Israeli study: Pfizer's booster shot very effective in preventing severe disease, hospitalization

A booster dose of the Pfizer COVID-19 vaccine, received five months after a second shot, reduces an individual's risk of hospitalization by 93%, according to a recent observational study out of Israel.

The study, which appeared last week in The Lancet medical journal, was conducted by Clalit Health Services – the small country's largest HMO – with assisted funding from the Harvard University Medical School. 

Researchers used data from 728,321 people who had received a booster shot and compared it with data from a similar number of people who had just received two regular doses of the Pfizer vaccine. 

The scientists found that the booster shot was 93% effective at preventing hospital admission for COVID patients. Twenty-nine hospitalizations were recorded in individuals who had received a third Pfizer dose, compared to 231 hospitalizations in those who received two-doses. 

The study additionally indicates that the Pfizer booster works to prevent serious disease caused by the virus, with just 17 cases recorded in the group that had received the third shot, and 157 cases in the control group.

"Vaccine effectiveness evaluated at least 7 days after receipt of the third dose, compared with receiving only two doses at least 5 months ago, was estimated to be 93% (231 events for two doses vs 29 events for three doses; 95% CI 88–97) for admission to hospital, 92% (157 vs 17 events; 82–97) for severe disease, and 81% (44 vs seven events; 59–97) for COVID-19-related death," reads the study.

Study participants had a median age of 52 and were about equally comprised of men and women. 

Researchers also pointed out that it is difficult to gage true vaccine efficacy in individuals under 40 "due to the relative scarcity of events" in that age range. 

Johns Hopkins Bloomberg School of Public Health infectious disease epidemiologist David Dowdy wrote on Twitter that "If you have no other medical problems, this (getting a third dose) is not an urgent priority."

According to Dowdy's calculations, the risk of severe disease without a booster shot and no coexisting health conditions sits at about 1 in 32,000, or three cases per every 100,000.