The case of a 66-year-old woman with no recent travel history becoming infected, confirmed by the government in a briefing on Wednesday, dashes hope that the city had successfully contained the virus after nearly four months of school closures and social distancing measures.

Her five-year-old granddaughter has also been confirmed to be infected, while six other family members have displayed symptoms and are now in isolation in hospital, said Chuang Shuk-kwan, head of the health department’s communicable disease branch.

The woman, who lives in the neighborhood of Tsuen Wan, first developed a fever last Friday and tested positive on Tuesday, she said. Officials don’t yet know how she picked up the infection.

The emergence of hidden local transmission chains is a setback to the government’s plan to reopen schools and loosen border restrictions with mainland China as cases dwindle. After months of social distancing measures, Hong Kong was on verge of reverting to normal life. Instead, the Asian financial hub now faces a potential resurgence of cases as it’s unclear how many people the woman came into contact with before her infection was identified.

Health officials are testing all residents in the two housing blocks that the woman visited, said Chuang.

Besides the woman and her granddaughter, a third case of a traveler returning from Pakistan was also reported on Wednesday.

“This is the challenge of Covid-19, the fact that so many cases are asymptomatic,” said Nicholas Thomas, an associate professor at the City University of Hong Kong. “Even though there have been no cases for nearly three weeks, that doesn’t mean that the virus is not in circulation. This is a problem not just for Hong Kong but for all countries as they seek to move out of lockdown and restart their economies.”

The resurgence of cases will complicate the political debate in Hong Kong on how quickly social distancing measures should be eased. While Hong Kong leader Carrie Lam’s administration eased limits on social gatherings to eight people earlier this month, the rule has come under pressure for being arbitrarily designed to forestall street protests, which in any case have re-emerged.

Separately, Hong Kong’s education chief said on Wednesday that the government wouldn’t push back the reopening of schools, currently slated to restart on May 27, for now.

“If the goal is just to eradicate the virus then Hong Kong should go back into shutdown mode,” said Thomas, who has edited an academic book series titled “Health Security and Governance.” “But it is about the virus against the backdrop of the economic and social needs of the community and for that reason this one case is unlikely to prevent the restarting of Hong Kong.”

Hong Kong’s been lauded for reacting quickly to the epidemic, which kept its outbreak small despite its proximity to mainland China. Many of its moves made as early as January — almost universal mask-wearing and the closure of schools and offices — have since been adopted globally as the pandemic widened.