The study, published Tuesday in the New England Journal of Medicine, looked at the health records of more than 1.25 million vaccinated people in Israel who were 60 or older from January through March 2022, a time when the Omicron coronavirus variant was the dominant strain.
The rate of severe Covid-19 infection in the fourth week after a fourth dose of vaccine was lower than in people who got only three doses by a factor of 3.5.
However, protection against severe illness did not seem to wane in the six weeks after the fourth shot, though the study period wasn’t long enough to determine exactly how long this protection lasts.
The rate of confirmed infection in the fourth week after the fourth dose was lower than in the three-dose group by a factor of 2. There seemed to be maximum protection against Omicron in the fourth week after vaccination, but the rate ratio fell to 1.1 by the eighth week, suggesting that “protection against confirmed infection wanes quickly,” the study says.
The protection provided by any vaccine naturally wanes, but a vaccine primes the immune system to make protective antibodies if it encounters threats later on.
The currently available Covid-19 vaccines were made to protect against the original strain of the virus. Omicron is significantly different, and therefore, vaccines have lost some of their effectiveness. Israel and the US are among the governments that have suggested fourth vaccine shots for certain people who are at high risk of severe illness, like those who are older.
There are limits to what the new research can say about the need for a fourth vaccine dose. It compares only protection provided by third and fourth doses, so it doesn’t include unvaccinated people for comparison. It also doesn’t add to the debate over whether people under the age of 60 may need a fourth dose. Previous research in Israel showed that a fourth dose didn’t do much in younger healthy populations, at least in terms of protection from infection.
For confirmed infection, “a fourth dose appears to provide only short-term protection and a modest absolute benefit,” the researchers wrote. But when it comes to severe infection, the fourth shot seems to help.
“Overall, these analyzes provide evidence for the effectiveness of a fourth vaccine dose against severe illness caused by Omicron variant as compared with a third dose administered more than four months later,” the study says.