A study on 1.3 million people aged 60 and older looked at data from the Israeli Ministry of Health database between January 10 and March 2, when the Omicron variant was predominant.
A fourth dose of the Pfizer/BioNTech vaccine has lowered rates of Covid-19 among the elderly but the protection against infection appeared short-lived, a study in Israel has found.
The second booster's protection against infection dwindled after four weeks, Israeli researchers showed in their study published on Tuesday in the New England Journal of Medicine.
Protection against severe illness did not wane during the six weeks after the dose but more follow-up study was needed to evaluate its longer-term protection, the researchers said.
"Overall, these analyses provided evidence for the effectiveness of a fourth vaccine dose against severe illness caused by the Omicron variant, as compared with a third dose administered more than 4 months earlier," they said.
"For confirmed infection, a fourth dose appeared to provide only short-term protection and a modest absolute benefit," they added.
The study comes ahead of a meeting by the US Food and Drug Administration on Wednesday to discuss the need for additional boosters.
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Last week, the United States authorised a second booster shot for people aged 50 and older amid a spread of the Omicron sub-variant BA.2.
European health ministers have also urged the bloc's governments to back a fourth dose for people over 60.
In Asia, South Korea started giving out fourth doses of Covid-19 vaccines in February and Singapore has said a second booster dose is planned for those aged 80 and older.
Another study from Israel showed last month that seniors who received a second booster of the Pfizer Inc/BioNTech SE COVID-19 vaccine had a 78 percent lower mortality rate than those who got just one.
Israel started offering a second booster in January.