Getting a fourth shot of the COVID-19 vaccine will boost a person’s ability to fight the virus, but it will not prevent an infection from the omicron variant of the disease, a preliminary study out of Israeli suggests.

The study, conducted at Israel’s Sheba Medical Center, followed people who were given a second booster shot of the COVID-19 vaccine from Pfizer and its German partner, BioNTech, as well as people who were given the Moderna COVID-19 vaccine.

In the trial, 274 medical workers at the hospital received a fourth dose of the vaccine in December. A little more than half of those in the study, 154, got the Pfizer/BioNTech vaccine, and the rest got the Moderna vaccine, according to a story from Reuters.

Everyone included in the study had previously gotten three Pfizer/BioNTech shots, according to The Times of Israel.

According to the study results, both groups saw an increase in antibodies, but researchers said the boost did not prevent the spread of omicron.

“Despite increased antibody levels, the fourth vaccine only offers a partial defense against the virus,” said Dr. Gili Regev-Yochay, director of the hospital’s infectious disease unit. “The vaccines, which were more effective against previous variants, offer less protection versus omicron.”

Israel is currently administering a fourth shot of the vaccines, despite the study results.

The director of Israel’s Health Ministry, Dr. Nahman Ash, told Channel 13 TV in Israel the campaign would continue because the second booster “returns the level of antibodies to what it was at the beginning of the third booster. That has great importance, especially among the older population.”

The findings of the study have not been published in a medical journal.

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