Health

Covid-19: Why we can expect vaccinated people to be infected anyway

Covid-19: Why we can expect vaccinated people to be infected anyway

While vaccination is presented by health authorities around the world as the main hope for a return to normality after more than a year of pandemic, it is not a miracle cure for Sars-CoV-2.


At the height of the Covid-19 vaccination campaign, the case had attracted the attention of the national media. A cluster of coronavirus cases was reported at the beginning of April in an Ehpad of Biscarosse, in the Landes, where 97% of the residents had been vaccinated. At a time when the use of vaccines is presented by health authorities around the world as the main hope for a return to normality after more than a year of pandemic, the appearance of cases of Covid-19 in a vaccinated population is cause for concern. But this is actually normal and should not make you hesitate to get vaccinated.

Because it takes time for vaccines to be fully effective

The first thing to clarify is that a patient can still get the disease until he or she has completed the vaccination process. With the exception of the Johnson & Johnson vaccine, which requires only one dose and injections only began on Saturday, all Covid-19 vaccines licensed in France need to be given in two doses to be fully effective.

“In the case of messenger RNA vaccines [such as those from Pfizer-BioNTech and Moderna], it takes 12 to 15 days after the first injection for the first antibodies to appear. Before that, it is quite possible to be infected,” warns Professor Yves Buisson, president of the Covid-19 watchdog unit at the French National Academy of Medicine. Above all, the vaccines are fully effective two weeks after the injection of the second dose.

“In many situations, post-vaccination infections occur within 14 days of the first injection. These cases are often attributed to a form of relaxation linked to a false sense of security provided by the first stage of vaccination,” notes Anne-Claude Crémieux, professor of infectious diseases at the Saint-Louis Hospital in Paris. A study published at the end of March in the New England Journal of Medicine shows that 379 cases of Covid-19 were identified among the 36,659 Californian caregivers who received at least one dose of vaccine. In 71% of the cases, these infections occurred in the first two weeks following the first injection.

Because vaccines mainly prevent the development of a severe form of the disease

Even if an overwhelming majority of the population were vaccinated tomorrow, the positive PCR tests would probably not immediately disappear like magic. No vaccine is 100% effective,” says Anne-Claude Crémieux. Messenger RNA vaccines, which are the most effective today, provide after the second dose a 90% protection against symptomatic forms of the disease, and 80% against asymptomatic forms.”

Even after a complete vaccination, there is still a small risk of developing a symptomatic or non-symptomatic form of the disease.
Anne-Claude Crémieux, infectiologist at franceinfo

Among the profiles most at risk of contracting the disease once vaccinated are the elderly and so-called immunocompromised patients, whose immune defenses may be weakened by the use of dialysis, who have just undergone a transplant or a chemotherapy-based treatment, lists the specialist. It is in this sense that the Directorate General of Health recommended on April 11 (PDF) the injection of a third dose of messenger RNA vaccine for immunocompromised people.

In any case, the interest of vaccination does not lie in the guarantee of never contracting a mild form of Covid-19. “What you have to consider is that it protects against severe forms or death from the virus. It is not because it is possible to contract the disease that one should not be vaccinated: on a collective scale, the vaccine makes it possible to reduce the number of hospitalizations”, agrees Paul Loubet, infectiologist at the CHU of Nîmes (Gard).

To demonstrate the effectiveness of vaccination, Didier Couteaud, the departmental director of the ARS of New Aquitaine in the Landes, notes that at the beginning of the year, when the campaign began, 300 cases of contamination had been noted in about twenty Ehpad for a total of 25 deaths. In March, once almost all the residents had been vaccinated, and apart from the case of the Ehpad de Biscarosse, we counted 13 cases in seven establishments,” he explains. Among the contaminated people, there were no serious cases and no deaths.”

Because some variants of Sars-CoV-2 appear to be more resistant to vaccines

The effectiveness of vaccination could also be undermined by the rise of more resistant Covid-19 variants. “We know today that even if they remain very effective against it, the messenger RNA vaccines have seen their neutralizing capacity diminished by the so-called South African variant,” confirms Anne-Claude Crémieux.

In Israel, where the population has been widely vaccinated, a pre-publication study compared the situation of a group of 400 Covid-positive people who received one or two doses of vaccine with that of 400 people who were also positive but not vaccinated. The prevalence rate of the South African variant was eight times higher among those who received two doses of vaccine than among the non-vaccinated (5.4% vs. 0.7%).

“Compared to the British variant, which is sensitive to the vaccine, the more vaccine-resistant variants have a very low incidence of new cases in France for the time being,” reassures Yves Buisson. For the president of the Covid-19 watchdog unit at the French National Academy of Medicine, the scenario of a vaccination campaign rendered futile by an uncontrollable number of contaminations by a vaccine-resistant variant “is for the moment a fantasy”.

Because the duration of the protection conferred by vaccines is still poorly known

Finally, it is possible that people who have received two doses can contract Covid-19 once the immunity induced by the vaccine has worn off. Unfortunately, due to a lack of experience, there is little certainty on this subject. We don’t know how long the vaccine will last, because the first countries involved only started their campaigns last December,” admits Dr Paul Loubet. The Pfizer laboratory recently communicated that its vaccine was very effective six months after the injection, but immunity will most likely last longer.”

We know that natural infection with Covid-19 confers protection for at least 8 to 9 months. We have no reason to believe that vaccination protects for less time.
Paul Loubet, infectiologist to franceinfo

On the duration of this protection will depend the need to make a possible recall, like what the French know with the flu vaccine. “We still do not know what is the precise threshold of residual neutralizing antibodies necessary to ensure good immunity, especially against variants. Once this is known, we will be able to determine when and with which vaccine to do a booster”, concludes Anne-Claude Crémieux. Until we can hope for a total extinction of the virus.