Preliminary research from Israel suggests that a fourth COVID-19 mRNA vaccine booster shot may be ineffective against breakthrough infection from the Omicron variant.

The authors of a study that looked at the effectiveness of a fourth Pfizer-BioNTech or Moderna shot against Omicron said they were releasing data early on Monday to keep the public up to date with the latest developments in vaccine research.

“Despite a significant increase in antibodies after the fourth vaccine, this protection is only partially effective against the Omicron strain, which is relatively resistant to the vaccine,” Dr. Gili Regev-Yochay, the lead researcher on the study, told reporters Monday.

The study included 154 health care workers at Sheba Medical Center who received their fourth Pfizer shot. Another 120 workers received a fourth dose of the Moderna vaccine, and a control group of 6,000 workers were not given a fourth booster shot of either vaccine.

Regev-Yochay said that a third shot resulted in "much higher antibodies, neutralization and the antibodies were not just higher in quantity but also in quality" than the second shot — but the fourth shot did not produce similar results.

"These are very preliminary results. This is before any publication, but we're giving it out since we understand the urgency of the public to get any information possible about the fourth dose," Regev-Yochay explained.

"We have a follow-up of the Pfizer vaccine for two weeks now, and we have a follow-up of the Moderna vaccine just for one week at this time point. And what we see is that the Pfizer vaccine, after two weeks, you see an enhancement or increase in the number of antibodies and neutralizing antibodies — a pretty nice increase. It's even a little bit higher than what we had after the third dose," she said. "Yet, this is probably not enough for the Omicron."

Regev-Yochay added that slightly fewer infections were observed among those who got the fourth vaccine shot compared to the control group, which may indicate there's a small benefit to letting the people most vulnerable to COVID-19 get a fourth booster.

"I think that the decision to allow the fourth vaccine to vulnerable populations is probably correct," she said. "It may give a little bit of benefit, but probably not enough to support the decision to give it to all of the population, I would say."

International bodies are warning governments and public health authorities against requiring a fourth vaccine shot. At a press briefing last week, the European Medicines Agency said there was no need for a second booster, even warning that repeated vaccine doses could actually weaken people's immune systems.

Boosters “can be done once, or maybe twice, but it’s not something that we can think should be repeated constantly,” Marco Cavaleri, the EMA head of biological health threats and vaccines strategy, said, according to Bloomberg.

“We need to think about how we can transition from the current pandemic setting to a more endemic setting,” Cavaleri added.

The EMA advised countries to leave more time between booster programs and to tie them to the cold season on each hemisphere.

The World Health Organization has also warned that repeated booster doses of COVID-19 vaccines are "not a sustainable global strategy," raising concerns about the supply of vaccine doses.

“With near- and medium-term supply of the available vaccines, the need for equity in access to vaccines across countries to achieve global public health goals, programmatic considerations including vaccine demand, and evolution of the virus, a vaccination strategy based on repeated booster doses of the original vaccine composition is unlikely to be appropriate or sustainable,” the WHO said last week.