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.