This research article was originally published by Reynolds et al. in Science Immunology, 23 December 2020. Further details can be found in our publication section.
Understanding how immunity to COVID-19 is generated in individuals who have experienced either mild or no symptoms (asymptomatic) is important in allowing us to know whether they are at risk of reinfection. UK Coronavirus Immunology Consortium (UK-CIC) Theme Lead Professor Mala Maini and Postdoctoral Research Fellow, Dr Leo Swadling, both from University College London have collaborated with researchers from across London to better understand the different components of the immune response to COVID-19 at four months after infection and how this relates to protective immunity.
The immune system is extremely complex and there are many different potential routes whereby it can generate immunity to a disease after an infection. For this study, researchers from several institutions worked together to analyse immune responses in a group of 136 London healthcare workers, 76 of which had a mild or asymptomatic COVID-19 infection, and a matched group of uninfected controls. These healthcare workers were part of a larger cohort followed from the start of UK lockdown in March 2020. Each week, every healthcare worker took a PCR test, had their blood analysed and kept a symptom diary to allow the researchers to record any COVID-19 infection and the subsequent antibody and T cell responses generated.
Out of the 76 healthcare workers tested at 16-18 weeks post mild or asymptomatic COVID-19 infection, 89% of them had neutralising antibodies present in their blood. This is important because this is the type of antibody that stops the virus entering cells, thereby potentially protecting against reinfection. In fact, 66% had high enough levels of this protective immune cell to suggest they would be protected in the case of re-exposure. The majority of individuals also had a variety of T cells in their blood that could recognise different parts of the SARS-CoV-2 virus. T cells can help to direct effective antibody responses as well as providing a direct added layer of protection by removing any cells that the virus has managed to infect.
When the researchers delved deeper into the results, they found a complex pattern of immune responses between individuals – about half of people had mismatched antibody and T cell responses. This suggests that these two arms of protective immunity can act in a complementary manner, with some individuals showing T cell immunity but no evidence of antibodies, and vice versa.
The study also shows some hints of differences in immune response depending upon severity of infection - T cell responses were higher in mild cases than asymptomatic cases, but neutralising antibodies were just as likely to be detectable in individuals whose infection had been completely asymptomatic.