A fourth dose of Pfizer’s COVID-19 vaccine cuts the risk of infection and severe illness in older adults when compared to a third dose given four months earlier, a new study from Israel finds. How long this protection lasts remains to be seen, and whether repeat boosters are a good strategy for future COVID-19 mitigation is still a matter of debate, the authors say.

Investigators analyzed data from Israel’s largest healthcare organization from Jan. 3 to Feb. 18, 2022, when omicron was a dominant variant. Participants were 60 years or older and had received a third dose (first booster shot) at least four months earlier.

A fourth dose of Pfizer’s vaccine increased protection against SARS-CoV-2 infection by up to 54% and against symptomatic illness by as much as 64%, between 14 and 30 days after receipt of the shot. It also reduced the risk of COVID-19-related hospitalization and death by up to 79% and 91% in the same timeframe, the researchers reported. 

Further studies will be needed to assess how long this fourth-dose protection lasts, the researchers noted. The efficacy of a third dose against infection and adverse COVID-19 outcomes has been shown to wane after about four months, according to the Centers for Disease Control and Prevention.

For now, “the results of our real-world study suggest that a fourth vaccine dose is, at least initially, effective against the omicron variant,” they said.The Food and Drug Administration in March recommended that adults aged 50 years and older receive a second booster shot from either the Pfizer-BioNTech or Moderna mRNA vaccines at least four months after receipt of their first booster.

Full findings were published in the New England Journal of Medicine.