• The coronavirus variant first discovered in South Africa is able to evade some of the protection of the Pfizer-BioNTech vaccine, according to a new Israeli study.
• The researchers found the prevalence of B.1.351 among patients who received two doses of the vaccine was about eight times higher than those who were unvaccinated.
• The researchers in the study noted the main caveat of the study was the small sample size.

The coronavirus variant first discovered in South Africa is able to evade some of the protection of the Pfizer-BioNTech vaccine, according to a new Israeli study, which has not yet been peer-reviewed.

Researchers at Tel Aviv University and Clalit, the largest health-care organization in Israel, examined nearly 400 people who had tested positive for Covid-19 after receiving at least one dose of the vaccine. They compared them to the same number of people who were infected and unvaccinated.

The researchers found the prevalence of the variant from South Africa, known as B.1.351, among patients who received two doses of the vaccine was about eight times higher than those who were unvaccinated. The data, published online over the weekend, suggest the B.1.351 is better able to "break through" the protection of the vaccine than the original strain, the researchers wrote in the study.

"Based on patterns in the general population, we would have expected just one case of the South African variant, but we saw eight," Professor Adi Stern, who headed the research, told The Times of Israel. "We can say it's less effective, but more research is needed to establish exactly how much."

CNBC has reached out to Pfizer for comment on the study.

The new data comes as public health officials grow concerned that highly contagious variants, which studies have shown can reduce the effectiveness of vaccines, could stall the world's progress on the pandemic.

Last month, CDC Director Dr. Rochelle Walensky issued a dire warning, telling reporters that she worried the United States is facing great peril as variants spread and daily Covid-19 cases begin to rebound once again, threatening to send more people to the hospital.

"I'm going to pause here, I'm going to lose the script, and I'm going to reflect on the recurring feeling I have of impending doom," she said March 29. "We have so much to look forward to, so much promise and potential of where we are and so much reason for hope, but right now I'm scared."

Israel launched its national vaccination campaign in December prioritizing people 60 and older, health-care workers, and people with underlying health issues. By February, it was leading the world in vaccinations per capita, inoculating millions of its citizens against the virus.

In January, Pfizer and the Israeli Ministry of Health entered into a collaboration agreement to monitor the real-world impact of its vaccine.

The researchers noted the main caveat of the study was the small sample size. B.1.351 made up only about 1% of all Covid-19 cases, they said. B.1.1.7, the variant identified in the U.K., is more prevalent.

As variants spread, drugmakers said they are testing whether a third dose would offer more protection.

In February, Pfizer and BioNTech said they were testing a third dose of their Covid-19 vaccine to better understand the immune response against new variants of the virus.