FRIDAY, Jan. 28, 2022 (HealthDay News) — Receipt of three doses of an mRNA COVID-19 vaccine is associated with protection against delta and omicron compared with being unvaccinated and receipt of two doses, according to a study published online Jan. 21 in the Journal of the American Medical Association.

Emma K. Accorsi, Ph.D., from the U.S. Centers for Disease Control and Prevention COVID-19 Response Team in Atlanta, and colleagues conducted a test-negative case-control analysis among adults aged 18 years or older with COVID-19-like illness tested Dec. 10, 2021, through Jan. 1, 2022. Data were included for 23,391 cases (13,098 omicron and 10,293 delta) and 46,764 controls.

The researchers found that 18.6, 6.6, and 39.7 percent of omicron cases, delta cases, and controls, respectively, reported prior receipt of three mRNA vaccine doses; 55.3, 44.4, and 41.6 percent, respectively, reported prior receipt of two vaccine doses; and 26.0, 49.0, and 18.6 percent, respectively, reported being unvaccinated. For three doses versus unvaccinated, the adjusted odds ratios were 0.33 and 0.065 for omicron and delta, respectively; for three versus two vaccine doses, the adjusted odds ratios were 0.34 and 0.16 for omicron and delta, respectively. For three versus two vaccine doses, median cycle threshold values were significantly higher for both omicron and delta.

“Although these findings provide evidence supporting that three-dose schedules are protective and that booster doses are more protective than primary series alone, the significantly higher odds ratio for omicron suggests that booster doses are less protective against omicron than against delta,” the authors write.

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