Antibodies from past Covid-19 infection provide 83% protection against reinfection for at least five months according to a large study of NHS staff which gives ‘the clearest picture yet’ of ongoing immunity.
But early evidence from the Public Health England (PHE) study which has monitored healthcare workers using PCR and antibody testing every 2-4 weeks suggests that some of those with antibodies may still be able to pass the virus on.
Data from 6,614 participants who tested positive for antibodies found 44 potential Covid-19 reinfections between June and November, suggesting that most individuals are protected for least for the five-month study period.
Of those, two were ‘probable’ and 42 ‘possible’ reinfections with a median interval of more than 160 days between primary infection and reinfection, the interim analysis of the SIREN study reported.
This compared with 318 new PCR positive Covid-19 infections and 94 antibody seroconversions in the negative cohort of 14,173 participants.
The degree of protection conferred by prior Covid-19 infection – at least in the short to medium term – appears to be similar to that as the currently licensed vaccines being rolled out, they added.
Primary infection also reduces the risk of asymptomatic infection and thus onward transmission, the PHE analysis suggests.
But some initial evidence from the next stage of the study suggests that a small number of people with antibodies carry high levels of virus and could continue to transmit the virus to others.
The SIREN study is continuing to investigate how long antibody responses last, reinfection rates with the new strain, and the impact of Covid-19 vaccines.
Professor Susan Hopkins, senior medical advisor at Public Health England and the SIREN study lead said: ‘We now know that most of those who have had the virus, and developed antibodies, are protected from reinfection, but this is not total and we do not yet know how long protection lasts. Crucially, we believe people may still be able to pass the virus on.
‘This means even if you believe you already had the disease and are protected, you can be reassured it is highly unlikely you will develop severe Covid-19 infections but there is still a risk that you could acquire an infection and transmit to others.
‘Now more than ever it is vital we all stay at home to protect our health service and save lives.’
Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading, said the study had major implications for how to get out of the current crisis.
‘The good news is that this study gives further weight that reinfections of Covid are rare, at least at this stage, and that having antibodies will provide protection for a meaningful amount of time, although it may not be lifelong immunity.
‘The concerning finding is that some people who have Covid antibodies appear to still be able to carry the coronavirus and could spread it to others.
‘This means that the vast majority of the population will either need to have natural immunity or have been immunised for us to fully lift restrictions on our lives, unless we are prepared to see many more people being infected and dying from Covid-19.’
The news comes as PHE is launching a study into the effect of Covid vaccination which will be tracking a subset of recipients to find out whether vaccination prevents virus transmission.
It will also research the efficacy after just one dose; how long protection lasts; as well as how the vaccine works in people with underlying conditions.
A version of this story first appeared on our sister publication, Pulse.