Top 5 Takeaways

1. Third Dose Effectiveness: A third dose of mRNA vaccines was highly effective at preventing COVID-19–associated emergency department (ED) and urgent care (UC) encounters and hospitalizations during both Delta (94% and 94%) and Omicron (82% and 90%) variant predominance periods.
2. Decline in VE Over Time: Vaccine effectiveness (VE) significantly declined for those who received their second dose ≥180 days before medical encounters compared to those vaccinated more recently.
3. Immunocompromised Individuals: VE after the third dose was lower among immunocompromised patients compared to non-immunocompromised patients.
4. Public Health Recommendation: All unvaccinated persons should get vaccinated as soon as possible, and all adults who have received mRNA vaccines should receive a third dose when eligible.
5. Omicron Impact: VE estimates were significantly lower during the Omicron period compared to the Delta period, indicating possible increased immune evasion by the Omicron variant.

Original Article Author and Citation

Corresponding Author

Mark G. Thompson, isq8@cdc.gov

Suggested Citation

Thompson MG, Natarajan K, Irving SA, et al. Effectiveness of a Third Dose of mRNA Vaccines Against COVID-19–Associated Emergency Department and Urgent Care Encounters and Hospitalizations Among Adults During Periods of Delta and Omicron Variant Predominance — VISION Network, 10 States, August 2021–January 2022. MMWR Morb Mortal Wkly Rep 2022;71:139–145. DOI: http://dx.doi.org/10.15585/mmwr.mm7104e3

Summary

This study analyzed the effectiveness of a third dose of mRNA COVID-19 vaccines in preventing COVID-19–associated emergency department (ED) and urgent care (UC) encounters and hospitalizations among adults during periods of Delta and Omicron variant predominance. The analysis included 222,772 encounters from 383 EDs and UC clinics and 87,904 hospitalizations from 259 hospitals across 10 states. The findings indicated that VE was significantly higher among patients who received their second dose less than 180 days before medical encounters compared to those vaccinated 180 days or more earlier. A third dose was highly effective in preventing COVID-19–associated ED and UC encounters and hospitalizations during both Delta and Omicron periods.

Methods

The study used a test-negative design to estimate VE, comparing the odds of a positive SARS-CoV-2 test result between vaccinated and unvaccinated patients. Medical encounters were defined as those among adults aged ≥18 years with a COVID-19–like illness diagnosis who had received molecular testing for SARS-CoV-2 within 14 days before or 72 hours after the admission or encounter. The study period was from August 26, 2021, to January 5, 2022, and analyses were stratified by the period before and after the Omicron variant became predominant.

Discussion

The study found that VE against laboratory-confirmed COVID-19 declined during the Omicron period compared to the Delta period. VE was significantly lower among patients who received their second dose ≥180 days before medical encounters. However, VE increased following a third dose and was highly effective during both periods at preventing COVID-19–associated ED and UC encounters and hospitalizations. The study also noted that immunocompromised adults had lower third dose VE compared to non-immunocompromised adults.

Conclusion

The findings underscore the importance of receiving a third dose of mRNA COVID-19 vaccine to prevent both moderately severe and severe COVID-19, especially during the Omicron variant predominance. All unvaccinated persons should get vaccinated as soon as possible, and all adults who have received mRNA vaccines should receive a third dose when eligible.

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