Positive PCR test after being vaccinated, is it possible?

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The case of people who obtained a positive result in a PCR test within days of being vaccinated against Covid-19 has been around on social networks. Few find a very simple explanation.

Is it possible to get vaccinated and then test positive for Covid-19 via RT-PCR test a few days later? Yes, as evidenced by the very rare cases identified in recent weeks. Some see it as proof that available vaccines are not effective, but the reality is quite different, as Inserm explains, thanks to data from vaccine clinical trials and knowledge gained so far. The organism indicates that yes, it can become infected with SARS-CoV-2 in the days after vaccination, if it forgets to take into account two elements: the incubation time of the virus and the time necessary to develop protection.

Consider the incubation time

In fact, the average incubation time is 4 to 5 days, which can go up to 14 days. "In May 2020, a study showed that more than 90% of people who develop symptoms do so within 11.5 days of being exposed to the virus," Inserm says. Therefore, it is theoretically possible for a person to be vaccinated relatively soon after being infected with the virus without knowing it, during the incubation period. In this case, it is possible that you have no clinical signs of the disease and that you do not test positive until after you have received your dose of vaccine: this happened, for example, to the French Minister of Culture Roselyne Bachelot.

A crucial time lag between vaccination and protection

Although it should be noted that, while vaccines cannot produce a positive PCR, "they activate the immune system and produce antibodies against the virus." Therefore, a serological test can be positive after vaccination, "Inserm notes.

Regarding the protection provided by vaccines, those currently on the market are characterized by very high efficacy. For the two mRNA vaccines, Inserm notes that phase 3 clinical trial data available in November 2020 showed an efficacy of 94% in protecting people from symptomatic infections for the Moderna vaccine and 95% for the Pfizer vaccine.

This effectiveness is obtained 7 days after the second injection of the Pfizer-BioNTech vaccine and 14 days after the second injection of the Moderna vaccine in subjects without prior SARS CoV-2 infection. For the Janssen vaccine, protection begins 14 days after the single injection: therefore, it is possible to contract Covid-19 within this time period.

The great effectiveness of these vaccines in preventing serious forms that require hospital treatment should also be underlined: in phase 3 clinical trials of the Pfizer, Moderna or AstraZeneca vaccines, it was rare for a patient to develop severe Covid-19 after both doses .

"Real-life studies in countries where vaccination has been massive (Israel, UK) show that vaccination can also protect against asymptomatic infections, with varying effectiveness between studies but generally between 80 and 95%. These data are very encouraging. to hope to end the epidemic. ", adds Inserm.

Finally, it should be noted that no vaccine ever reaches 100% efficacy and that, at present, hindsight is still insufficient to know how long the protection conferred by these vaccines lasts. Therefore, studies are continuing to determine if an additional dose will be necessary to prevent infection in the vaccinated in the longer term, or even if annual vaccination should be considered.

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