Past Covid-19 infection provides some immunity, but reinfections are seen after eight months

Published: 20 January 2021

Researchers at St George’s have found that among people infected with Covid-19 in the first wave of the pandemic, there was a 94% rate of protection from reinfection in the second wave. Although those with immunity may be vulnerable to catching the virus again.

The study, led by Dr Aodhán Breathnach, Consultant Medical Microbiologist at St George’s Hospital and honorary senior lecturer at St George’s, University of London, identified 66,001 patients who had been tested for new Covid-19 infections, as well as the presence of antibodies, before the end of July 2020.

Of the 10,727 patients who were found to have been infected by Covid-19, eight contracted the virus again between 1 August and 30 December, more than three months after their first infection. This represents a 94% rate of protection from reinfection.

This compares to 713 of the 55,274 patients who had no laboratory evidence of Covid-19 in the first wave subsequently becoming infected in the second wave.

The study found no reinfections within the first seven months after the peak of the first wave; all eight patients with reinfections were diagnosed in December.
These results support other recent studies, including the SIREN study – which found that naturally acquired immunity as a result of past infections provides 83% protection against reinfection for at least five months.

Dr Aodhán Breathnach said: “These results are a sign of hope – and it is important to give people hope in these difficult times. The vast majority of people who have had Covid-19 develop immunity that lasts for at least several months. I see this, and other studies showing evidence of immunity, as small steps on the road back to normality.

“However, the small number of reinfections we saw in December shows that immunity may decrease with time – an important reminder that we must all continue to follow the guidelines carefully, even if you have had the virus before.”

This immunity study extends a portfolio of research at St George’s to tackle the coronavirus pandemic. The University has launched a Coronavirus Action Fund to raise money for vital research into the pandemic and is actively seeking support for a broad research programme involving all parts of the University.

Please see our Covid-19 Guidance webpages for more information on the University’s response to the pandemic.