A fourth dose of the coronavirus vaccine increases antibodies fivefold in patients who receive the shots, preliminary findings from an Israeli study show.

Israel’s Prime Minister Naftali Bennett announced the news at the Sheba Medical Center near Tel Aviv on Tuesday, alongside Dr. Gili Regev-Yochay, a lead researcher on the study. The results have not been peer reviewed.

“We know that a week after administration of a fourth dose, we see a five-fold increase in the number of antibodies in the vaccinated person,” Bennett said, according to Haaretz. “This most likely means a significant increase against infection and …hospitalization and [severe] symptoms.”

Israel began administering fourth doses of the coronavirus vaccine this week as a second booster shot to adults ages 60 and up, as well as medical workers who have gone at least four months since their first booster shot. Nearly 100,000 people already received or were scheduled to receive a fourth dose of the coronavirus vaccine as of Tuesday, The Times of Israel reported.

The World Health Organization, Dr. Anthony Fauci and others have expressed concern about the aggressive booster program.

Professor Galia Rahav, the head of Sheba’s infectious diseases center, spoke to an Israeli news channel to address the criticism on Tuesday.

“We’re always ahead of the rest. That’s been the case the whole way. We learned a lot about the third shot before the rest. And we saw its amazing effect — reducing illness in the most impressive way,” Rahav told Channel 12, according to The Times of Israel.