Full vaccination, defined as receipt of 3 doses of a mRNA COVID-19 vaccine, protected vaccine recipients against both the Omicron and Delta variants. This is in contrast to patients who were unvaccinated and those treated with only 2 doses.1
A wrinkle in the results was that the vaccinations provided better protection against the Delta variant than against the Omicron variant, according to Emma Accorsi, PhD, and colleagues from the CDC COVID-19 Response and the Epidemic Intelligence Service, CDC, Atlanta.
The investigators assessed the vaccine performance against the Omicron variant via a test-negative case-control analysis. All patients in the study were adults and all underwent testing for a COVID-like illness from December 10, 2021, through January 1, 2022. The testing was done by a national pharmacy-based testing program in 49 states. A total of 23,391 cases (13,098 Omicron; 10,293 Delta) and 46,764 controls were included.
The main outcome measure was to determine the association between symptomatic SARS-CoV-2 infection with Omicron or Delta variants and vaccination; receipt of 3 doses was compared with both no vaccinations and with 2 doses.
When broken down by variant, the results showed that prior receipt of 3 mRNA vaccine doses was reported for 18.6% (n = 2,441) of Omicron cases, 6.6% (n = 679) of Delta cases, and 39.7% (n = 18,587) of controls; the respective findings for 2 mRNA vaccine doses were 55.3% (n = 7,245), 44.4% (n = 4,570), and 41.6% (n = 19,456); and the respective findings among unvaccinated were 26.0% (n = 3,412), 49.0% (n = 5,044), and 18.6% (n = 8,721).
The investigators commented, “These findings suggest that receipt of 3 doses of an mRNA vaccine, relative to being unvaccinated and to receipt of 2 doses, was associated with protection against both the Omicron and Delta variants, although the higher odds ratios for Omicron suggest less protection for Omicron than for Delta.”
Accorsi EK, Britton A, Fleming-Dutra KE, et al. Association between 3 doses of mRNA COVID-19 vaccine and symptomatic infection caused by the SARS-CoV-2 Omicron and Delta variants. JAMAPublished online January 21, 2022; doi:10.1001/jama.2022.0470