Covid-19: what we know and do not know about the coronavirus
Reports that a woman in Japan has tested positive for the Covid-19 disease for a second time, after seemingly recovering, will alarm scientists and public health experts trying to control the spreading epidemic, and underlines how much we still do not know.
There are a number of possible explanations for the second positive test of the woman, in her late 40s, a resident of Osaka who worked as a tourbus guide. She first tested positive for coronavirus in late January and was discharged from hospital on 1 February after recovering. She tested negative again on 6 February.
It is possible, say experts, that when the woman was released, she had not cleared the virus. But if so, that means it lingered dormant in her body longer than the 14-day quarantine period. She will have been in contact with more people than have been traced, which poses worrying questions about the length of time people should be isolated after a positive test.
Alternatively, she may have been wrongly diagnosed with Covid-19 the first time round. But nobody is ruling out the possibility of reinfection. Once the immune system has fought off viral or bacterial infections, it generally recognises them and can block them the next time they are encountered – but not always and the protection may not last.
There have been reports of a few cases of reinfection in China, but doctors will hope it occurs in just a very few individuals, if at all.
After more than 82,000 cases of Covid-19, the unknowns still outnumber the knowns. Although there is clearly human-to-human transmission, we don’t know whether that happens only through droplets from coughs or sneezes or whether there are other forms of transmission as well. There have been reports of airborne transmission in China, although the World Health Organization (WHO) says it is generally not happening.