Three mRNA Vaccine Doses Protective Against Delta, Omicron

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.