A fourth dose of the Pfizer-BioNTech Covid-19 vaccine drives a five-fold increase in antibodies and likely significantly improves protection against coronavirus infections, according to preliminary findings from a study at Israel's Sheba Medical Center.

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About the study

For the study, researchers examined 154 medical workers who had received their third vaccine dose before Aug. 20, 2021. Prior to the fourth shot, the participants' antibody levels had waned, suggesting they no longer had robust protection against Covid-19 infection.

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By one week after their fourth shot, participants' antibody levels increased by five-fold on average.

In a statement, Israeli Prime Minister Naftali Bennett said the results indicated "a very high likelihood that the fourth dose will protect vaccinated people to a great degree against infection to some degree and against severe symptoms."

Side effects from the fourth dose were minor and similar to those after earlier vaccine doses, including "low fevers" and "sore arms," according to a spokesperson for the medical center.

For a forthcoming study, researchers also intend to study the effects of a booster dose of the Moderna Covid-19 vaccine for those who previously received Pfizer vaccine doses.

Some experts question whether fourth doses are truly needed

Even before the trial results were released, the Israeli government had authorized fourth doses for the immunocompromised, people aged 60 and older, and medical workers who are four months from their third dose.

Some health experts, however, voiced skepticism about the need for fourth doses—especially for the general population.

"That it raises the antibody levels is good news, but first we need to know if there is a need for another shot," said Hagai Levine, an epidemiologist at Hebrew University and chairman of the Israeli Association of Public Health Physicians.  

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Professor Andrew Pollard, the director of the Oxford Vaccine Group and head of the U.K.'s Committee on Vaccination and Immunization, worried about the viability of repeated, population-wide boosters. "We can't vaccinate the planet every four to six months," he said. "It's not sustainable or affordable. In the future, we need to target the vulnerable."

He also noted that, even as Israel considers fourth doses, "less than 10% of people in low-income countries have even had their first dose."

Will the U.S. follow Israel's lead?

So far, the United States has not acted to make available fourth vaccine doses to the general population. The exception is for immunocompromised individuals over the age of 12, for whom CDC guidelines recommend a three-dose primary vaccination series followed by a booster dose.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, in late December said it was "premature" to consider fourth doses for the broader U.S. population.

The first step, Fauci said, is to understand "the durability of the protection is following the third dose of an mRNA vaccine," such as those produced by Pfizer-BioNTech and Moderna.

If the protection proves to be durable, "then we may go a significant period of time without requiring a fourth dose," Fauci added. (Kennedy and Guy, CNN, 1/5; Lieber, Wall Street Journal, 1/4; Hendrix, Washington Post, 1/4)