An Israeli hospital said Monday that preliminary studies suggest a fourth dose of a coronavirus vaccine would provide only limited defense against the omicron variant that is ravaging the world.

Sheba Hospital last month began administering the fourth vaccine to more than 270 medical staff — 154 of them received the Pfizer-BioNtech vaccine and another 120 received the Moderna vaccine. All had 3 previous Pfizer-BioNtech vaccines.

The clinical trial found a “slightly higher” increase in antibodies in both groups compared to last year’s third vaccine. But it said the increased antibodies did not stop the spread of omicron.

“Despite the increase in antibody levels, the fourth vaccine provides only partial defense against the virus,” said Dr. Gili Regev-Yochay, chief of the hospital’s infectious diseases department. “These vaccines are more effective against previous variants and offer less protection than omicron.”

The preliminary results have raised questions about Israel’s decision to give its population over the age of 60 a second booster shot — and a fourth overall. The government said more than 500,000 people had received a second booster in recent weeks.

Dr. Nahman Ash, director of the Israeli Ministry of Health, said the study did not mean that the fourth vaccine study was wrong. “It restores antibody levels to where they were at the start of the third booster. This is very important, especially in the elderly population,” he told Channel 13 TV.

But he said the study would be considered when authorities debate whether to expand the extra booster campaign to a wider population.

Israel was one of the first countries to vaccinate its population broadly last year, becoming the first to offer a booster last summer. The latest booster campaign for Old Israelis is also believed to be the first of its kind in the world.

Aggressive vaccination efforts in Israel have failed to stem the omicron outbreak in recent weeks. The variant has led to record infection levels and is sending more and more people to hospitals, although the number of critically ill patients is still lower than in previous waves.

It has also forced large numbers of Israelis into quarantine, putting pressure on schools and businesses.

Omicron already dominates many countries and can also infect those who have been vaccinated or who have previously been infected with the virus. However, early research suggests that it is less likely to cause serious disease than previous delta variants. Vaccinations and booster immunizations are still effective in preventing serious illness, hospitalization, and death.

On Tuesday, the Israeli government said it would shorten the mandatory quarantine period from seven days to five days to help keep the economy afloat.

Prime Minister Naftali Bennett said: “This decision will allow us to continue to maintain public health on the one hand, and to keep the economy afloat at this time, despite the difficulties, so that we can safely ride out this wave wave.” .