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Covid-19. Is it necessary to do a serology before being vaccinated? Is it really useful?

“Why aren’t we doing a serological test to measure the level of antibodies before any new vaccine? This is the BA-BA in medicine…” Pierre, who asks us this question, seems not to understand this choice.

Serological tests have been developed since the first wave of the pandemic, in March 2020. Their objective is to identify the production of antibodies against the SARS-CoV-2 virus by a patient. So, what is a serological test used for? Why is it not systematized? Is it really useful before getting vaccinated? Ouest-France answers you.

What are the existing serological tests?

The tests offered during vaccination are “rapid diagnostic orientation tests” (TROD). Thanks to a drop of blood taken from the tip of the finger, they indicate whether the patient has already been in contact with the virus, but not the level of antibodies.

If the rapid diagnostic orientation test (TROD) is positive, “the patient’s primary vaccination schedule will be considered complete after the first injection”.

There are also quantitative serological tests which are carried out in a medical biology laboratory by blood sampling. They make it possible to give a level of antibodies, but do not make it possible to know if the patient is immune against the infection.

Read also: Covid-19. Symptoms, contagiousness, dangerousness… Are we sicker with the BA.5 sub-variant?

Who is affected by serological tests?

These TRODs are only offered during a primary vaccination, in “a logic of saving doses” and to collect “epidemiological data”, writes the High Authority for Health (HAS). To qualify, you must be between the ages of 5 and 55. Persons with proof of past infection with Covid-19 (positive PCR, antigen or serology test result dating back more than 2 months) are not concerned. Similarly, the vaccine strategy for immunocompromised people is not based on serological tests.

For people over 55, it is possible to perform a serological test “at their request and according to the assessment of a doctor in a vaccination center”says the Ministry of Health.

Serological tests were particularly recommended when the vaccination was introduced for 5-11 year olds. “The use of TRODs will identify children previously infected with SARS-CoV-2 so that they receive only a single dose of vaccine”, notes the Ministry of Health. However, TRODs are not mandatory and are subject to parental consent.

In summary, if serological tests before vaccination are not mandatory, they may be recommended depending on their age and their vaccination course. In a press release dated March 2022, the Ministry of Health indicated that it wanted to integrate serological tests into this course. Serological tests are offered “in a vaccination center, in an outpatient vaccination relay or at a city professional authorized to prescribe vaccination”.

The limits of serology

Serology still has limits, reminds us of the High Authority for Health. “There are no data yet to define correlates of protection, that is to say the existence of a level of protection in relation to a measured antibody level. Furthermore, the results of the serological tests do not make it possible to decide on the protection conferred, whether on the level of protection or on its duration over time. »

A positive serological test is therefore not necessarily an argument for not receiving a booster dose of the vaccine. Questioned by our colleagues from Marianne Stéphane Paul, immunologist at Saint-Etienne University Hospital and member of the Covid-19 vaccine committee, explains that “When we have defined this antibody threshold for protection, the serological screening strategy will make sense. »

Finally, it is not dangerous to receive a booster when you already have antibodies. “There is no risk of having too many antibodies”says the doctor.

Covid-19. Is it necessary to do a serology before being vaccinated? Is it really useful?



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