Immunity is not eternal, the protection conferred by Covid vaccines decreases over time. The data analyzed by Israeli scientists allow us to learn more about the effect of the decrease in immunity on the parameters of the epidemic.
As early as December 2020, Israel launched a major vaccination campaign against Covid-19. In three months, half of the eligible adults received both doses of the Pfizer vaccine. As a result, the number of cases drastically decreased throughout the country, until mid-June 2021 when it soared again. This resurgence of infections is part of a particular context marked by the arrival of the Delta variant, against which vaccines are losing some of their effectiveness, in the country. As of June 14, it accounted for 68 percent of infections in Israel, then 98 percent just a fortnight later.
But the Delta variant alone does not explain the more than 8,000 confirmed infections in the country at the end of August against only 40 in May of the same year. The other parameter to consider is the gradual disappearance of vaccine immunity. This issue is still subject to debate. Previous work has concluded that vaccine immunity could last for several years or even decades. However, data analyzed by Israeli scientists in a publication in The New England Journal of Medicine suggests that vaccine immunity gradually vanishes within a few months.
Vaccination as the test of time
This conclusion is drawn from the analysis of data from Israelis over the age of 16 and having a complete vaccination schedule before June 2021. The incidence of positive cases and severe forms of Covid-19 among vaccinees between 11 and 31 July are compared to those of past months (February, March, and April-May). The goal is to track the evolution of the incidence over the months. Here are the results for the age group of 60 years and older, the first to have been vaccinated in early 2021.
During the study period (July), the incidence rate is 3.3 confirmed cases per 1,000 people fully vaccinated in January, 2.2 confirmed cases per 1,000 people vaccinated in February, and 1.7 confirmed cases per 1,000 people who received both doses of vaccine in March. In other words, the virus circulates more easily in people who have been vaccinated for six months.
The same reasoning was made for severe forms of the disease. Scientists listed 0.34 severe forms of the disease per 1,000 people vaccinated in January; 0.26 severe forms per 1,000 people vaccinated in February; 0.15 per 1,000 people vaccinated in March. Again, severe forms are more common in people who have been vaccinated for six months.
An essential booster dose?
The biological mechanism behind these observations could be the gradual disappearance of neutralizing antibodies from vaccination. Nevertheless, the vaccine confers significant protection – even after six months – compared to the unvaccinated, and this study provides additional arguments in favor of a third dose. Israel has taken the plunge since the end of the summer and more than 40% of the population has already received a booster dose.
No comments:
Post a Comment