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

Authors: Mark G. Thompson, Karthik Natarajan, Stephanie A. Irving, Elizabeth Rowley, Eric P. Griggs, 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 J. Embí, Eduardo Azziz‐Baumgartner, Ousseny Zerbo, Catherine H. Bozio, Sue Reynolds, Jill M. Ferdinands, Jeremiah Williams, Ruth Link‐Gelles, Stephanie J. Schrag, Jennifer R. Verani, Sarah W. Ball, Toan C. Ong - MMWR Morbidity and Mortality Weekly Report 2022 cited by 431

to January 5, 2022. Analyses were stratified by the period before and after the Omicron variant became the predominant strain (>50% of sequenced viruses) at each study site. During the period of Delta predominance across study sites in the United States (August-mid-December 2021), VE against laboratory-confirmed COVID-19-associated ED and UC encounters was 86% 14-179 days after dose 2, 76% ≥180 days after dose 2, and 94% ≥14 days after dose 3. Estimates of VE for the same intervals after vaccination during Omicron variant predominance were 52%, 38%, and 82%, respectively. During the period of Delta variant predominance, VE against laboratory-confirmed COVID-19-associated hospitalizations was 90% 14-179 days after dose 2, 81% ≥180 days after dose 2, and 94% ≥14 days after dose 3. During Omicron variant predominance, VE estimates for the same intervals after vaccination were 81%, 57%, and 90%, respectively. The highest estimates of VE against COVID-19-associated ED and UC encounters or hospitalizations during both Delta- and Omicron-predominant periods were among adults who received a third dose of mRNA vaccine. All unvaccinated persons should get vaccinated 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.