, Hanson KE, et al., "Interim Estimates of 2022–23 Seasonal Influenza Vaccine Effectiveness — Wisconsin, October 2022–February 2023," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Licence : CC0 1.0 (domaine public) · Adapté de www.cdc.gov
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