A fourth dose of the same mRNA vaccines won't stop the omicron variant, infectious disease experts said, even as Israeli doctors began tests of more booster shots in vaccinated medical workers.

The fourth jab follows the initial two-shot mRNA vaccines and the booster. But with new variants raging in the United States, boosting the old vaccine may not help, according to some epidemiologists.

“You’re boosting people with [antibodies to fight] the original Wuhan strain vaccine, and the circulating variants are delta and omicron,” said Dr. Amesh Adalja, a senior scholar at the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health. “Your body is kind of primed towards the original strain.”

Researchers at Sheba Medical Center near Tel Aviv gave the first round of fourth doses to a test group of 150 healthcare workers on Monday in the first major study to determine whether another shot will stave off infection from the omicron variant. The results of the study will be sent to the Israeli Health Ministry in about two weeks, Reuters reported. The study in Israel, an early leader in the global vaccination drive, will be closely watched by other countries as the omicron variant takes over.

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Vaccine experts and public health officials have stepped up calls for people to get their booster doses of an mRNA vaccine, which have shown to bolster waning immunity to the virus in the months following the second round of shots. While the rapid speed of omicron’s spread has alarmed health officials in the United Kingdom, where outbreaks have often proven a harbinger of what’s to come in the U.S., government officials are not committed to the theory that another dose is necessary so soon after giving out third shots.

“When you get the benefit of a boost, it's to allow the immune system in the interval between one injection and another to give it a chance to … rev itself up to be optimally responsible to the next boost,” Dr. Anthony Fauci, the top government infectious disease expert, said on Monday.

Boosting too often and too frequently, Fauci added, deprives the immune system of the time needed for B cells generated by vaccines to make antibodies capable of targeting new variants, such as omicron.

Many public health experts, meanwhile, have pointed out that the booster doses already being administered in the U.S. are doing what they were meant to do: keep vaccinated people from becoming seriously ill due to this new variant.

“We got a booster so that [circulating antibodies] counteract and neutralize any virus that got into our respiratory system much more quickly than the immune system recognizing that the virus is there and then creating antibodies, so we had them preexisting,” said Dr. Susan Hassig, an epidemiologist at the Tulane University School of Public Health and Tropical Medicine.

The antibodies generated by vaccines can protect a person from severe infection for about four to six months before dropping off.

Concern of omicron-driven case spikes mounted in the lead-up to the Christmas holiday weekend when people were likely to gather indoors with others in large numbers, vastly increasing the risk of viral transmission. Average daily cases have exceeded 243,000 for the first time since January 2021. Caseloads have ballooned in the Northeast, where omicron has a strong foothold, and strained hospitals in the Midwest and New England are still struggling to care for the volume of patients seriously ill from the delta variant.

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Still, meteoric rises in new cases linked to omicron nationwide have not translated to steep increases in hospitalizations, heartening scientists who believe illness due to the strain will be milder.

“If we're having low hospitalization rates … it starts to fall under the heading of ‘don't fix something that's not broken,’” said Dr. George Rutherford, an epidemiologist at the University of California, San Francisco.

Pfizer and Moderna have touted the benefits of the technological platform of their mRNA vaccines, which could be altered to better target the spike proteins on a mutated strain of COVID-19. An omicron-specific booster engineered to protect against that variant is not out of the question, but for right now, the initial booster doses are sufficient, Rutherford said.