Top 5 Takeaways

  1. Rapid Outbreak: The University of Michigan experienced a rapid increase in influenza A(H3N2) cases from October 6 to November 19, 2021, with over 95% of cases detected in November.
  2. Demographics: The median age of those infected was 19 years, with 54.1% being female and 60% residing off-campus.
  3. Vaccine Effectiveness: The similar vaccination rates among those with positive and negative influenza test results suggest low protection against mild infection with the 2a.2 subgroup of H3N2 viruses.
  4. Public Health Measures: Increasing influenza vaccination coverage, especially among high-risk groups, and using antiviral medications are critical strategies to mitigate severe influenza and reduce strain on healthcare services.
  5. Nonpharmaceutical Interventions: Measures such as physical distancing, masking, and hand hygiene, used for COVID-19 prevention, may also help protect against influenza.

Original Article Author and Citation

Corresponding Author

Emily T. Martin, etmartin@umich.edu

Suggested Citation

Delahoy MJ, Mortenson L, Bauman L, et al. Influenza A(H3N2) Outbreak on a University Campus — Michigan, October–November 2021. MMWR Morb Mortal Wkly Rep 2021;70:1712-1714. DOI: http://dx.doi.org/10.15585/mmwr.mm7049e1

Summary

In October and November 2021, the University of Michigan experienced a significant outbreak of influenza A(H3N2), marking one of the first substantial influenza activities during the COVID-19 pandemic. The outbreak saw 745 laboratory-confirmed cases, predominantly among young adults with a median age of 19 years. The investigation highlighted the rapid spread of the virus and the relatively low effectiveness of the 2021–22 influenza vaccine against the 2a.2 subgroup of H3N2 viruses.

Methods

The investigation involved testing individuals with COVID-19–like or influenza-like illness for SARS-CoV-2, influenza, and respiratory syncytial viruses using a rapid multiplex molecular assay. Data on demographics, genetic characterization of viruses, and influenza vaccination history were reviewed to understand the outbreak’s dynamics and control measures.

Discussion

The findings underscore the need for increased vigilance and public health measures to mitigate influenza’s impact, especially given the ongoing COVID-19 pandemic. The similar vaccination rates among those with positive and negative influenza test results suggest that the vaccine’s protection against mild infection with the 2a.2 subgroup of H3N2 viruses was low. However, vaccination remains crucial for preventing severe outcomes like hospitalization and death.

Conclusion

To reduce the strain on healthcare systems, it is essential to improve influenza vaccination coverage, especially among high-risk groups, and to use antiviral medications for treatment and prophylaxis. Nonpharmaceutical interventions for COVID-19 prevention may also help reduce influenza transmission.

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