“Hello, how do you explain a negative serological test despite a strong Covid-19 infection and a positive PCR test? The serological test and the PCR test were carried out in the same laboratory by my husband and myself”, asks Jocelyne. While 182,006 new cases were recorded on Tuesday July 12, can serology help to know if we still have antibodies? Can it turn out to be negative despite an infection in the past? West France answers you.
Serological tests have developed since the first wave of the pandemic, in March 2020. They aim to identify antibody production once morest the SARS-CoV-2 virus by a patient. It is true that a negative serology means, in the vast majority of cases, that the patient has not encountered the Covid-19 virus. But several things can explain a negative serological test despite a Covid-19 infection in the past.
What is a serological test?
There are two types of serological tests: rapid diagnostic orientation tests (TROD) and quantitative serological tests. For the first, a drop of blood is taken from the fingertip to indicate whether the patient has already been in contact with the virus, without obtaining the antibody level.
For the second, you have to go to a medical biology laboratory in order to carry out a blood sample. The test can give a level of antibodies, but does not indicate if the patient is immune to infection.
Read also: Covid-19. Is it necessary to do a serology before being vaccinated? Is it really useful?
Why can a serology be negative?
One of the explanations may be that the subject, who had a Covid-19 infection, took the serological test too early. In this case, it is possible that the antibodies have not yet appeared. This is why a serological test should not be done when the patient has symptoms. It should preferably be carried out at least fifteen days following the onset of symptoms, the time necessary for the antibodies to develop.
Another possibility is that the antibodies have already disappeared. It can happen at the most vulnerable people, especially the elderly. In this case, the patient no longer has an immune response and is therefore no longer protected.
It is also not relevant to do a serology following a vaccination in order to be reassured regarding the level of antibodies present in the body. The High Authority for Health (HAS) justifies it as follows: “ The efficacy of vaccines and their very good protection once morest severe forms of the disease have been proven by data from clinical trials and those obtained in real life. Here once more, the antibodies take between ten and fifteen days to appear.
A positive serology is not always synonymous with immunity
Serology results should be interpreted with caution. Current scientific knowledge does not make it possible to determine the duration of protection of antibodies. A positive serological test is therefore not necessarily synonymous with lasting and systematic immunity.
“There are no data yet to define correlates of protection, i.e. the existence of a level of protection in relation to a measured antibody level”recalled the High Authority for Health (HAS) in an opinion issued on June 23, 2021.
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