Top 5 Takeaways

1. Low Vaccine Effectiveness: The 2021–22 seasonal influenza vaccine showed only 16% effectiveness against medically attended outpatient ARI caused by influenza A(H3N2) viruses.
2. Predominance of A(H3N2): Most influenza viruses detected this season were A(H3N2), with a genetic clade 3C.2a1b subclade 2a.2.
3. Continued Vaccination Recommendation: Despite low effectiveness, CDC recommends continued vaccination as it can prevent severe outcomes and protect against other circulating influenza viruses.
4. Antiviral Medications: CDC advises the use of antiviral medications as an adjunct to vaccination, especially in the context of reduced vaccine effectiveness.
5. Study Limitations: VE estimates were limited due to low influenza test positivity and potential misclassification of vaccination status.

Original Article Author and Citation

Corresponding Author

Jessie R. Chung, jchung@cdc.gov

Suggested Citation

Chung JR, Kim SS, Kondor RJ, et al. Interim Estimates of 2021–22 Seasonal Influenza Vaccine Effectiveness — United States, February 2022. MMWR Morb Mortal Wkly Rep 2022;71:365–370. DOI: http://dx.doi.org/10.15585/mmwr.mm7110a1

Summary

This report provides interim estimates of the effectiveness of the 2021–22 seasonal influenza vaccine in the United States. The study included data from 3,636 children and adults with acute respiratory infection (ARI) during October 4, 2021–February 12, 2022. The vaccine showed only 16% effectiveness against outpatient ARI caused by influenza A(H3N2) viruses, which have predominated this season. Despite the low effectiveness, CDC continues to recommend vaccination to prevent severe outcomes and protect against other influenza viruses that may circulate later in the season.

Methods

The study used data from seven sites of the U.S. Influenza Vaccine Effectiveness Network. Participants aged ≥6 months with ARI and seeking outpatient medical care were enrolled. Data collection included demographic information, health status, symptoms, and vaccination status. VE was estimated using the test-negative design and adjusted for study site, age group, days from illness onset to enrollment, and month of illness onset.

Discussion

The findings indicate that the 2021–22 influenza vaccine did not significantly reduce the risk of outpatient medically attended illness with influenza A(H3N2) viruses. These results align with previous evidence of low protection against A(H3N2) subclade 2a.2 viruses. Despite the low VE, vaccination is still recommended to prevent severe outcomes and protect against other influenza viruses. The study highlights the importance of ongoing diagnostic testing, antiviral treatment, and preventive measures.

Conclusion

The 2021–22 seasonal influenza vaccine showed low effectiveness against outpatient ARI caused by influenza A(H3N2) viruses. However, vaccination remains crucial to prevent severe outcomes and protect against other influenza viruses. Health care providers should continue to administer influenza vaccines and consider antiviral medications as an adjunct to vaccination.

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