Collective immunity, a dangerous mirage

Sometimes presented as a cause for hope at the start of the Covid-19 pandemic and a way to avoid generalized lockdowns, the idea of ​​letting the virus circulate to achieve collective immunity appears more and more clearly as a dangerous mirage , explain many scientists.

The idea is that once a certain proportion of the population has been affected, the epidemic would stop on its own, for lack of victims to strike. But after months of pandemic, “we are very far from the account”, notes to AFP Frédéric Altare, specialist in immunity at Inserm. The World Health Organization (WHO) has stated it bluntly: “Never, in the history of public health, has collective immunity been used as a strategy to respond to an epidemic, and even less to a pandemic. It is scientifically and ethically problematic “, declared its managing director, Tedros Adhanom Ghebreyesus. “Giving free rein to a dangerous virus, which we do not understand everything, is simply unethical. It is not an option,” he insisted, recalling that about 10% of the population could have been infected with the virus in most countries.

Billions of people therefore remain in theory at the mercy of a virus more dangerous, more lethal and more contagious than the seasonal flu. And for which there is no vaccine. As early as May, the WHO warned that countries relying on collective immunity were engaged in “a really dangerous calculation”. Regularly, and even very recently, US President Donald Trump has defended this idea. At the beginning of October, scientists defended it in a call – “The Great Barrington Declaration” – to let the virus circulate in young and healthy populations while protecting the most vulnerable. A call supported – according to the American press – by the White House. For its defenders, the main benefit of this “strategy” would be to avoid the economic, social, health damage, generalized confinements even though some countries, like Spain, had to reintroduce local containment measures, or like the France, curfew.

It is “a mistake”, answered Thursday 80 scientists in an open letter published by the medical journal The Lancet. “Uncontrolled transmission among the youngest would run significant risks in terms of health and mortality for the entire population”, they note, also noting the risk of saturation of health systems.

Sweden, which has refused to confine its population and close schools, bars and restaurants, now seems to be caught up in the pandemic. Compared to its population, Covid-19 mortality is among the 15 highest in the world, according to data from Johns Hopkins University. Another problem: it is not known how long the immunity against Covid lasts and cases of re-infections, although very rare, have been reported. “It is possible that the antibodies weaken over time,” said WHO official Maria Van Kerkhove last week. “Reinfections show us that we cannot rely on the immunity acquired through natural infection to achieve group immunity,” wrote Prof. Akiko Iwasaki, immunity specialist at Yale University (States -United).

Some supporters of natural herd immunity also argue that its threshold – usually estimated at around 60 to 70% – is in fact lower, in particular because not everyone is equally susceptible to catching the virus (between country people and city dwellers for example or young and old).

What we have also discovered over time is that some people are protected from Sars-CoV-2 even though they have not encountered it, explains Frédéric Altare (Inserm). Rather than antibodies (those sought with serological tests) specifically directed against this virus, they have developed another immunity, called “cellular”, thanks to a certain type of white blood cell. No doubt because they have already encountered infectious agents resembling Sars-CoV-2, these blood cells identify it as a danger and attack it, he says. “Which means that the figures that we announce (5 to 10% of possible immunity), it is probably a little underestimated but we do not know to what extent”, notes Frédéric Altare.

But even “pushing the statistics, taking into account (this) other immunity, the fact that people do not all get infected the same, that the elderly populations are less likely to become infected because they protect themselves more “, we manage” to bring the percentages down to 50%, not below, “he said. While noting that reaching this threshold would result in a considerable number of deaths. Collective immunity must therefore go through “safe and effective vaccines,” says Prof. Iwazaki.

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