Studies: 3 or 4 COVID vaccine doses protective against Omicron

Three new studies show that recipients of three or four mRNA COVID-19 vaccine doses received substantially better protection against infection with the Omicron variant than those who received only two.7% vs 20% infection rate after 4, 3 dosesIn the first study, published yesterday in JAMA Network Open, researchers in Israel studied the effectiveness of a fourth dose of Pfizer/BioNTech mRNA COVID-19 vaccine against infection in 29,611 healthcare workers (HCWs) at 11 general hospitals who had received three vaccine doses in August or September 2021.

The Omicron surge in Israel began in December 2021.The researchers tested workers for COVID-19 using polymerase chain reaction (PCR) only after symptoms emerged or they were exposed to the virus.Of all participants, 5,331 (18%) received a fourth dose in January 2022, the start of a fourth-dose vaccination campaign for Israeli HCWs, and were not infected in the first week after vaccination.

Average participant age was 44 years, and 65% were women. Participants were followed until Jan 31, 2022.The rate of breakthrough infection was 7% in workers who received four vaccine doses, compared with 20% in the 4,802 of 24,280 who received only three (relative risk [RR], 0.35; 95% confidence interval [CI], 0.32 to 0.39).

A matched analysis by day of receipt of the third dose found comparable reductions in breakthrough infections (RR, 0.61; 95% CI, 0.54 to 0.71).