(NEXSTAR) — The Pfizer/BioNTech vaccine may be less effective against the South African strain of the coronavirus than other variations circulating in the population, a new study out of Israel has found.

Using data from Israel’s largest health provider, researchers studied 400 fully vaccinated individuals who had contracted the virus against 400 members of the unvaccinated infected population. Researchers found the South African variant, known as B.1.351, was about eight times more likely to be the cause of the infection in the vaccinated group relative to the unvaccinated population.

“We found a disproportionately higher rate of the South African variant among people vaccinated with a second dose,” Tel Aviv University’s Adi Stern told Reuters. “This means that the South African variant is able, to some extent, to break through the vaccine’s protection.”

Researchers cautioned that the study was a relatively small sample size and more research is needed. The work has not yet been fully peer-reviewed, according to Reuters. The findings were revealed in a preprint of the study results.

Concerns over variants have vaccine makers tweaking their original formulations to broaden protections. The Associated Press reports that some Americans have already received a third shot, one aimed specifically at variants.

All the major vaccine makers are tweaking their recipes in case an update against that so-called B.1.351 virus is needed. Now experimental doses from Moderna and Pfizer are being put to the test in live patients.

Viruses constantly evolve, and the world is in a race to vaccinate millions and tamp down the coronavirus before even more mutants emerge. More than 119 million Americans have had at least one vaccine dose, and 22% of the population is fully vaccinated, according to the Centers for Disease Control and Prevention. Much of the rest of the world is far behind that pace.

“We need to be ahead of the virus,” Dr. Nadine Rouphael of Emory University, who is helping to lead a study of Moderna’s tweaked candidate, told the Associated Press. “We know what it’s like when we’re behind.”

It’s not clear if or when protection would wane enough to require an update but, “realistically we want to turn COVID into a sniffle,” she added.

Already an easier-to-spread version found in Britain just months ago has become the most common variant now circulating in the United States, one that’s fortunately vaccine-preventable.

The Moderna and Pfizer vaccines, like the majority of COVID-19 vaccines being used around the world, train the body to recognize the spike protein that is the outer coating of the coronavirus. Those spikes are how the virus latches onto human cells.

Research funded by the National Institutes of Health is examining whether to boost vaccines for those who have already received shots, and how best to protect those who haven’t yet received any kind of COVID-19 vaccination.

The Associated Press contributed to this report.