Top 5 Takeaways

1. Influenza A(H3N2) virus, specifically clade 3C.2a1b.2a.2, emerged in Chile much earlier than in pre-pandemic seasons, resulting in 1,002 hospitalizations in 2022.
2. The influenza vaccination was found to reduce the risk of hospitalization due to A(H3N2) by 49%.
3. Influenza activity in 2022 was higher than in 2020–21 but lower than in pre-pandemic years, indicating ongoing disruptions due to the COVID-19 pandemic.
4. Over 90% of Chile’s prioritized population was vaccinated, and the vaccines used were effective in preventing hospitalizations during the predominant A(H3N2) season.
5. The findings suggest that Northern Hemisphere countries might experience similar atypical influenza seasons and should prepare accordingly, emphasizing vaccination and preventive measures.

Original Article Author and Citation

Corresponding Authors

Rodrigo A. Fasce, rfsace@ispch.cl;
María Fernanda Olivares Barraza, maria.olivares@minsal.cl.

Suggested Citation

Olivares Barraza MF, Fasce RA, Nogareda F, et al. Influenza Incidence and Vaccine Effectiveness During the Southern Hemisphere Influenza Season — Chile, 2022. MMWR Morb Mortal Wkly Rep 2022;71:1353–1358. DOI: 10.15585/mmwr.mm7143a1.

Summary

The study focused on the influenza season in Chile during 2022, which started earlier than usual and was dominated by the influenza A(H3N2) virus, clade 3C.2a1b.2a.2. The cumulative incidence of influenza-attributable pneumonia and influenza hospitalizations was 5.1 per 100,000 person-years. This incidence was higher compared to 2020–21 but lower than during the 2017–19 influenza seasons.

Methods

Data was collected from Chile’s Ministry of Health hospital discharge data and viral surveillance data. The approach included using CDC RT-PCR protocols for influenza virus testing and applying methods to estimate cumulative incidence of influenza hospitalization. The vaccine effectiveness was assessed using a test-negative, case-control design.

Discussion

The report highlights the significant influenza activity in Chile in 2022, marked by an earlier onset and a different strain compared to previous years. This change is partially attributed to the altered patterns of virus transmission due to the COVID-19 pandemic. The effectiveness of the influenza vaccine, at 49%, emphasizes the importance of vaccination in controlling the spread and severity of the disease.

Conclusion

The 2022 influenza season in Chile demonstrates the ongoing impact of the COVID-19 pandemic on influenza patterns. These findings suggest the need for continued vigilance and preparedness, particularly in the Northern Hemisphere, for potentially atypical influenza seasons. Emphasis on vaccination and preventive measures remains crucial for public health.

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