Covid-19: Why France needs to vaccinate 90 percent of adults to return to ‘normal’ life

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A new study published this week by France's Pasteur Institute concludes that since the British variant of Covid-19 is now dominant in France, a full 90 percent of adults will need to be vaccinated before the country can get back to normal life without risking a fresh surge of the virus.

According to new modelling released Tuesday on the Pasteur Institute's website, the British variant's proliferation in France has put a clear damper on the prospect of getting back to life as we knew it pre-pandemic this autumn, the horizon some scientists had put forward only months ago.

The study's authors conclude that since the British variant, known as B.1.1.7, is now dominant over other coronavirus strains in France, 90 percent of the country's adult population would need to be vaccinated by summer's end before French residents can dispense with social distancing measures without causing a new spike in cases.

The impact of the British variant

That high level of inoculation among adults is a very tall order in notoriously vaccine-sceptic France, where recurrent delivery delays have further plagued the country's initially sluggish vaccination rollout. Specialists tell FRANCE 24 that the 90-percent goal would be virtually impossible to meet.

"The main message of our work is that vaccinations will allow us to come out of the crisis, but we have to expect to live with certain constraints in the autumn," Pascal Crépey, co-author of the study and a researcher in epidemiology and biostatistics at France's EHESP School of Public Health in Rennes, told FRANCE 24.

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So how did the researchers arrive at the 90-percent figure when, until recently, the generally accepted aim was a 60- to 70-percent rate of protection would offer the much-discussed "herd immunity" levels necessary to stifle Covid-19?

The answer lies in how thoroughly B.1.1.7 has upped the stakes. Specifically, the issue is its impact on what is known as the R0 (pronounced R-nought or R-zero), the reproduction rate that determines how transmissible a virus is in terms of how many people a single infected individual will infect on average. If the R0 is less than 1, each infected person infects at most one other individual and infection rates should eventually fall.

"The R0 of the majority strain circulating in France has changed. It was around 3 for the original novel coronavirus and it is estimated at between 4 and 5 for the 'British' variant," Amaury Lambert, a mathematics professor at Paris's Sorbonne University, told FRANCE 24.

The leap in the R0 was calculated "taking into account the scientific estimates according to which the British variant propagates 60 percent more easily", Crépey said.

Vaccinating children helps everyone

On that basis, the increased R0 "automatically increases the vaccination threshold needed to envisage a return to normal life without risking a new epidemic surge", Lambert said. Hence the 90-percent figure given for inoculating individuals over the age of 18 given the dominance of the B.1.1.7 strain in France.

The researchers recognise, however, that the necessary threshold would not be as high if children were to be vaccinated, too. Indeed, "if only adults are vaccinated, a considerable epidemic is nevertheless expected in children, contributing to the infection of unprotected parents and grandparents", the Pasteur study's authors note.

By including candidates under the age of 18, "the vaccination of 60 to 69 percent of 0 to 64-year-olds and 90 percent of those over 65 could allow for the complete slackening of control measures" as early as September, the authors suggest.

However, that discussion remains purely theoretical since "no vaccine has received market authorisation for children in France", noted Crépey. In fact, the results of clinical tests that could show the Moderna, BioNTech/Pfizer and AstraZeneca vaccines are as safe and effective for minors as they are for adults have yet to be published. "It is clear that if we knew the effectiveness of these vaccines in children that this would allow the vaccination strategy to be adapted," epidemic modelling specialist Jean-Stéphane Dhersin, scientific deputy director of France's National Institute of Mathematical Sciences, told FRANCE 24.

Another parameter that could influence the vaccination campaign's priorities is the level of protection the injections provide. "At the start, we knew that the vaccines strongly reduced the number of severe Covid-19 cases, but we lacked the data to know whether they were also effective in limiting transmission (infectiousness) or to prevent the infection of the person vaccinated (susceptibility). That's why it was decided that the priority would be to vaccinate fragile and older populations to limit the number of deaths and hospitalisations," Crépey explained.

Scientists have since been able to scrutinise data from Covid-19 vaccination campaigns rolled out around the world that suggest that available vaccines do offer good protection against the risk of infection. The model retained by the Pasteur study indicates that in the case of a vaccine that is effective both for preventing serious cases and protecting against infection, "the order of priority of the populations to be vaccinated becomes less important", Crépey pointed out.

In other words, expanding vaccination to the entire adult population confers the same advantages – limiting the number of deaths and hospitalisations – as does putting the priority on vaccinating the highest-risk populations.

"Indeed, the vaccination of younger people, who have a lower risk of developing severe forms of the disease but who play an important role in transmission, reduces the circulation of the virus and therefore indirectly protects the most fragile," the study's authors write.

"If it is confirmed that the vaccines have a real impact on the susceptibility to infection, we could think about expanding vaccination to younger people sooner," Crépey said. That would make quicker work of reaching the 90-percent population goal.

But even if that threshold is not reached by autumn, "we will still reap the benefits of the vaccination campaign, since every person who is vaccinated reduces the speed at which the virus propagates", explained Dhersin. The authorities should, in the mathematician's opinion, slowly but surely lift Covid-19 constraints or adapt them.

"One could imagine a reopening of cultural venues where the public would be obliged to wear a mask, for example," Dhersin offered.

For residents of France, it might just be a matter of redefining the concept of "normal life".

This article has been translated from the original in French.