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 2021-2022

"COVID-19 mRNA vaccine effectiveness (VE) in preventing COVID-19 might decline because of waning of vaccine-induced immunity or variant immune evasion. 

VE was significantly higher among patients who received their second mRNA COVID-19 vaccine dose <180 days before medical encounters compared with those vaccinated ≥180 days earlier. During both Delta- and Omicron-predominant periods, receipt of a third vaccine dose was highly effective at preventing COVID-19–associated emergency department and urgent care encounters (94% and 82%, respectively) and preventing COVID-19–associated hospitalizations (94% and 90%, respectively).

All unvaccinated persons should start vaccination as soon as possible. All adults who have received mRNA vaccines during their primary COVID-19 vaccination series should receive a third dose when eligible, and eligible persons should stay up to date with COVID-19 vaccinations." - 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

Additional Authors: Manjusha Gaglani, Nicola P. Klein, Shaun J. Grannis, Malini B. DeSilva, Edward Stenehjem, Sarah E. Reese, Monica Dickerson, Allison L. Naleway, Jungmi Han, Deepika Konatham, Charlene McEvoy, Suchitra Rao, Brian E. Dixon, Kristin Dascomb, Ned Lewis, Matthew E. Levy, Palak Patel, I-Chia Liao, Anupam B. Kharbanda, Michelle A. Barron, William F. Fadel, Nancy Grisel, Kristin Goddard, Duck-Hye Yang, Mehiret H. Wondimu, Kempapura Murthy, Nimish R. Valvi, Julie Arndorfer, Bruce Fireman, Margaret M. Dunne, Peter Embi, Eduardo Azziz-Baumgartner, Ousseny Zerbo, Catherine H. Bozio, Sue Reynolds, Jill Ferdinands, Jeremiah Williams, Ruth Link-Gelles,Stephanie J. Schrag, Jennifer R. Verani, Sarah Ball, Toan C. Ong