Top 5 Takeaways

1. High Overall Vaccine Effectiveness: COVID-19 vaccines showed high effectiveness against hospitalization (86%) and emergency department/urgent care (ED/UC) encounters (82%) during the Delta variant predominance.
2. Age-Related Differences: Vaccine effectiveness (VE) against hospitalization was lower in adults aged ≥75 years (76%) compared to those aged 18–74 years (89%).
3. Vaccine-Specific Effectiveness: Moderna vaccine recipients had the highest VE against hospitalization (95%) and ED/UC encounters (92%), followed by Pfizer-BioNTech, and Janssen had the lowest VE.
4. Laboratory-Confirmed Infections: Among hospitalized patients, 18.9% of unvaccinated and 3.1% of fully vaccinated patients had laboratory-confirmed SARS-CoV-2 infections.
5. Limitations Noted: The study did not examine VE by time since vaccination or for partial vaccination, and findings might not be generalizable to the entire U.S. population.

Original Article Author and Citation

Corresponding Author

Mark G. Thompson, isq8@cdc.gov

Suggested Citation

Grannis SJ, Rowley EA, Ong TC, et al. Interim Estimates of COVID-19 Vaccine Effectiveness Against COVID-19–Associated Emergency Department or Urgent Care Clinic Encounters and Hospitalizations Among Adults During SARS-CoV-2 B.1.617.2 (Delta) Variant Predominance — Nine States, June–August 2021. MMWR Morb Mortal Wkly Rep 2021;70:1291–1293. DOI: http://dx.doi.org/10.15585/mmwr.mm7037e2

Summary

This study evaluated the effectiveness of COVID-19 vaccines against hospitalizations and emergency department/urgent care (ED/UC) encounters during the Delta variant predominance in nine states from June to August 2021. Data from the VISION Network showed that vaccines remained highly effective, with overall VE of 86% against hospitalization and 82% against ED/UC encounters. The study highlighted differences in VE by age and vaccine type, with Moderna showing the highest effectiveness.

Methods

The study used a test-negative design to estimate VE, analyzing 32,867 medical encounters from 187 hospitals and 221 ED/UC clinics. Eligible encounters were among adults aged ≥18 years with SARS-CoV-2 molecular testing and a COVID-19–like illness diagnosis. VE was adjusted for age, geographic region, calendar time, and virus circulation, and weighted for inverse propensity to be vaccinated or unvaccinated.

Discussion

The findings indicate that COVID-19 vaccines continue to provide high protection against moderate and severe COVID-19 during the Delta variant predominance. However, VE was lower among older adults and varied by vaccine type. The study suggests potential factors such as changes in the virus and waning immunity over time. Further research is needed to understand the differences in VE between vaccine products and the potential waning of vaccine-induced immunity.

Conclusion

The study reaffirms the high effectiveness of COVID-19 vaccines in preventing hospitalizations and ED/UC encounters due to COVID-19, especially during the Delta variant predominance. It underscores the importance of full vaccination and continuous monitoring of VE over time and across different populations.

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