A fourth dose of the Pfizer COVID-19 vaccine provided modest protection against infection in older adults, as well as greater protection against more severe outcomes, real-world data from Israel showed.

In a cohort of 182,122 matched pairs of adults ages 60 and up, relative vaccine effectiveness ranged from 52% against asymptomatic infection to 76% against COVID-related death 14 to 30 days after the fourth dose, reported Noa Dagan, MD, of Clalit Health Services in Tel Aviv, and colleagues in the New England Journal of Medicine.

This study filled in some gaps about the effectiveness of a fourth dose that were not covered by other recent real-world studies from Israel, mainly regarding protection against more severe outcomes, such as COVID-related death.

Dagan’s group examined data from January 3 to February 18 on adults ages 60 and up who were eligible to receive a fourth dose of Pfizer vaccine and had no prior PCR-confirmed SARS-CoV-2 infection using the largest healthcare database in Israel. Healthcare workers, long-term care facility residents, and people who “interacted with the health system” during the previous 3 days were excluded.

Those who received a fourth dose (cases) were matched with those yet to receive a fourth dose (controls). Among both groups, median age was 72 years, 53% were women, and 37% had three or more CDC-defined risk factors for severe COVID.