The protection from a second booster dose of Pfizer's COVID-19 vaccine against infection wanes after four weeks, according to a study from Israel published April 5 in The New England Journal of Medicine. Meanwhile, the additional shot's effectiveness against severe illness appeared to last longer.
Using the Israeli Ministry of Health database, researchers analyzed records on more than 1.2 million people at least 60 years old who were eligible for a fourth dose during a period in which omicron was dominant. They compared the rate of confirmed infections and severe cases among those who received a second booster to those who had gotten just three doses.
Findings showed protection against infection peaked about four weeks after receipt of the fourth shot, with the rate of infection about twice as high among those who didn't get the second booster. However, the added protection from a fourth dose nearly disappeared by the eighth week after receipt, with the infection rate nearly the same among the cohorts.
Meanwhile, the additional booster's effectiveness against severe COVID-19 appeared to stay put longer. Four weeks after getting the second booster, the rate of severe disease was 3.5 times lower than that of the three-dose group. After six weeks, this protection did not appear to wane, though the study period was too short to determine whether it would hold up in the long-term.
"For confirmed infection, a fourth dose appeared to provide only short-term protection and a modest absolute benefit," researchers said.
The findings follow the FDA's authorization of a second booster for adults 50 and older. The CDC stopped short of explicitly recommending the additional shots, instead saying those eligible may do so, drawing criticism from some physicians who have said they've been bombarded with questions and provided with little guidance.