9 out of 10 people who have had mild or asymptomatic Covid-19 carry neutralizing antibodies 16-18 weeks after infection.
This is according to new research published in Science Immunology by UCL, Imperial College London, Queen Mary University of London, Bart’s and the London School of Medicine and Dentistry and Public Health England (PHE), and supported by the NIHR UCLH BRC.
The study analysed antibody and T cell responses in 136 London healthcare workers with mild or asymptomatic infection dating back to the start of lockdown in March 2020.
89 per cent of those analysed carried neutralizing antibodies 16-18 weeks following infection, usually complemented by a multi-pronged T cell response.
However, the research also suggests that these two arms of protective immunity do not always persist in harmony, with some individuals showing T cell immunity but no evidence of antibodies, and vice versa.
From the outset of the pandemic, scientists across the globe have been working to understand how our immune system protects us against SARS-CoV-2 and how long this protection lasts.
Much of the debate around protective immunity has focussed on the different roles of B cells, which make antibodies, and T cells, white blood cells which work in several different ways to help protect from viruses, including direct killing. Some previous studies emphasised the point that antibodies wane quite quickly while T cells may provide more durable protection.
Understanding how this careful choreography of immune responses works in people with mild or asymptomatic infection is particularly important as they represent the largest infected group. With this in mind, the study team set out to establish whether individuals in this category would still show SARS-CoV-2 specific antibody and T cell responses more than four months after infection.
16-18 weeks following infection, the team found that 89 per cent still had neutralizing antibodies capable of blocking virus entry, with 66 per cent showing high levels. Most also had T cells capable of recognising multiple different parts of the virus.
Overall, however, just over half of the healthcare workers had mismatched antibody and T cell responses and did not produce a T cell response specific to virus spike protein, with 15 per cent not producing a response to 'N', a key virus protein.
Importantly, the team found that T cell responses tended to be higher in those with the classic, defining symptoms of COVID-19 disease in comparison to those with asymptomatic infection. Asymptomatic infection resulted in less strong T cell immunity than symptomatic infection, but equivalent neutralizing antibody responses.
The study findings suggest that understanding protective immunity in the population will require careful and simultaneous scrutiny of both T cell and antibody responses.