By Maayan Lubell

The study was released on the medRxiv pre-print site on April 9 and has not been peer reviewed. It compared almost 400 people who had tested positive for COVID-19, after they received one or two doses of the vaccine, against the same number of unvaccinated patients with the disease.

It matched age and gender, among other characteristics.

The South African variant, B.1.351, was found to make up about 1% of all the COVID-19 cases across all the people studied, according to the study by Tel Aviv University and Israel’s largest healthcare provider, Clalit.

But among patients who had received two doses of the vaccine, the variant’s prevalence rate was eight times higher than those unvaccinated – 5.4% versus 0.7%.

This suggests the vaccine is less effective against the South African variant, compared with the original coronavirus and a variant first identified in Britain that has come to comprise nearly all COVID-19 cases in Israel, the researchers said.

The researchers said the study was not intended to assess overall vaccine effectiveness against any variant, since it only looked at people who had already tested positive for COVID-19, not at overall infection rates.

Separate real-world Israeli studies on the vaccine’s overall effectiveness, including by Clalit, have shown the Pfizer shot to be more than 90% effective.

“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,” said Tel Aviv University’s Adi Stern.

In an update to the study posted on April 16, the researchers noted that within the group of people who received two doses, which comprised eight people, all of the B.1.351 infections occurred within a week to 13 days after the second shot. None of them tested positive for it 14 days or more after the second dose.

“This may imply that there is a short window of susceptibility to B.1.351 infection limited to the immediate two weeks after the second dose – but we cannot be confident that this is indeed the case,” Stern and Clalit’s Ran Balicer said in an email to Reuters on Sunday.

The researchers cautioned, though, that the study only had a small sample size of people infected with the South African variant because of its rarity in Israel.

“The B.1.351 incidence in Israel to-date remains low and vaccine effectiveness remains high among those fully vaccinated,” the study said.

Pfizer declined to comment on the Israeli study. Pfizer and BioNTech said on April 1 that their vaccine was around 91% effective at preventing COVID-19, citing updated trial data that included participants inoculated for up to six months.

They have been testing a third dose of their shot as a booster, and have said they could modify the shot to specifically address new variants if needed.

In respect to the South African variant, they said that among a group of 800 study volunteers in South Africa, where B.1.351 is widespread, there were nine cases of COVID-19, all of which occurred among participants who got the placebo. Of those nine cases, six were among individuals infected with the South African variant.

Some previous studies have indicated that the Pfizer/BioNTech shot was less potent against the B.1.351 variant than against other variants of the coronavirus, but still offered a powerful defence.

While the results of the study may cause concern, the low prevalence of the South African strain was encouraging, according to Stern.

“Even if the South African variant does break through the vaccine’s protection, it has not spread widely through the population,” said Stern, adding that the British variant may be “blocking” the spread of the South African strain.

More than half of Israel’s 9.3 million population has received both Pfizer doses. Israel has largely reopened its economy in recent weeks while the pandemic appears to be receding with a sharp drop in infections and hospitalizations. (Reporting by Maayan Lubell; Editing by Pravin Char, Frances Kerry and Daniel Wallis)