People Who Have Had Mild Covid-19 Appear To Have Some Immunity

Even people who experienced mild symptoms of Covid-19 could still have acquired immunity for several weeks or more, according to a new study in French healthcare workers. The news is promising as it suggests mild Covid-19 cases might hold some protection against re-infection, but it’s worth remembering that immunity can be a very fiddly and fickle thing, especially when it comes to this new and little understood virus.

Researchers from Strasbourg University Hospital and the Pasteur Institute studied 160 French healthcare workers who contracted Covid-19, but only experienced mild symptoms and did not require hospitalization. Blood samples revealed that almost all of the patients developed antibodies to Covid-19 within 15 days of the infection, while up to 98 percent of them still had neutralizing antibodies after 28 days. 

The neutralizing activity of antibodies also appeared to increase over time, with neutralizing antibodies detected in 79 percent of samples 13-20 days after the onset of symptoms, and then in 92 percent of samples after 21-27 days, followed by 98 percent after 28-41 days.

“It was known that people with severe forms of the disease developed antibodies within 15 days of the onset of the signs. We now know that this is also true for those who have minor forms, even if the antibody levels are probably lower,” Arnaud Fontanet, one of the authors of the study and head of the Global Health department at the Institute Pastor, said in a statement. 

The study, which has not yet been peer-reviewed, was recently released by the researchers and available to read on the pre-print server MedRxiv.

The researchers say their findings suggest people who have been infected with Covid-19, even if they experienced only mild symptoms, have protection against reinfection of SARS-CoV-2, the coronavirus that causes Covid-19, for “at least up to 40 days after the onset of signs.” However, it remains unclear whether this protection lasts for longer. 

Immunity to any virus isn't simply black-and-white, but rather a spectrum – one can experience a continuum of infection levels from zero to full infection and everything in between. Even if you have antibodies to a specific virus, it doesn’t necessarily mean you are totally protected from the infection. Equally, levels of immunity can progressively slip as time goes on. Furthermore, immunity is not all about antibodies; the immune response to a pathogen also relies on a well-trained army of B cells (the white blood cell that secretes antibodies) and T-cells (which directly kill cells that have been infected by an invader).

This new research looks promising but is not a guarantee that people who have had a mild Covid-19 infection will have any lengthy, meaningful immunity to reinfection. The team is hoping to investigate this further by looking at the patients’ antibody response and its associated neutralization capacity in the long term.

“Yes, we have good results for a month but we have no way of predicting how long this will last,” Kevin Ariën, professor of virology at the Institute of Tropical Medicine Antwerp, who was not directly involved in this study, told the Financial Times. “We presume that people infected in this first wave of Covid-19 will be protected if there’s a second wave in the autumn, but we don’t really know what will happen.”