Fourth COVID-19 shots increase antibody levels in patients for some 13 weeks, according to a new peer-reviewed Israeli study, with the increased protection against infection alone seemingly fading by 15 weeks.
The study focused on first-generation Pfizer vaccines, not the more recent ones that have been updated for Omicron and are being phased into circulation in Israel.
Importantly, it is unclear what antibody levels mean for protection against serious illness, as the study did not look at that metric. The vaccine may well offer defense against severe forms of the disease for longer than the three months in which antibody levels are high.
Still, study authors at Sheba Medical Center said the research should prompt healthcare providers to time booster campaigns wisely. As added protection against infection peaks for a relatively short period, boosters should be given when spates of high infection are on the horizon, or when specific patients are facing circumstances that heighten their risk, they argued.
“With protection clearly waning after four months, individuals and health systems must plan their booster timing wisely,” said Prof. Gili Regev-Yochay, director of infectious diseases at Sheba and one of the lead authors.
They “should take into consideration not only surges in infection but also personal medical conditions, upcoming events and travel, and higher-risk seasons.”
The study, published in the New England Journal of Medicine and conducted by Sheba researchers along with experts from the Health Ministry and Dr. Michal Canetti of Columbia University, was based on 6,000 Sheba employees and volunteers.
Some COVID experts have responded to the study by emphasizing that while the boost in antibodies and infection reduction seems short, it has value. Bar Ilan University epidemiologist Prof Michael Edelstein, who wasn’t involved in the study, told The Times of Israel: “Although the timeframes are quite short, they may be enough to protect vulnerable individuals during higher risk times like peaks of transmission.”
The study found that antibodies’ rise after fourth shots was less than after third shots.
The researchers said they couldn’t report on severe outcomes, as there were none among participants.
Edelstein said that it illustrates that fourth dose benefits are “more transient” than those of previous doses. However, he stressed that fourth doses can “still be critical to protect those vulnerable at times of high risk such as increased circulation of the virus.”
He said that more research is needed to provide more detail, such as how fourth doses impact severe illness. “We cannot fully assess the effectiveness of vaccines only by measuring antibody levels and infection rates — vaccines work through a range of mechanisms and we don’t have the full picture yet.”