“Discussion: The omicron variant is genetically divergent from the ancestral SARS-CoV-2 strain for which the BNT162b2 vaccine was tailored. The results presented here indicate that as compared with three vaccine doses given at least 4 months earlier, a fourth dose provides added short-term protection against confirmed infections and severe illness caused by the omicron variant. The incidence rate for confirmed infection was lower by a factor of 2 and the rate of severe disease lower by a factor of 3 among persons in the fourth week after receiving the fourth dose than among eligible persons who did not receive the fourth dose.
Comparing the rate ratio over time since the fourth dose (Figure 2) suggests that the protection against confirmed infection with the omicron variant reaches a maximum in the fourth week after vaccination, after which the rate ratio decreases to approximately 1.1 by the eighth week; these findings suggest that protection against confirmed infection wanes quickly. In contrast, protection against severe illness did not appear to decrease by the sixth week after receipt of the fourth dose.”
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