Older men and people with immunosuppression are at a higher risk of developing a severe COVID-19 breakthrough infection than other groups if six months have passed since they received a second dose of the Pfizer/BioNTech vaccine, an Israeli study has shown.
The new study from Sheba Medical Center in Israel examined how antibody levels in different population groups wane over six months after the second vaccine dose.
With the data they gathered, researchers were able to construct a model that predicts antibody levels according to sex, age, BMI (body mass index) and immunosuppression.
Published in the New England Journal of Medicine (NEJM) on Thursday, the study was led by Professor Gili Regev-Yochay, director of the Infectious Diseases Unit at Sheba Medical Center. It was carried out together with Israel’s Ministry of Health and the Gertner Institute, where Regev-Yochay is also associated.
The study found that antibody levels decreased fastest in people over 65 years old, older men and those with immunosuppression, suggesting that these groups are more likely to develop a severe breakthrough case after six months have passed from vaccination.
Regev-Yochay presented the findings of the research at a news conference on Tuesday.
“The whole world is looking for what is the threshold of antibodies required to protect you from being infected altogether, from having severe disease or for mortality,” Regev-Yochay said.
“Once we know these thresholds, the large table of probabilities will help assist with what is the right preventive measure for different subpopulations,” she said.
Nearly 5,000 health care workers from Sheba Medical Center who were vaccinated with two Pfizer/BioNTech doses and who underwent serological testing took part in the study. Researchers monitored both neutralizing antibody levels and IgG (Immunoglobulin G) levels over a period of six months.
They found that both neutralizing antibodies and IgG levels significantly decreased after the six months had elapsed.
The whole world is looking for what is the threshold of antibodies required to protect you from being infected altogether, from having severe disease or for mortality
Among healthy individuals, 6% of women and 15% of men over 65 years old had very low neutralizing antibody levels (16 or fewer units of neutralizing antibodies) six months after they had received their second dose. For immunosuppressed people, these figures were significantly higher in the same age group: 30% of women and 50% of men had low antibody levels. Finally, for individuals who are considered to be obese (BMI greater than 30), 4% of women and 10% of men over 65 had low antibody levels.
The last series of data surprised researchers, Regev-Yochay noted, and they are looking into it further.
In July, Israel became the first country in the world to begin offering third vaccine doses. So far, nearly 3.6 million Israelis have received a booster shot.
One of the primary goals of the research, which is ongoing, is to give scientists the ability to determine which subpopulations might require additional doses in the future.
In fact, once COVID-19 becomes endemic, vaccines could then be administered only to specific groups that are at higher risk of developing a serious breakthrough case, Regev-Yochay told The Media Line.
“Our study will eventually answer that [question],” she said. “Right now, we’re in a pandemic so we really want to vaccinate even people who are not at a very high risk of severe disease, but also people who might be infectious, because they allow the pandemic to go on.”
Researchers from Sheba Medical Center had previously shown that neutralizing antibody levels strongly correlated to breakthrough COVID-19 infections and severe disease in vaccinated individuals.
“The reason that we’re giving boosters is to altogether stop this surge that we had,” Regev-Yochay said, referring to the delta variant outbreak. “We obviously saw a lot of breakthrough [cases] with mild or even moderate disease among younger people who are not immunosuppressed. We see a lot of disease altogether.”
A separate study published earlier this week in The Lancet medical journal showed that the Pfizer vaccine’s effectiveness in preventing infection dropped from 88% to 47% six months after the second dose. Nevertheless, the data also showed that the vaccine continued to successfully prevent hospitalization and death over that same period at a rate of 90%.
Last month, a serological study conducted in Israel found that the antibody levels detected in the body after a third Pfizer dose were ten times higher than after the second. The study, published in the NEJM, showed that individuals were 11.3 times less likely to get infected with COVID-19 12 days after receiving a third jab.
Researchers from the Weizmann Institute of Science, the Ministry of Health, the Technion-Israel Institute of Technology, the Hebrew University of Jerusalem, Sheba Medical Center and the KI Institute collaborated on the study.
“None of us knows yet if we’ll need boosters every six months,” Dr. Sharon Alroy-Preis, who heads the Health Ministry’s public health services, said in a video statement that was shared with The Media Line. “It also depends on whether this pandemic will at some point fade entirely, and then maybe we won’t need to take anything at all. It depends on how often this virus mutates and whether we’ll need to update our protection against it.”
European Union drug regulators on Monday backed giving boosters to people with weakened immune systems. The US Food and Drug Administration (FDA) has meanwhile approved third doses of the Pfizer/BioNTech vaccine for older Americans and those in high-risk groups.