Effectiveness of COVID-19 mRNA Vaccines Against COVID-19–Associated Hospitalization

Five Veterans Affairs Medical Centers, United States, February 1-August 6, 2021

Kristina L. Bajema, MD; Rebecca M. Dahl, MPH; Mila M. Prill, MSPH; Elissa Meites, MD; Maria C. Rodriguez-Barradas, MD; Vincent C. Marconi, MD; David O. Beenhouwer, MD; Sheldon T. Brown, MD; Mark Holodniy, MD; Cynthia Lucero-Obusan, MD; Gilberto Rivera-Dominguez, MD; Rosalba Gomez Morones, MD; Alexis Whitmire, MPH; Evan B. Goldin; Steve L. Evener, MPH; Maraia Tremarelli, MSPH; Suxiang Tong, PhD; Aron J. Hall, DVM; Stephanie J. Schrag, DPhil; Meredith McMorrow, MD; Miwako Kobayashi, MD; Jennifer R. Verani, MD; Diya Surie, MD


Morbidity and Mortality Weekly Report. 2021;70(37):1294-1299. 

In This Article


Among U.S. veterans hospitalized at five VAMCs, mRNA vaccines were 86.8% effective in preventing COVID-19–associated hospitalizations and remained highly effective during a period of Delta variant predominance. The mRNA vaccines were effective against COVID-19–associated hospitalization among all age groups, although lower effectiveness (79.8%) was observed among veterans aged ≥65 years. These findings support current evidence that COVID-19 mRNA vaccines are highly effective in preventing COVID-19–associated hospitalization[1–3] and reinforce the importance of vaccination, including among veterans, who are at high risk for COVID-19 hospitalization because they are older and have a higher prevalence of underlying medical conditions compared with persons in the general U.S. population.[2,4]

Consistent with national trends,§§ Delta became the predominant SARS-CoV-2 variant in this cohort in July 2021. Protection against COVID-19–associated hospitalization remained high despite the emergence of Delta as the predominant variant in the United States; protection was similar during periods before (February–June 2021; 84.1%) and during (July–August 2021; 89.3%) Delta variant predominance. Recent reports have shown that COVID-19 vaccine protection against SARS-CoV-2 infection is lower in areas with increasing Delta variant transmission;[7,8] however, protection against severe disease outcomes, including hospitalization, remains high.[7,9]

Although the observed vaccine effectiveness in this study is similar to that reported by other studies measuring protection against COVID-19–associated hospitalization, significantly lower vaccine effectiveness among older adults has not previously been observed.[1,2,9] This might be a result of differences in the populations evaluated; periods of vaccine effectiveness assessment, including differences in vaccine coverage, variant circulation, and time since vaccination; and variability in unmeasured confounding. Decreased immunogenicity with increasing age has been reported after vaccination with COVID-19 mRNA vaccines.[10] Because one fourth of adults included in this evaluation were aged ≥75 years, age-related differences in immunogenicity might have significantly contributed to lower estimated effectiveness in older persons. Additional evaluations of vaccine effectiveness across age groups, including the relationship between age and duration of protection, are warranted.

The findings in this report are subject to at least four limitations. First, although the five VAMCs included in this assessment were in diverse geographic locations, they are not representative of the entire veteran population or the general U.S. population. Second, despite the inclusion of 1,175 participants, the statistical power was insufficient to detect potential differences in vaccine effectiveness among all subgroups. Third, vaccine effectiveness estimates might be confounded by certain unmeasured behaviors, including mask use or time spent in congregate settings. Finally, the number of veterans in this sample who received the Janssen COVID-19 vaccine was too small to assess the effectiveness of this vaccine in preventing COVID-19–associated hospitalization.

These findings show that the COVID-19 mRNA vaccines remain highly effective for preventing COVID-19–associated hospitalization in this older, racially diverse population of predominantly male U.S. veterans, including during periods of widespread circulation of the SARS-CoV-2 Delta variant. However, vaccine effectiveness was lower among veterans aged ≥65 years than among those aged 18–64 years. Additional evaluations, particularly among older adults with high prevalences of underlying conditions, are important to assess vaccine effectiveness in these populations. COVID-19 vaccination of all eligible persons is essential to prevent COVID-19–associated hospitalizations.