Antibody levels decrease rapidly after two doses of the Pfizer coronavirus vaccine, a study by researchers at the Sheba Medical Center published Wednesday in the New England Journal of Medicine showed.
The research also showed the probability that different groups of individuals – based on age and general health status – will find themselves below a certain antibody threshold after a period of six months.
The hope is that these findings will help identify the levels associated with different clinical outcomes, for example, offering good protection against serious symptoms, Prof. Gili Regev-Yochay, one of the authors of the paper, said.
Over 4,800 staff members of Sheba participated in the study. They were invited to undergo periodical serological tests that measure the level of antibodies in the blood for a period of six months after receiving the two doses of the Pfizer vaccine with an interval of three weeks.
All participants underwent between one and seven tests.
“We saw that the decline in antibody level is very rapid,” Regev-Yochay said at a press briefing.
Although women and younger people tend to start at a higher level of antibodies, the decline is similar regardless of gender and age.
“One of our goals was to understand how likely different groups of individuals are to find themselves below a certain threshold after a period of time,” Regev-Yochay said.
According to the study, after six months, a healthy woman between the ages of 18 and 45 has a 2.5% probability of having an antibody level below 16 – where antibodies are generally considered absent – and 68% to have one below 256.
The starting point is on average several hundred.
The percentages of the probability of having an antibody level under 16 and under 256 become 5% and 79%, respectively, for women ages 45-65, and 6% and 81% for a woman older than 65.
For healthy men, the numbers are moderately higher in the first age group (respectively 4% and 75%) and the gap with women widens as they are older: A healthy man between 45 and 65 has an 11% probability of having an antibody level below 16, and 89% probability of having one below 256. For men over 65, the percentages are 15% and 92%.
When it comes to immunocompromised people, all age groups, men and women, have between 93% and 99% probability of having an antibody level under 256 after six months, and one in two men over 65 has an antibody level below 16.
However, obese individuals, surprisingly, who are known to be at risk of a more severe form of disease, appeared to have a less significant decrease in antibody level than healthy individuals. Whether this means that they are also better-protected remains to be seen, Regev-Yochay noted.
“Eventually, I hope we will understand the correlation between the antibody level and the degree of disease,” she said. “We know that there is some form of correlation, but we are looking for thresholds. For example, we would like to be able to know that people whose level is under 256 might not be protected anymore from getting infected but are protected from severe disease, or those who are under 32 might get ill enough to be hospitalized in intensive care, but not to die, and so on and so forth.”
In the future, these evaluations might help understand who needs further shots and when.
Asked about whether the rapid decline in antibody level is also going to occur after the booster, the professor said the study does not offer answers.
“We can see that after the booster the starting antibody level is much higher than after two shots, which is an encouraging sign,” Regev-Yochay remarked.
The expert expressed support for Israel’s decision to give a booster to all the population in light of the epidemiological situation it found itself.
In addition, she said that other studies, especially conducted in the United Kingdom, appear to suggest that a longer interval between the first two doses might offer longer protection.