A look at how the Pfizer vaccine is performing in the real-world suggests it may not be as effective against the coronavirus variant that was first discovered in South Africa.

A recently released study looked at roughly 400 people in Israel who had tested positive for COVID-19 roughly two weeks or more after they received one or two doses of the vaccine, according to Reuters and Al Jazeera.

Those patients were compared to the same number of unvaccinated patients with COVID-19.

The variant, identified as B.1.351, was responsible for about 1% of all the COVID-19 cases total in the study.

However, the percentage of patients with the B.1.351 variant among the group who had received two doses of the vaccine was eight times higher than the percentage of the unvaccinated group with the variant.

About 5.4% of those who had COVID-19 after receiving the vaccine had the variant, while just 0.7% of the unvaccinated group who had COVID-19 had the variant.

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

Researchers warn about the small sample size, since the variant first identified in South Africa is not very common in Israel right now. The variant first identified in Britain is dominant in the country.

The study was published on the medRxiv website and has not been peer reviewed yet.

More than 50% of Israel’s 9.3 million people have received both doses of the Pfizer-BioNTech vaccine.