The preliminary analysis compares data from about 400,000 people aged 60 and over who received a fourth dose in January and some 600,000 people in the same age group who got only three doses — with the third shot administered four months or more previously.

The researchers found that those who had the fourth dose had twice the protection from infection as the others, and at least three times the protection from severe illness.

Israel began administering a fourth dose to older adults, health workers and people with compromised immune systems in late December as the omicron variant spread quickly through the population. Since then, almost 600,000 Israelis have received the extra dose. The great majority of people in Israel have received the Pfizer Inc. vaccine.