Covid: why can people vaccinated even so catch the virus?

Published by Cécile D. · Published on 27 April 2021 at 11h54
Covid-19 vaccines are a crucial asset amid the fight against the pandemic, and so far, our best ally to make things return to normal as soon as possible. Yet, these vaccines are not magic: they do not totally prevent people immunized from falling sick and do not eradicate the virus either.

Catching Covid-19 after being vaccinated? It can be, and it does not mean vaccines are not working. Early April, a story disturbed medias and people who heard it: 97% of residents in a nursing home have caught coronavirus although they were all vaccinated. The interest of vaccines has been therefore called into question.

Doctors and medical experts then intended to reassure people: yes, vaccines are effective and necessary to fight against Covid-19. Yet, they have breaches you need to know about, to make sure you do not panic if you are sick after being vaccinated. Several health professionals explained to France Info why one can catch the virus even though they have been immunized.

Most vaccines available in France (excluding Johnson & Johnson) require two shots. Between the two doses, the body is not fully protected: one’s body is still vulnerable to the virus. “In the case of mRNA vaccines [Pfizer and Moderna], 12 to 15 days are necessary after the first injection to make the first antibody appear”, National Medicine Academy Covid-19 monitoring unit president Pr. Yves Buisson explains.

Furthermore, Paris Saint-Louis hospital infectious disease professor Anne-Claude Crémieux noticed some patients are less cautious with the virus once they have been given the first dose of the vaccine, increasing infection risks. “In many situations, post-vaccination contaminations occur within the 14 days following the first injection. These cases are often attributed to a sort of slackening caused by a false feeling of safety provided by the first step of vaccination”, she deciphers.

It should also be noticed – as Crémieux does – that novaccine is 100% effective. MRNA vaccines – which are the most effective today – bring after the second dose a 90% protection against symptomatic forms of the disease, and 80% against asymptomatic forms”. Furthermore, vaccines do not prevent from falling sick. They mostly enable to prevent someone vaccinated from developing severe infection. “Even after complete vaccination, there is still a low risk of developing symptomatic form or not of the disease”, the expert says.

Therefore, some people whose immune defense are weak may catch Covid-19 even though they are vaccinated. This is why the Direction Générale de la Santé recommended – this past April 11 – that immunocompromised people be given a third dose of mRNA vaccine.

Nîmes University Hospital infectious disease specialist Paul Loubet insists on the importance of vaccination, developed to save lives: “What we need to take into account is it protects from severe diseases and even virus-caused death. It’s not because it’s possible to fall sick you should not be vaccinated: on the collective basis, the vaccine enables to make hospitalizations decrease”.

Resisting variants

One of the weaknesses of these vaccines is variants. Some novel Covid-19 forms seem to be very resisting to vaccines, and even to antibodies some people infected have developed. For instance, it seems the South African variant can go through the barriers of the mRNA vaccines.

In France, this situation does not worry doctors yet. Buisson explains that “compared to the UK variant, sensitive to vaccine, variants resisting to vaccination the most have – so far – a very low incidence in terms of new cases in France”. The expert does not think variants resisting to vaccines can boom in France, making all our health endeavors useless.

But what concerns experts is how long vaccines are working for. So far, searchers struggle to assess how long people vaccinated or recovered remain immunized. “We lack of background as for the lifespan of vaccinal protection, because the first countries it applies to have started their campaign only this past December. The Pfizer laboratory recently claimed their vaccine was working for six months post injection, but immunity would be probably much longer”, infectious disease specialist Loubet explains. Adding that we know “Covid-19 natural infection gives protection for at least 8 to 9 months. We have no reason to think vaccination protects less longer”.

For this very reason – and among others – the health authorities consider a booster shot will be necessary, like for the flu vaccine.

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