A military nurse prepares to administers the coronavirus vaccine in Tel Aviv, January 18, 2021.

Over 10,000 Israelis test positive within two weeks of receiving the first dose, while antibody tests on health workers show 'the vaccine works wonderfully' after the second dose

Over 12,400 Israeli residents have tested positive for COVID-19 after being vaccinated, among them 69 people who had already gotten the second dose, which began to be administered early last week, the Health Ministry reported.

This amounts to 6.6 percent of the 189,000 vaccinated people who took coronavirus tests after being vaccinated.

According to the ministry’s data, 5,348 people were found infected up to a week after getting the vaccine, of the 100,000 people who were vaccinated and then tested a week later – an infection rate of 5.4 percent of those vaccinated during that time.

Another 5,585 people tested positive for the virus between the eighth and 14th day after getting the first vaccine – 8.3 percent of the 67,000 people who were vaccinated and tested during that post-vaccination period.

People were also testing positive more than two weeks after getting the first dose, between the 15th and 21st day, a period during which partial immunity is already meant to be in effect. Of the 20,000 people tested during this post-vaccine period, 1,410 people tested positive, some 7.2 percent. Of the 3,199 people who took coronavirus tests between day 22 and day 28 after the first vaccine, 84 were found to be positive (2.6 percent), including 69 people who had already been vaccinated twice.

Some 2.15 million people have been vaccinated in Israel over the past month, of whom 300,000 have already gotten a second dose.

At this stage it is still difficult to draw definitive conclusions about the effectiveness of the vaccine, for better or worse. In some groups of inoculated people during the different post-vaccine periods, the ratio of positive tests corresponds to the ratio of positive tests in the general population, the overwhelming majority of whom were not yet vaccinated. There could also be issues related to the fact that most of those vaccinated first and who have thus accumulated the most post-vaccination time are over 60 years old.

Meanwhile, serological tests done on employees of Sheba Medical Center at Tel Hashomer a week after they had received the second dose of the Pfizer vaccine showed that of 102 employees tested, 100 had antibody levels between 6 to 20 times higher than they had presented a week earlier.

Sheba staffers are very encouraged by these results, and according to Prof. Gili Regev-Yohai, director of the hospital’s infection prevention and control unit, “It’s exciting. It means that the vaccine works wonderfully and that we are hoping to see a drop in the incidence of infection in a few days.”

The infection statistics among those who have been vaccinated in the general population illustrate the significance of the passage of time in building immunity. According to Pfizer, the big jump in immunity is meant to come between day 15 and day 21, when the vaccine’s effectiveness is meant to go from 52 percent to 89 percent, after which the second dose is meant to bring the vaccinated percent to a state of 95 percent protection.

But these statistics, encouraging as they are, cannot determine the level of protection the vaccine grants each individual person. Moreover, the more people are vaccinated, the more likely gaps will be found between Pfizer’s results in its clinical trials and results in the field, not just with regard to the level of the vaccine’s effectiveness for individuals, but regarding its general effectiveness and ability to provide “herd immunity.” This is especially true since it still isn’t clear to what degree the vaccine protects against mutations of the virus.

Israelis aged 60 and older are at this point the vast majority of those “veteran” vaccine recipients, who have gotten the shot two or more weeks ago. To date, 86 percent of those aged 70 to 79 have been vaccinated, along with 80 percent of those 80 or older and 68 percent of those aged 60 to 69. Pfizer had reported that the vaccine had been highly effective among those 65 or older, providing 95 percent protection among the 7,000 participants in that age group in its trials. That statistic will have to prove itself in real life, since one’s immune system erodes over time and becomes less powerful compared to that of a younger person.

One can see this in the flu shot, for example, which is less than 60 percent effective among those 65 and over, while being 80 to 90 percent effective among younger people, even when the vaccine component is tailored to the strains of flu most common during a given season.

But age isn’t the only factor that plays a role. The erosion of one’s immune system can be affected by heredity, lifestyle, environmental exposure and more. That’s why over the years there have been various indices developed to measure “immune age” separately.

“The difference in immune reactions between people is enormous,” says Prof. Yoram Reiter, an expert on molecular immunology at the Technion – Israel Institute of Technology. “In most cases we administer vaccines but we cannot predict the strength of the response or the level of protection, either at the antibody level or at the level of cellular protection.”

The most prominent difference that is evident to date is the response and side effects experienced by those vaccinated who are aged 55 and under compared to those who are older. The Pfizer trials found the incidence of side effects to be different for different age groups.

“This isn’t unique to the coronavirus vaccine,” says Reiter, noting that the body’s “activation mechanism,” or the way the immune system responds a few weeks after a vaccine, is different for younger people than for older people. “With older people it is generally on a lower level in the first place, which is why the exposure to the second dose, which mimics the virus, generates a less powerful response. That’s why we see fewer reactions or side effects that express the immune system’s active response.”

However, he stressed, the lack of reactions or side effects does not indicate that the vaccine is less effective or that one’s immune system is deficient.

According to Prof. Zvika Granot, an immunology expert at the Hebrew University Medical School, “The immune system of an older person is less efficient and that is apparently the reason for the differences in reactions. A 70-year-old will react differently to a virus than a 20-year-old. The response to the second dose, in contrast to the first, is already the body’s response to the virus, or in this case the protein that the cell produces that mimics the virus.”

He added, “In general, an acquired immune mechanism is less flexible among older people and in general less responsive. This means a slower and less powerful reaction. That doesn’t mean that the vaccine is less effective. But it certainly explains why among older people a bombastic immune system reaction with side effects is less common.”