The scientific community continues to search for a vaccine or treatment against Covid-19. The question of immunity is also regularly raised. And for good reason: if antibodies develop after a coronavirus infection, it is not yet clear whether they give us real immunity.
One question therefore remains unanswered: can we be affected twice by Covid-19? “There is currently no evidence that people who have recovered from Covid-19 and who have antibodies are immune to a second infection,” affirmed the World Health Organization (WHO) in April 2020.
Suspicions of reinfection had also been identified in China, Japan and South Korea. Why did these recovered patients test positive several weeks later? Specialists put forward several hypotheses: they were re-infected, their body still contained a small amount of virus that the tests failed to detect, the screening tests were faulty or poorly performed.
Since then, studies on the subject have multiplied and suggest that immunity is possible, even if it is limited in time. To find out, scientists from Harvard University (United States) carried out an experiment by inoculating Sars-CoV-2 into macaques. A month later, the monkeys were reinfected and thanks to the antibodies they had developed, they did not get sick.
Immunity limited in time?
A British study published in July 2020 in the scientific pre-publication journal MedRxiv has also looked into the matter. She suggests that the antibodies provide potential immunity to patients, but that it decreases or even disappears after a few months.
To find out, researchers at King’s College London (United Kingdom) looked at the immune response of 90 patients and healthcare professionals from the Guy’s and St Thomas’ NHS Foundation Trust who were affected by Covid-19. While 60% of the volunteers had developed strong antibodies in response to the infection, the researchers found that only 17% of the patients had retained that same level of antibodies three months later.
These results therefore suggest that the antibodies would have a limited lifespan and that patients already affected by Covid-19 could therefore be re-infected. An observation that once again calls into question the idea of a potential collective immunity.
Covid-19: even shorter immunity in asymptomatic patients
Another study published in June 2020 in the journal Nature Medicine and conducted by Chinese researchers already suggested that the immune response in patients infected with the virus decreased a few weeks after their recovery.
To find out, the researchers followed 37 asymptomatic patients tested by PCR, a sample of nasal cells using a swab. Aged 8 to 75, 22 of the participants were women and 15 were men. All had been hospitalized in Wanzhou province (China) and placed in isolation. Scientists from Chongqing Medical University noticed that the median duration of viral shedding was 19 days in them, compared to 14 days in symptomatic patients.
To study the immune response of these two groups, the researchers compared asymptomatic subjects with 37 people infected with Covid-19, and who had presented symptoms. Eight weeks after the onset of infection, they found that the level of antibodies had decreased by 81.1% in asymptomatic. As for the symptomatic, their immune response was reduced by 62.2%.
Covid-19: what immunity for patients with mild forms?
A French study had also been carried out on immunity, but in patients with mild forms of the coronavirus. Teams from the Institut Pasteur and the Strasbourg University Hospital analyzed the profiles of 160 people who had been struck by minor forms of the disease.
As part of the research, the scientists focused on the hospital services of two sites of the University Hospitals of Strasbourg. In these patients, the researchers had identified antibodies fifteen days after the onset of infection. “Antibodies were found in almost all of them: 159 out of 160. And, more interestingly, we were looking for neutralizing antibodies which we know are protective against, for example, reinfection. And there, from one month, it is found in 98% of people who had been infected with SARS-CoV-2. Results which are indeed good news “, had reported Professor Arnaud Fontanet, one of the leaders of the study, on France Inter.
Mild forms immunity could last, scientists say “from a few weeks to a few months”. After this period, the subjects will have to be tested again to see if the antibodies persist over time.. “These results are also good news for future vaccine strategies” , completed Pr Fafi-Kremer, first author of the study and head of the virology service of the University Hospitals of Strasbourg.