Top 5 Takeaways

1. Higher Effectiveness After Third Dose: Vaccine effectiveness (VE) against COVID-19-associated emergency department/urgent care (ED/UC) visits and hospitalizations was higher after the third dose compared to the second dose.
2. Waning Effectiveness Over Time: VE decreased with time since vaccination, particularly during the Omicron-predominant period.
3. Omicron vs. Delta: VE was generally lower during the Omicron-predominant period compared to the Delta-predominant period.
4. Protection Against Hospitalizations: VE against hospitalizations was higher than VE against ED/UC visits during both variant periods.
5. Public Health Recommendation: All eligible persons should stay up to date with recommended COVID-19 vaccinations to best protect against COVID-19-associated hospitalizations and ED/UC visits.

Original Article Author and Citation

Corresponding Author

Jill M. Ferdinands, zdn5@cdc.gov

Suggested Citation

Ferdinands JM, Rao S, Dixon BE, et al. Waning 2-Dose and 3-Dose Effectiveness 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;71:255–263. DOI: http://dx.doi.org/10.15585/mmwr.mm7107e2

Summary

This study examined the effectiveness of mRNA COVID-19 vaccines against emergency department/urgent care (ED/UC) visits and hospitalizations among adults during periods of Delta and Omicron variant predominance. The analysis found that vaccine effectiveness (VE) was higher after a third dose than after a second dose but waned over time. VE was lower during the Omicron period compared to the Delta period. The study underscores the importance of staying up to date with COVID-19 vaccinations to maintain protection against severe outcomes.

Methods

The study used a test-negative case-control design with data from eight VISION Network sites across 10 states. It included adults aged ≥18 years with COVID-19-like illness who received molecular testing for SARS-CoV-2. VE was estimated by comparing the odds of a positive SARS-CoV-2 test result between vaccinated and unvaccinated patients, adjusting for various factors including age, local virus circulation, and immunocompromised status.

Discussion

The findings indicate that VE against COVID-19-associated ED/UC visits and hospitalizations was higher after a third dose but waned over time, particularly during the Omicron period. The study highlights the need for additional doses to sustain or improve protection. Limitations include the short median interval from the third dose to medical encounters and potential biases due to unmeasured behaviors and residual differences between vaccinated and unvaccinated groups.

Conclusion

Receiving a third dose of mRNA COVID-19 vaccine is crucial for preventing COVID-19-associated ED/UC encounters and hospitalizations among adults. The waning protection observed over time emphasizes the importance of staying up to date with recommended vaccinations to best protect against severe outcomes.

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