Moshe Geva Rosso, 62 years old, receives a fourth dose of the coronavirus disease (COVID-19) vaccine after Israel’s Health Ministry approved a second booster for the immunocompromised, at Sheba Medical Center in Ramat Gan, Israel, December 31, 2021. REUTERS/Nir Elias

A fourth dose of the COVID-19 vaccine increases the number of neutralizing antibodies, but the boost is probably not enough to provide protection against infection of the Omicron variant, findings of a preliminary study conducted by Israel’s Sheba Medical Center concluded.

“We know by now that the level of antibodies needed to be protected from Omicron and not to get infected is probably too high for this vaccine, even if it is a good vaccine,” said Prof. Gili Regev-Yochay, the lead researcher of the study, during a briefing Monday. “Thus, the vaccines (including Pfizer & Moderna), which were extremely effective against the earlier variants, are not as effective versus Omicron.”

Two weeks after administering a fourth Pfizer vaccine shot to 154 medical personnel from Sheba Medical Center, Regev-Yochay reported a “pretty nice” increase in the number of antibodies. A week ago, an additional 120 adults received the Moderna shot after previously getting three doses of the Pfizer vaccine.

Earlier this month, Regev-Yochay said that one week after the administration of a fourth Pfizer dose, the number of antibodies rose fivefold among trial participants. The trial seeks to test the safety of a second round of boosters and its efficacy in producing antibodies amid the rapid spread of the Omicron variant.

“The rise in antibody levels that we saw with both Moderna and Pfizer are slightly higher than what we saw after the third booster vaccine,” said Regev-Yochay. “However, as we have seen within the realm of our ongoing serological studies amongst Sheba personnel and the rising numbers of personnel who are being infected with Omicron, despite increased antibody levels, the fourth vaccine only offers a partial defense against the virus.”

More than 500,000 Israelis over the age of 60 have so far received the fourth dose of the COVID-19 vaccine. Earlier this month, Israel initiated a campaign for getting a fourth dose to all adults over 60, and medical workers, who had received a previous jab at least four months ago.

“I think that the decision to allow the fourth vaccine to vulnerable populations is probably correct,” conceded Regev-Yochay. “It may give a little bit of benefit, but probably not enough to support the decision to give it to all of the population.”

Regev-Yochay cautioned that the findings are based on ” very preliminary results, before any publication.”

“We decided to share them because we understand the urgency of the public to receive information,” she added.