Pfizer’s COVID-19 Booster Dose Effective in Preventing Severe Disease: Israeli Study

A pharmacist prepares a syringe of the Pfizer vaccine for COVID-19 in Seattle, Wash., on Jan. 8, 2021. Ted S. Warren/AP Photo

A booster dose of Pfizer-BioNTech’s COVID-19 vaccine five months after a second dose reduces an individual’s risk of hospitalization by 93 percent, according to an Israeli study published on Oct. 28.

The study, published in The Lancet (pdf), was conducted by Israel’s largest HMO, Clalit Health Services, with funding from Harvard Medical School. Researchers compared data from 728,321 people aged 12 and up who had received a booster shot, as well as a similar number of people who received just two doses of Pfizer’s vaccine at least five months prior.

The researchers found that a vaccine booster dose was 93 percent effective at preventing hospital admission for COVID-19, the disease caused by the CCP (Chinese Communist Party) virus, with 29 hospitalizations recorded for those who had received a third Pfizer vaccine dose, compared to 231 hospitalizations for two-dose participants.

The study also suggests that Pfizer’s COVID-19 booster dose is effective in preventing severe disease, with 17 cases recorded in the group who had received a booster dose, 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,” the researchers wrote.

The study suggests that a third dose of Pfizer’s vaccine is “effective in protecting individuals against severe COVID-19-related outcomes, compared with receiving only two doses at least 5 months ago.”

All individuals involved in the study received a third vaccine dose between July 30, 2020, and Sept. 23, the study notes. Participants had a median age of 52 years, and 51 percent of them were female.

The researchers noted that the optimal time to achieve maximum protection against COVID-19-related outcomes after a third dose of the vaccine isn’t currently known.

“The results demonstrate in a very convincing way that the third dose of the vaccine is extremely efficient,” Ran Balicer, Clalit’s chief innovation officer and senior author of the study, said in a statement.

However, researchers did note several limitations, including that they couldn’t evaluate vaccine effectiveness in those younger than 40 years old, “due to the relative scarcity of events” in individuals in that age group.

“This vaccine effectiveness study did not explore potential adverse clinical events and excess health-care [utilization] associated with the administration of a third dose,” they wrote. “Finally, we excluded populations (health care workers, those living in long-term care facilities, and those medically confined to their homes) that are likely to be targeted early to receive the booster dose.”

Ben Reis, director of the Predictive Medicine Group at Harvard Medical School and the Boston Children’s Hospital Computational Health Informatics Program, said in a statement that the study “provides reliable information on third-dose vaccine effectiveness, which we hope will be helpful to those who have not yet decided about vaccination with a third dose.”

Responding to the findings, David Dowdy, an infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health, wrote on Twitter that the risk of severe disease without a booster dose and no coexisting health conditions is “1 in 32,000 (3 per 100k).”

“If you have no other medical problems, this is not an urgent priority,” Dowdy said.