It's very rare to catch Covid-19 twice

A few readers have asked us whether you can catch Covid-19 twice. This follows reports at the end of February that a Japanese woman had tested positive a second time.  A recent article in the Daily Mail and two articles in the Sun also suggested in their headlines and their early paragraphs that this might be the case.

The evidence so far shows that catching the disease twice is very rare, and that most infected people recover and develop immunity against it. However, it is not yet clear how long this immunity will last. 

When a journalist raised the case of the woman in Japan at the Prime Minister’s press conference on 16 March, the Chief Scientific Advisor, Sir Patrick Vallance said: “In any infectious disease there are cases where people can catch something again. They’re rare. There’s nothing to suggest that this is a common occurrence in this disease, but we are learning as we go along.”

A report from the European Centre for Disease Control and Prevention on 25 March says, “there is emerging evidence from early studies suggesting that individuals develop antibodies after infection and are likely to be immune from reinfection in the short term”. 

During a live question-and-answer session on 25 March, Professor Jimmy Whitworth from the London School of Hygiene and Tropical Medicine said: “There have been a few isolated examples where [reinfection] has been reported. That people were positive, then they were negative, then they were positive again. 

“It looks like, in the great majority of cases, this doesn’t happen. That people get infected once. My suspicion is that those discrepant test results that we get are to do with, actually, the sampling. It’s not straightforward to take a sample from the back of the throat and make sure you catch virus every time. So I suspect it’s a technical issue, rather than repeat infection… It looks like you are immune for getting it again, but for how long, we don’t know yet.”

The articles in the Daily Mail and the Sun cite several reports from China, which describe a small number of recovered Covid-19 patients testing positive for the disease again. 

As the Mail and Sun articles say, based on local reports, the patients who tested positive twice in Wuhan showed no symptoms the second time and none apparently passed the disease to others. 

The same was true of a group of patients who tested positive twice in Guangdong province. The Sun quotes a doctor involved in the research saying  the results were in the “weak positive” range, suggesting that the virus may no longer have been active. He also added that there may have been problems with sampling.

The Mail quotes the director of Wuhan’s Tongji Hospital, Wang Wei, saying: “It’s possible that these recovered patients tested negative before because of false results... The accuracy of a nucleic acid test is 30 to 50 per cent.”

In short, these cases do not show that it is common for people to contract Covid-19 twice, as it is possible that the virus had not yet left these patients’ bodies after their first infection.