Top 5 Takeaways

1. Rapid Outbreak Following School Event: An outbreak of influenza A(H3N2) occurred among attendees of a school banquet, with 41% of students reporting symptoms within a median of 47 hours post-event.
2. High Testing Coverage and Results: Out of 184 attendees, 95% were tested for respiratory pathogens, and 32% tested positive for influenza A(H3N2), while no SARS-CoV-2 was detected.
3. Impact of Lifted COVID-19 Mitigation Measures: The outbreak coincided with the lifting of mask use and physical distancing mandates, factors that likely contributed to the rapid spread of the virus.
4. Low Vaccination Rates Among Attendees: Only 16% of respondents reported receiving an influenza vaccine before the event, highlighting low vaccination coverage as a contributing factor.
5. Recommendations for Future Prevention: Recommendations were made for mask use following the outbreak and immediate antiviral treatment for those testing positive, underscoring the need for continued vigilance against influenza.

Original Article Author and Citation

Corresponding Author

Lello Tesema, ltesema@ph.lacounty.gov.

Suggested Citation

Tesema L, Sullivan D, Pulido M, et al. Notes from the Field: Influenza A(H3N2) Outbreak Following a School Event — Los Angeles, California, March 2022. MMWR Morb Mortal Wkly Rep 2022;71:745–746. DOI: http://dx.doi.org/10.15585/mmwr.mm7122a4 .

Summary

The MMWR article discusses an influenza A(H3N2) outbreak among 184 attendees of an off-campus school banquet in Los Angeles, reported to the Los Angeles County Department of Public Health. The outbreak resulted in 72 students showing symptoms, though no hospitalizations occurred. The school closed for in-person attendance due to the high attack rate.

Methods

The Los Angeles County Department of Public Health obtained a line list of all banquet attendees, conducted a survey for symptoms and exposures, and implemented testing for various respiratory pathogens using a multiplex polymerase chain reaction assay. An environmental assessment of the event space was also performed.

Discussion

This outbreak occurred shortly after local mandates on mask use and physical distancing were lifted, at a time when community influenza activity was rising. The increase in influenza activity and the relaxed mitigation measures likely facilitated the virus’s spread. The low influenza vaccination rate among attendees also played a significant role in the outbreak’s magnitude.

Conclusion

This event underscores the potential for influenza viruses to cause significant outbreaks, particularly when preventive measures like vaccination and mask wearing are underutilized. As COVID-19 measures are lifted, vigilance against influenza and other respiratory viruses must be maintained.

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