The standard regimen for messenger RNA-based COVID-19 vaccines is two doses, but some governments, including Israel’s, have started administering third ‘booster’ shots. The latest study evaluated 1.1 million Israelis over the age of 60 who had received their first two doses at least five months earlier. Twelve or more days after receiving a third jab, participants were about 19.5 times less likely to have severe COVID-19 than were people in the same age group who had received only two jabs and were studied during a similar time period.
“It’s a very strong result,” says Susan Ellenberg, a biostatistician at the University of Pennsylvania in Philadelphia, who adds that the data might be the most robust she has seen in favour of boosters. But potential biases in the data leave some scientists unconvinced that boosters are necessary for all populations—and the data do not dispel concerns about vaccine equity when billions of people are still waiting for their first jab.