They found that most infected people had T cells capable of recognizing the virus 4 months later
MEDICAL WRITING
New research has found evidence of protective immunity in people up to four months after mild or asymptomatic Covid-19 . The study, published in Science Immunology, analyzed antibody and T-cell responses in 136 healthcare workers in London who had a mild or asymptomatic Covid-19 infection dating back to March 2020.
The team, which included researchers from Queen Mary University of London, Imperial College London and University College London, found that 89 percent of the healthcare workers tested carried neutralizing antibodies 16-18 weeks after infection .
The researchers found that most also had T cells capable of recognizing multiple different parts of the virus . However, the two responses did not always persist in harmony; some individuals showed T-cell immunity but no evidence of antibodies, and vice versa.
“Our study of SARS-CoV-2 infection in healthcare workers in London hospitals reveals that four months after infection, around 90 percent of people have antibodies that block the virus . Even more encouraging, in 66 percent of healthcare workers we see that the levels of these protective antibodies are high and that this robust antibody response is complemented by T cells that react to various parts of the virus,” explains Joseph Gibbons , a postdoctoral research assistant at Queen Mary.
"This is good news . It means that, if you have been infected, it is very likely that you have developed antibodies and T cells that can provide some protection if you encounter the virus again ," he says.
Incompatible immune responses
Since the start of the pandemic, scientists around the world have been working to understand how our immune system protects us against SARS-CoV-2 and how long this protection lasts. Much of this debate about protective immunity has focused on the different roles of B cells and T cells.
In this study, the researchers show that, while protective antibody responses were generally complemented by a T cell response, more than half of the healthcare workers had mismatched antibody and T cell responses , and did not produce a specific T cell response to proteins found on the outside of the SARS-CoV-2 virus's outer layer.
They also found that T cell responses tended to be higher in those with classic, defining symptoms of Covid-19, whereas asymptomatic infection resulted in weaker T cell immunity than symptomatic infection , but equivalent neutralizing antibody responses.
Reassuring evidence
Understanding how this careful choreography of immune responses works in people with a mild or asymptomatic infection is particularly important, as they represent the largest infected group.
The new study also provides reassurance for vaccination efforts, suggesting that even after a mild infection, individuals carry antibody and T-cell immunity to many parts of the virus , known as epitopes. While new variants are emerging, changes in the virus do not necessarily occur within these epitopes, so the vast majority of immune recognition is expected to continue unabated.
“Our study of asymptomatic and mild cases provides a positive view of the durability of immunity to SARS-CoV-2 four months after infection. A remarkable number—around 90%—of individuals have a combined strength of strong antibodies that prevent the virus from entering, along with T-cell responses to various parts of the virus that interfere with its survival. This is an important finding, as mild or even no symptoms of Covid-19 are very common and representative of most infections in the community. Such abundant immune responses also give hope for the long-lasting efficacy of vaccines,” said Corinna Pade , a postdoctoral research scientist at Queen Mary University of London.