Are the announcements around a “decrease in vaccine effectiveness” really worrying? – Release

A slight decrease in the effectiveness of vaccines has been observed over the months in the general population, but protection is still high. For the time being, the recommendation for a booster dose only concerns a certain category of people.

Question asked on October 4, 2021

According to Israeli data released at the end of July, people vaccinated last winter were much more likely to develop severe Covid than those vaccinated in the spring. Some commentators believed they could deduce a drastic drop in vaccine efficacy in just a few months, from more than 90% to almost 50%. Several newspapers quickly alerted to the fact that the comparison did not, in reality, make much sense: the first-time vaccinated were among them the people most at risk, either by their age (immunity is less valiant in the most elderly), or by their level of exposure to the virus (nursing staff in particular). The real question was therefore: in the same category of population, vaccinated at the same time and exposed to the same risks, does the vaccine efficacy decrease significantly over the months?

As we recalled in a previous article, the long-term follow-up of participants in clinical trials of vaccines reveals a slight decline in vaccine protection, with a few additional severe cases from one month to the next. But this decline is much less pronounced than that allegedly depicted by the Israeli data: for the follow-up of the volunteers of the Pfizer trial, which was 96% effective during the first two months post-vaccination, both were reduced to 90%. following months, then 84% at six months. Data from the field in Qatar lead to an estimate of an efficiency rate of around 90% at six months.

Recontextualize the wealth of experimental data

In these studies, the appearance of alpha and beta variants does not seem to have changed the situation much – with a more marginal decrease in efficacy than initially expected. It could be otherwise with the delta variant. UK data released Thursday in Nature (pre-published in August) suggest a decrease in the efficacy of the vaccine of the order of 10 to 13% for that of Pfizer and of 16% for that of AstraZeneca (compared to the efficacy against alpha).

These field observations are essential to recontextualize the numerous experimental data on the evolution of immunity conferred by vaccination. Admittedly, the level of antibodies produced in the presence of the vaccine antigen decreases by a factor of ten in six months. But these antibodies are not the only weapon in the immune arsenal, and cells that confer immune memory (and the ability to react quickly in the event of new infections) develop in vaccinees.

Beyond the question of vaccine effectiveness (which concerns the prevention of severe forms of Covid-19), several studies have also tried to quantify “in population” the evolution of prevention against the risk of infections, symptomatic and asymptomatic.

A general observation is drawn up: the proportion of positive PCR tests in the vaccinated increases over the months. But is this growth attributable, or only attributable, to a decline in immunity in the vaccinated?

“More social contacts”

Here again, the debate seems far from over. Thus, the authors of the Qatari study observed a drastic drop in protection against infection, six months after the second dose (which would only be around 20%), but pointed out that one of the causes of the problem could be to look for on the side of“Less adherence to sanitary measures” and “More social contacts” in vaccinated people. A hypothesis also raised by other observers to explain the increase in the rate of positive tests in the United Kingdom among the oldest vaccinated people (an increase which is far from being of the same intensity as that suggested by the Qatari data). The greater virulence of the delta variant is also advanced as an explanatory factor in a study carried out in the United States.

In fact, when circulating antibodies fail to stem the infection, the body of the vaccinee remains better armed against the virus. People infected despite vaccination thus tend to develop milder forms of Covid-19 and eliminate the virus more quickly, as illustrated by a study published in early September in The Lancet.

At the end of August, the High Authority for Health (HAS) recommended a third dose of vaccine for those over 65 and those at risk, “After a period of at least six months following the complete primary vaccination”, a decline in efficacy being more marked in these populations. For the time being, the available data do not seem to justify extending this recommendation to other categories of the population.

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