According to a survey of 124 families in Beijing published in BMJ Global Health, masks were 79% effective at halting spread of the virus if at least one person in the home started wearing a face covering before anyone got sick.

The families that were interviewed for the survey had a total of460 people.  This included many high-risk grandparents living in the same house their children and grandchildren. Each of the families had at least one Covid-19 case between February and March. The survey also concluded that the virus spreading throughout a household is not guaranteed.  About 25% of the patients’ family members were infected, with secondary transmission happening in about 33% of the families surveyed.