Both studies were completed before the advent of the Omicron variant.
In one study that included data on more than 4 million patients, led by Yinon M. Bar-On, MSc, of the Weizmann Institute of Science in Rehovot, Israel, the rate of confirmed SARS-CoV-2 infection was lower in the booster group than in the nonbooster group by a factor of about 10.
This was true across all five age groups studied (range among the groups [starting with age 16], 9.0-17.2).
The risk for severe COVID-19 in the primary analysis decreased in the booster group by a factor of 17.9 (95% confidence interval, 15.1-21.2), among those aged 60 years or older. Risk for severe illness in those ages 40-59 was lower by a factor of 21.7 (95% CI, 10.6-44.2).
Among the 60 and older age group, risk for death was also reduced by a factor of 14.7 (95% CI, 10.0-21.4).
Researchers analyzed data for the period from July 30 to Oct. 10, 2021, from the Israel Ministry of Health database on 4.69 million people at least 16 years old who had received two Pfizer doses at least 5 months earlier.
In the main analysis, the researchers compared the rates of confirmed COVID-19, severe disease, and death among those who had gotten a booster at least 12 days earlier with the rates in a nonbooster group.
The authors wrote: “Booster vaccination programs may provide a way to control transmission without costly social-distancing measures and quarantines. Our findings provide evidence for the short-term effectiveness of the booster dose against the currently dominant Delta variant in persons 16 years of age or older.”
Death risk down by 90%
A second study, led by Ronen Arbel, PhD, with the community medical services division, Clalit Health Services (CHS), Tel Aviv, which included more than 800,000 participants, also found mortality risk was greatly reduced among those who received the booster compared with those who didn’t get the booster.
Participants aged 50 years or older who received a booster at least 5 months after a second Pfizer dose had 90% lower mortality risk because of COVID-19 than participants who did not get the booster.
The adjusted hazard ratio for death as a result of COVID-19 in the booster group, as compared with the nonbooster group, was 0.10 (95% CI, 0.07-0.14; P < .001). Of the 843,208 eligible participants, 758,118 (90%) received the booster during the 54-day study period.
The study included all CHS members who were aged 50 years or older on the study start date and had received two Pfizer doses at least 5 months earlier. CHS covers about 52% of the Israeli population and is the largest of four health care organizations in Israel that provide mandatory health care.
The authors noted that, although the study period was only 54 days (Aug. 6–Sept. 29), during that time “the incidence of COVID-19 in Israel was one of the highest in the world.”
The authors of both original articles pointed out that the studies are limited by short time periods and that longer-term studies are needed to see how the booster shots stand up to known and future variants, such as Omicron.
None of the authors involved in both studies reported relevant financial relationships.
A version of this article first appeared on Medscape.com.