Science

Covid-19: three questions on post-vaccination contamination

Two people aged 53 and 68 died in Guyana from a severe form of Covid-19 “after having been vaccinated a few weeks previously with two doses of Pfizer”. This is what the Regional Health Agency (ARS) unveiled on Monday.

This double case could “slow down the vaccination campaign in Guyana”, feared AFP the director of the Institut Pasteur in Guyana, Dr Mirdad Kazanji, while vaccination is now open from the age of 18. But this situation is actually quite normal, as vaccination is not a miracle cure for the virus.

  • What is the profile of the deceased?

These two patients, two men according to a health source, died at the Cayenne hospital center. They had received their second injection of the Pfizer / BionTech vaccine at the end of February, said ARS Guyana.

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“These two people had a history of serious comorbidities and a very fragile state. One of them had an immune system disorder, which could cause a lesser response to vaccination,” reports the ARS.

In this French South American community considerably exposed to the Brazilian variant, the “samples are being sequenced to know the exact strain of the virus in question”, further indicates the ARS.

  • Why can vaccinated people still be infected?

It is not impossible to be infected with the coronavirus while having been vaccinated. A retired couple living in Marseille was thus contaminated by Covid-19 after being vaccinated with the formula developed by Pfizer, reported Provenceon April 21. If the husband was to receive the second injection in mid-April, his wife had received her two doses of the vaccine earlier this year. At the end of March, twelve residents of an Alsatian nursing home had also tested positive, while among them seven had received the double injection of the Pfizer / BioNtech vaccine.

But these contaminations mainly concern people who have received only one dose, and often occur less than fifteen days after the injection of the first dose. “After the first dose, the antibodies take two to three weeks to arrive. But this first protection remains insufficient for optimal protection. After the second dose, the second wave of antibodies arrives much faster. Fifteen days after this second, the people are well protected “, specifies West France Frédéric Altare, immunologist and research director at Inserm.

Vaccines are not 100% effective against the disease. According to clinical studies, that of Pfizer / BioNtech shows record efficiency (95%), ahead of another RNA messenger vaccine administered in two doses, Moderna (94.1%). Two studies published on March 23 in the New England Journal of Medicineshowed the effectiveness of Moderna and Pfizer vaccines, and that infections after vaccination are very rare. One of that work showed that only four of 8,121 employees at the University of Texas Southwestern Medical Center in Dallas tested positive for Covid-19 after being vaccinated with both doses.

Against severe forms, however, AstraZeneca’s vaccine appears to be more effective. According to a large-scale Scottish study dated late February involving 490,000 people, the AstraZeneca vaccine reduced the risk of hospitalizations by 94%, more than with Pfizer, which reduced the risk by 85%. “In most cases, vaccination drastically reduces the number of hospitalizations and serious cases”, underlines Mirdad Kazanji.

  • Why are frail people more at risk?

People in great fragility present an increased risk of contamination, despite vaccination. “Vaccine failure exists for all types of vaccination. It depends on the state of the immune system of the person vaccinated”, reacts Dr Mirdad Kazanji. For the practitioner, “vaccinated people can possibly have viruses which develop in the respiratory tract and, possibly, contaminate others” in the event of a “weakened” immune system and “major factors of comorbidities”.

The elderly are also more at risk. “Beyond 70-80 years, in some, even if they have been vaccinated, the immune response is weaker”, suggests Frédéric Altare to West France.

A few deaths have also been reported after an injection of the AstraZeneca vaccine. Five new cases of atypical thrombosis (clots) were recorded in France between April 9 and 15, while this vaccine has been reserved since March 19 for people aged 55 and over, because of this adverse effect. Since the start of vaccination, out of 3.2 million injections performed with AstraZeneca, the results are now 27 cases of thrombosis in France, including eight deaths. Very rare cases of thrombosis have also been observed in the United States after an injection of the Johnson & Johnson vaccine, already administered to 8 million people in this country. According to data presented on April 24, of 3.99 million women vaccinated, fifteen developed severe cases of blood clots, and three died.


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