Israeli research shows 50-fold jump in antibodies after third shot

A third dose of the Pfizer coronavirus vaccine led to 50 times more neutralizing antibodies in healthcare workers who received the shot eight months after their second dose, a new study has shown.

The report, published recently in Lancet Microbe by Dr. Esther Saiag, deputy director for information and operations at Tel Aviv Sourasky Medical Center, and her colleague Dr. David Bomze, examined the effect of the third dose on 346 healthy hospital employees.

“Healthcare workers are very unique,” Saiag told The Jerusalem Post.

She said that these workers tend to be healthier and more active than average citizens, and that because of the role they have played in the pandemic, they were among the first to take the vaccines.

Most of these workers received their second shot eight months before their third shot. Israel started its vaccination campaign on December 20, and medical personnel were among the first group to be vaccinated.

When the country opened up third doses to elderly people in August, Saiag asked if any of the older staff would be screened to check their level of antibodies before getting the shot. Some 346 people between the ages of 64 and 73 (215 women) complied.

The test found that in August their median baseline level of antibodies was only 3.67.

While antibodies do not tell the whole immunity story, as cellular memory is also important, Saiag said this number was very low.

Those who were screened before the shot returned 10 days after their third dose to get tested again. Almost all of them (95.7%) had a surge in antibodies more than 150.

Anti-spike protein concentrations were established with the ADVIA Centaur SARS-CoV-2 IgG assay, which provides an index value up to 150, the Lancet article explained. An index equal to or greater than one is considered reactive.

“We saw that very soon after having enough of the population getting the third booster that the fourth wave subsided,” Saiag said. “Now we see what was happening behind the scenes. We have this surge in antibodies. Maybe we all expected to find this, but now we have the data to prove it.”

Only two subjects did not respond at all and the level of antibodies in their blood remained negative. Nine additional subjects responded with only a moderate increase in the level of antibodies, despite the booster dose.

A follow-up study is now planned to trace possible causes for lack of response or non-maximal response among these subjects.

This is the largest study of its kind to examine the effect of the booster dose among healthcare workers.

Saiag said the plans are to continue to follow this group and re-screen them at various intervals to see what happens with their antibody levels. They will also check in with the staff to see if any of them contract COVID and, if so, if they have symptomatic or asymptomatic cases.

The results could help governments, including the Israeli government, make decisions about whether a fourth dose is needed, she said.