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(CNN) --
A fourth dose of Pfizer/BioNTech's COVID-19 vaccine appears to offer short-lived protection against infection overall, but protection against severe disease did not wane for at least several weeks, according to a new studio.
The study, published Tuesday in the New England Journal of Medicine, analyzed the health records of more than 1.25 million vaccinated people in Israel who were 60 years of age or older between January and March 2022, the time when the omicron variant of the coronavirus was the dominant strain.
The rate of severe COVID-19 infection in the fourth week after the fourth dose of the vaccine was lower than in people who received only three doses by a factor of 3.5.
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However, protection against severe disease did not appear to decrease in the six weeks after the fourth dose, although the study period was not long enough to accurately determine the duration of this protection.
The confirmed infection rate in the fourth week after the fourth dose was lower than in the three-dose group by a factor of 2. There appeared to be maximal protection against omicron in the fourth week after vaccination, but the rate ratio it fell to 1.1 by week eight, suggesting that "protection against confirmed infection declines rapidly," the study says.
The protection provided by any vaccine decreases naturally, but vaccines prime the immune system to produce protective antibodies if it encounters threats later on.
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Currently available COVID-19 vaccines were made to protect against the original strain of the virus.
The omicron variant is significantly different and therefore the vaccines have lost some of their efficacy.
Israel and the US are among the governments that have suggested giving a fourth dose of the vaccine to certain people who are at high risk of serious illness, such as the elderly.
Last week, the FDA expanded its emergency use authorization to allow adults age 50 and older to receive a second booster dose as early as four months after their first booster of a Pfizer or Moderna vaccine.
People with certain immunodeficiencies can also receive a fourth dose in the United States.
But the debate continues about whether one is necessary for the general population.
The independent vaccination committee of the US Food and Drug Administration (FDA) will meet this Wednesday to discuss the country's future booster strategy.
Some experts have suggested that the Covid-19 vaccine could become an annual flu shot.
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FDA advisers will discuss it this week
What the new research can say about the need for a fourth dose of the vaccine has its limits.
It only compares the protection provided by the third and fourth doses, so it does not include unvaccinated people with whom a comparison can be made.
Nor does it add new information to the debate about whether those under 60 may need a fourth dose.
Earlier research in Israel showed that a fourth dose did not have much of an effect in younger, healthy populations, at least in terms of protection against infection.
In the case of confirmed infection, "a fourth dose appears to provide only short-term protection and modest absolute benefit," the researchers wrote.
But when it comes to a serious infection, the fourth dose seems to help.
"Overall, these analyzes provide evidence of the efficacy of a fourth dose of the vaccine against severe disease caused by the omicron variant compared to a third dose given more than four months later," the study says.
Even with this news, it is unclear how many eligible people will receive a fourth dose of the COVID-19 vaccine.
About 66% of the US population has been vaccinated at least once against Covid-19, but only 30% of people are fully vaccinated with a booster dose, according to the US Centers for Disease Control and Prevention. United States.
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