Compared with a third vaccine dose, a fourth dose of the Pfizer/BioNTech COVID-19 vaccine lowered the risk of infection, symptomatic infection, hospitalization, severe illness, and death 52% to 76%—depending on the measure—amid the Omicron surge among older adults, finds a new Israeli study.

Protection against infection waned, however, after 5 weeks, but not protection against severe COVID-19. The findings were published yesterday in the New England Journal of Medicine.

Clalit Research Institute investigators in Tel Aviv led the study, which compared the effectiveness of a fourth vaccine dose 8 days earlier—after a third dose at least 4 months before—relative to a third dose only (control group) and a fourth dose 3 to 7 days earlier (internal control group), among 1,252,331 participants 60 and older from Jan 3 to Feb 18, 2022.