Dressed in a hazmat suit, two masks and a face shield, Du Mingjun knocked on the mahogany door of a flat in a suburban district of Wuhan on a recent morning.

Chinese doctors in Wuhan, where the virus first emerged in December, say a growing number of cases in which people recover from the virus, but continue to test positive without showing symptoms, is one of their biggest challenges as the country moves into a new phase of its containment battle.
 

Those patients all tested negative for the virus at some point after recovering, but then tested positive again, some up to 70 days later, the doctors said. Many have done so over 50-60 days.
 

The prospect of people remaining positive for the virus, and therefore potentially infectious, is of international concern, as many countries seek to end lockdowns and resume economic activity as the spread of the virus slows. Currently, the globally recommended isolation period after exposure is 14 days.

So far, there have been no confirmations of newly positive patients infecting others, according to Chinese health officials.
 

China has not published precise figures for how many patients fall into this category. But disclosures by Chinese hospitals to Reuters, as well as in other media reports, indicate there are at least dozens of such cases.

In South Korea, about 1,000 people have been testing positive for four weeks or more. In Italy, the first European country ravaged by the pandemic, health officials noticed that coronavirus patients could test positive for the virus for about a month.

As there is limited knowledge available on how infectious these patients are, doctors in Wuhan are keeping them isolated for longer.