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BACK in August 2020, a worrying report came in from Reno, Nevada. A 25-year-old man who had recovered from covid-19 in April had fallen ill with it again, and this time his symptoms were worse. He had tested negative for the virus in between bouts, so had been infected twice.

Other reports of reinfection were also circulating at the time, raising fears that infections don鈥檛 lead to long-lasting immunity.

Nine months on, however, those fears have receded. Not only is vaccination proving highly effective, a number of large studies in Europe and the US have now shown that while reinfection is possible, it is rare and usually produces mild disease at worst.

One such was carried out over four months in 100 care homes for older people in England. Between June and November 2020, Maria Krutikov at University College London and her colleagues took blood samples from 682 residents and 1429 staff and tested them for antibodies against SARS-CoV-2. Over the next four months, all the subjects were regularly screened for infection using PCR tests.

Initial blood tests found that 634 of the total 2111 people were antibody positive, meaning they had already been infected. Only 14 of them subsequently had a positive PCR test 鈥� a reinfection rate of just over 2 per cent.

In comparison, 204 of the 1477 (14 per cent) people whose blood test came back antibody negative subsequently caught the virus. Data from residents and staff who had been vaccinated more than 12 days before their samples were taken were excluded from the analysis (The Lancet Healthy Longevity, ).

Eleven of the 12 reinfected people for whom symptoms were recorded had symptoms such as a cough or fever, but none required hospitalisation. The researchers warn that the numbers are quite small, so it is hard to draw firm conclusions, but it seems that previous infection reduces the risk of reinfection by about 70 per cent. This is in line with another study of healthcare workers in England, also carried out between June and November 2020.

carried out in Denmark during the second wave of infections there found an even higher level of protection among people previously infected in the first wave, around 80 per cent. Meanwhile, a non peer-reviewed study of 50,000 healthcare employees in Ohio between December 2020 and April 2021 found a .

鈥淭he concern for covid-19 is that we might not see complete protection or that protection might fall with time,鈥� says Alexander Edwards at the University of Reading, UK. 鈥淚t鈥檚 therefore great news to see that reinfection wasn鈥檛 seen very often. However, it鈥檚 important to also confirm that reinfection does occur, so protection is not complete. We expect that natural infection should protect against severe infection, but we still don鈥檛 have enough data to know this.鈥�

We don鈥檛 know to what extent new variants change the picture. The care home study in England was done when the alpha variant was becoming dominant in the UK. 鈥淭he low number of reinfections suggest a good level of immunity against this variant following natural infection,鈥� says Krutikov. However, it predates the rise of the delta variant, which now causes 91 per cent of cases in the UK.

鈥淭he low number of reinfections suggest a good level of protective immunity against alpha鈥�

鈥淭he study does not inform directly the protection against reinfection with this variant,鈥� says Rowland Kao at the University of Edinburgh, UK.

Further information on reinfection is unlikely to be forthcoming because mass vaccination makes it difficult to study enough unvaccinated people, says Edwards.