Wearing face masks at home could help prevent the spread of COVID-19 infection among family members living in the same household, but only before the onset of the symptoms, according to a study.
The observational study of Chinese families, published in the journal BMJ Global Health, shows that wearing masks at home was 79 per cent effective at curbing transmission before symptoms developed in the first person infected.
However, the practice was not protective once symptoms had developed, according to the researchers, including those from Beijing Research Center for Preventive medicine in China.
The figures in China suggest that most of the person to person transmission of SARS-CoV-2, the coronavirus responsible for COVID-19 infection, has occurred in families, they said.
To explore whether face masks might make a difference, the researchers questioned 460 people from 124 families in Beijing, on their household hygiene and behaviours during the pandemic.
Each family had at least one laboratory-confirmed case of COVID-19 infection between late February and late March 2020, the researchers said.
Family members were defined as those who had lived with the infected person for four days before, and more than 24 hours after that person's symptoms first appeared, they said.
The researchers wanted to know what factors might heighten or lessen the risk of subsequently catching the virus within the incubation period -- 14 days from the start of that person's symptoms.
They found that during this time, secondary transmission -- spread from the first infected person to other family members -- occurred in 41 out of the 124 families.
A total of 77 adults and children were infected in this way, giving an 'attack rate' of 23 per cent or around 1 in 4, the study found.
Around a third of the children in the study caught the virus compared with more than two-thirds of the adults, the researchers said.
Daily use of disinfectants, window opening, and keeping at least one metre apart were associated with a lower risk of passing on the virus, even in more crowded households, they said.
Of all the behavioural and hygiene factors, four were significantly associated with the secondary transmission of the virus, they said.
Diarrhoea in the first person to become infected and close daily contact with them increased the risk of passing on the virus, according to the researchers.
They said diarrhoea was associated with a quadrupling in risk, while close daily contact, such as eating meals round a table or watching TV together, was associated with an 18-fold increased risk.
Frequent use of bleach or disinfectants for household cleaning and the wearing of a face mask at home before symptoms emerged, were associated with a reduced risk of viral transmission, the researchers said.
A face mask worn before symptoms started was 79 per cent effective, and disinfection 77 per cent effective, at stopping the virus from being passed on, they said.