Some individuals vaccinated against coronavirus can have a much lower level of antibodies than other inoculated individuals and are therefore more at risk to get infected, new groundbreaking research by the Sheba Medical Center in Tel Hashomer published in the New England Journal of Medicine on Wednesday has shown.

The finding might represent a key to understand who should receive a third vaccine, Dr. Gili Regev-Yochay, director of Infectious Disease Epidemiology Unit at Sheba and the lead author of the study, said.

In addition, the antibody level is directly related to the viral load in infected vaccinated individuals and to the risk that they infect others.

Sheba documented 39 breakthrough cases among about 11,000 of its workers who had been fully vaccinated – meaning they were at least ten days after the second shot of Pfizer – in the previous three months.

As explained by Regev-Yochay, the average age of the infected was 42.5. About one third of the cases were completely asymptomatic, 10% suffered from very mild symptoms, 21% experienced fever and another 19.4% presented so called “long-Covid 19 symptoms” – from the loss of taste and smell to exhaustion – for over six weeks.

In addition, almost 5,000 health workers routinely underwent serological tests.

The researchers were able to compare the antibody level of those who got infected – as measured right before the virus was detected – with that of other staff members featuring similar characteristics (sex, age, general health, etc.) who did not get infected.

“This was the most important part of the study,” Regev-Yochay said in a press briefing. “What we saw is that the people who got infected had three times lower neutralizing antibodies than those who did not get infected. If we considered the peak of antibodies recorded after the inoculation, the level of those infected was seven times lower than of those who didn’t.

The study marks the first time that such a correlation is proven with data from real patients.

“This is the reason why a study on a relatively small group of individuals has been published in such a prestigious journal,” Regev-Yochay noted.

In addition, the study also documented a connection between level of antibodies and viral load – amount of virus particles in the body – which is directly linked to the level of infectivity of a virus carrier.

“People who had a higher level of neutralizing antibodies also presented a lower viral load, which means they were more unlikely to infect other people,” the doctor remarked. “This also shows that vaccinated people are less likely to infect other people.”

About 85% of the cases considered in the study were infected with the Alpha variant, also known as British, as the research was carried out in a period when the currently dominant Delta variant was not present in Israel yet.

However, Regev-Yochay said that the correlation between antibodies and level of protection is still relevant.

While the research does not shed light on how the efficacy of the vaccine wanes as time passes, the connection between the level of antibodies and infection can help determine who is more in need of a third coronavirus vaccine, a move that the Israeli authorities are strongly considering for the elderly.

“I think that this data on the correlation between antibodies and infection is important to understand who is the population at risk and from that maybe start thinking about who should receive a third dose and when,” Regev-Yochay concluded.