Fourth Pfizer Dose Spurs Sharp Jump in Antibodies, Early Results From Israel Show: People who got the fourth shot had a fivefold increase in antibodies one week later

TEL AVIV—Early data from Israel suggests a fourth dose of the Covid-19 vaccine from Pfizer Inc. and BioNTech SE will provide safe and effective protection against infection and severe illness for those with waning immunity.

There was a fivefold increase in antibodies of individuals one week after they got their fourth shot, according to data from Israel’s Sheba Medical Center, which provided the dose to 154 medical workers of various ages. The personnel who took part in the trial had taken a third shot by Aug. 20 and their tests showed they lacked sufficient antibodies for good protection.

The findings were announced on Tuesday by Israeli Prime Minister Naftali Bennett during a visit to the medical center located in central Israel.

“Apparently, this will demonstrate a much higher level of protection than without the fourth dose, both in relation to infection and the spread of the virus and in relation to severe morbidity,” Mr. Bennett said in a video message.

Mr. Bennett and researchers for the trial said the results, the first of their kind, would help governments around the world decide whether they should prepare to offer citizens a fourth dose and how soon.

Israel itself didn’t wait for the results of the trial. On Monday, after much debate within the country, Israel became the first nation to offer a fourth dose to people aged 60 years and older as well as medical workers who are four months past their third dose. Immunocompromised people began receiving a fourth dose last week.

Health officials said the decision was due both to the expectation that Omicron would surge through the country—including among the vaccinated—and research that showed protection provided from the third dose begins to wane within three to four months.

Israel had one of the world’s quickest and widest vaccination campaigns at the start of the pandemic. Consequently, it also became the first country to carry out a wide-scale booster campaign, as its population suffered from waning protection first and was hit hard by the Delta variant.

Mr. Bennett said over 100,000 of Israel’s around 9.3 million residents had signed up for or taken a fourth dose.

Mr. Bennett said more time would be needed to understand with certainty how effective a fourth dose will be and that Israel would share the results with other countries in the coming days.

On Sunday, Sheba Medical Center said data from its trial showed side effects from the fourth dose were similar to previous rounds.

The Israeli medical center is also running a separate trial in which 150 of its medical personnel who have three shots of Pfizer’s will take a fourth dose of Moderna Inc.’s vaccine.

The Omicron strain has spread rapidly across the country, and Israel recorded 10,700 new cases on Monday, nearing its daily record since the pandemic began.