The estimated effectiveness of the fourth dose during days 7 to 30 after administration was 45% against infection, 55% for symptomatic disease, 68% for hospitalization, 62% for severe illness, and 74% for death compared with a third dose given at least fourth months earlier.

The study compared 182,122 individuals aged 60 and older who received a fourth dose and 182,122 very similar people who had received a third dose but not a fourth.

"The results of our real-world study suggest that a fourth vaccine dose is, at least initially, effective against the Omicron variant," the researchers said. "Additional follow-up will allow further assessment of the protection provided by the fourth dose over time." 

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