They repeated the theory that Covid-19 could be spread internationally on imports of frozen food, something many scientists have questioned.
Officials said the woman who tested positive had no history of travel.
They added they found traces of the virus on a package she received.
The infection comes less than three weeks before Beijing is set to host the Winter Olympics.
On Monday, China announced that it would not be selling tickets to members of the public for the Games. As part of virus control measures, only people invited will be allowed to attend. It is not yet clear how they will be selected or whether they will have to quarantine.
The case is, so far, the only one here to have been recorded with the highly contagious variant of Covid-19, the BBC’s Stephen McDonell reports from the Chinese capital.
Health official Pang Xinghuo told reporters on Monday the virus had been found on the surface of a letter the infected person had received from Canada, as well as inside the unopened letter, the AFP news agency reports.
Dozens of letters from the same batch were tested, and five showed positive traces of Covid-19, Ms Pang said, including samples from inside unopened letters.
Beijing’s Centre for Disease Control said the possibility that the woman was infected by a parcel from another country could not be ruled out.
If deliveries from abroad are opened, it has been suggested that this should be done outside, with the opener wearing gloves and disposing of the packaging.
What Beijing calls its “dynamic zero Covid” strategy combines mass vaccination with a regime of constant testing, nationwide monitoring of people’s movements, temperature-taking and phone apps to prove you don’t pose a threat, says the BBC’s Robin Brant.
All of that amounts to hyper vigilance about any new outbreak – if and when one is spotted the reaction is severe.
China has been wedded to this approach since the beginning because it’s likely the leaders from Xi Jinping down consider anything less would cause illness and death on a scale that would be politically, economically and socially unacceptable.