A fourth dose of the Pfizer vaccine lowered rates of COVID-19 among people aged 60 years or older but protection against the infection appeared short-lived, according to a large-scale study conducted in Israel.
The study, published in New England Journal of Medicine on Tuesday, used the Israeli Ministry of Health database to extract data on 1,252,331 persons 60 years of age or older and eligible for the fourth dose during a period in which the Omicron variant was predominant.
The researchers estimated the rate of confirmed infection and severe COVID-19 starting at eight days after receipt of a fourth dose as compared with that among persons who had received only three doses, and among those who had received a fourth dose 3 to 7 days earlier (control).
For the estimation of rates, the researchers, including those from the Weizmann Institute of Science in Israel, adjusted for age, sex, demographic group, and calendar day.
“Rates of confirmed SARS-CoV-2 infection and severe COVID-19 were lower after a fourth dose of BNT162b2 (Pfizer) vaccine than after only three doses," the authors of the study noted.
"Protection against confirmed infection appeared short-lived, whereas protection against severe illness did not wane during the study period," they added.
The number of cases of severe COVID-19 per 100,000 person-days was 1.5 in the aggregated four-dose groups, 3.9 in the three-dose group, and 4.2 in the internal control group, the researchers said.
The adjusted rate of severe COVID-19 in the fourth week after receipt of the fourth dose was lower than that in the three-dose group by a factor of 3.5, and was lower than that in the internal control group by a factor of 2.3, they said.
Protection against severe illness did not wane during the six weeks after receipt of the fourth dose, according to the researchers.
The number of cases of confirmed infection per 100,000 person-days was 177 in the aggregated four-dose groups, 361 in the three-dose group, and 388 in the internal control group, they said.
The adjusted rate of confirmed infection in the fourth week after receipt of the fourth dose was lower than that in the three-dose group by a factor of 2 and was lower than that in the internal control group by a factor of 1.8.
However, this protection waned in later weeks, according to the researchers.
