A study published in New England Journal of Medicine stated that 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.

182,122 matched pairs of adults aged 60 and above were included in the study. It was observed that the protection against the disease enhanced as the number of days after vaccination increased with the best protection occurring between days 14 and 30 after the fourth dosage. The relative vaccine effectiveness was 52 per cent for asymptomatic infection, 61 per cent for symptomatic COVID, 72 per cent for COVID-related hospitalization, 64 per cent for severe COVID, and 76 per cent for COVID-related death in days 14 to 30 after the fourth dose.

A fourth dose of the BNT162b2 vaccine reduced the risk of Covid-19-related outcomes in people who had already had a third dose at least 4 months before.

(Source: MedPage Today April 13, 2022); N Engl J Med 2022; DOI: 10.1056/NEJMoa2201688.