The news that a man in Nevada in the US has been infected with the coronavirus twice has made headlines around the world – but scientists say its significance is unclear.
While indicating that reinfection is possible, researchers stress it is only a single case and may not offer useful information as to whether reinfection is common.
Researchers reported in the journal The Lancet this week that the 25-year-old man, who tested positive in April and June, suffered more severe symptoms on reinfection and had to be hospitalised.
It was not the first confirmed reinfection, with at least three other people – a 33-year-old man in Hong Kong, a 51-year-old woman in Belgium and a 46-year-old man in Ecuador – having previously been reported to have had Covid-19 twice.
Every virus I've seen, as far as I know, you're never totally immune
John Oxford, professor emeritus at the University of London
The case in Nevada has sparked particular concern because the second infection was more severe than the first.
This could suggest the immune system, in producing antibodies against the first infection, ended up assisting rather than preventing the subsequent infection.
However, out of the other three confirmed reinfections, the second infection was less severe than the first in one case, while in another it was asymptomatic.
Data suggest reinfection is rare
Paul Hunter, a professor in medicine at the University of East Anglia in the UK, told the Science Media Centre that there may have been many reinfections where the second illness was less severe or asymptomatic, but these have simply been missed.
“Given the fact that to date over 37 million people have had the infection, we would have expected to have heard of many more incidents if such very early reinfections with severe illness were common,” he said.
“Nevertheless, repeat infections do occur with different strains and I suspect many more will be found over coming months as immunity declines in individuals after infection.”
John Oxford, professor emeritus at the University of London and co-author of the textbook Human Virology, cautioned against reading too much from the findings.
“Every virus I’ve seen, as far as I know, you’re never totally immune. When you get infected, you can never be 100 per cent [sure] you won’t be reinfected,” he said.
The question of whether people who have recovered from a coronavirus infection can be reinfected has been a main talking point during the pandemic.
Typically, the body produces antibodies against the coronavirus about a week after symptoms appear, although the degree to which they offer protection against reinfection has been unclear, according to the British Society for Immunology.
Second Covid-19 infection raises questions over herd immunity
As many as one in five people who have been infected produce few or no detectable antibodies in their blood.
The body’s immune response does not just involve antibodies, with white blood cells called T cells also playing an important role.
If people cannot be reinfected, it offers hope that as the numbers who have had Covid-19 increase, transmission rates will decline because more people are immune, eventually reaching what is called herd immunity.
The case in Nevada, confirmed as two separate infections because genetic differences were found between the viruses during the two positive cases, has been interpreted by some commentators as suggesting herd immunity may be less likely to develop.
As well as potentially having implications for herd immunity, if reinfections turned out to be common it could also cast doubt on the effectiveness of immunisation programmes, as it may indicate that a vaccinated person could still be at risk of infection.
Vaccines are not, though, expected to confer immunity on all who receive them, so isolated cases of reinfection may offer little insight into the likely effectiveness of such programmes.
Prof Oxford said that he was optimistic about the vaccines under development, some of which are expected to receive clearance for widespread use in the coming months.
“Despite these reinfections, I think some of them are going to work, I really do,” he said. “Which one is another question.”