While second COVID-19 vaccine booster shots provide additional protection against Omicron infections, they only did so for a very limited time, the study found, raising questions as to whether vaccines will have to be reformulated or whether protection against severe illness—not all infection—will be the primary focus of future public health efforts.

Using Israeli Ministry of Health data on more than 1.2 million people age 60 and over who were eligible for the fourth dose during the period when Omicron was the dominant variant, the study’s authors found that while a second booster of the BioNTech-Pfizer vaccine offered strong protection against severe infections for at least six weeks, the protection against all infections began to wane after four weeks and almost disappeared after eight.

“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 four months earlier,” the authors wrote. “For confirmed infection, a fourth dose appeared to provide only short-term protection and a modest absolute benefit.” 

Also from the study: After four weeks, people 60 and over who had only one booster were 3.5 times more likely to suffer severe illness than those who had the fourth dose. The comparative protection offered by the second booster even rose in the sixth week, to 4.3 times.

The likelihood of getting infected at all was another story. After four weeks, those with only one booster were twice as likely to get infected than those with a fourth dose, but that ratio fell to 1.1 times after eight weeks.

“Protection against confirmed infection appeared short-lived, whereas protection against severe illness did not wane during the study period,” the researchers wrote.

Because the study covered only a two-month period, it was unclear if protection against severe illness faded after eight weeks. “More follow-up is needed in order to evaluate the protection of the fourth dose against severe illness over longer periods,” the authors wrote.

The study, published on Tuesday in The New England Journal of Medicine, did not look at the effects of a fourth dose on those under 60, as the data was collected at a time when Israel had approved a fourth does only for people 60 and older, as well as for high-risk populations and health care workers.

Second booster needs

The analysis of the Israeli data will likely stoke debate over whether—and for whom—a fourth shot is needed.

“In the most likely scenario, it would be necessary to periodically revaccinate at least the most vulnerable, and place more emphasis on treatments,” said professor Fernando Rodríguez Artalejo, an epidemiologist at the Autonomous University of Madrid. “It may not be necessary to reformulate the vaccines since the responses to the vaccine boosters progressively generate greater protection against new types of coronavirus.”

Several doctors told Fortune in March that the necessity of a fourth shot will depend on the virulence of any new coronavirus variants that spread.

“I think depending on what the new variants bring, there could be additional rounds of vaccination that will be required,” said Dr. William Checkley, a pulmonary and critical care physician at the Johns Hopkins Hospital and associate professor of medicine at Johns Hopkins University.

“There’s certainly every reason to think that whatever variant that comes out in the future, our prior protection will still be somewhat protective against the future variant,” Dr. Gregg Miller, an emergency-room doctor in Washington State and chief medical officer at Vituity, a physician-owned health care company, told Fortune. But, he added, a fourth booster shot could improve chances at combating severe illness.

“The higher and higher you build up that [immunity] wall, the harder and harder it’s going to be for future variants to jump over it,” he said.