Wearing face masks at home might help ward off the spread of Covid-19 among family members living in the same household, but only before symptoms develop, suggests a study of Chinese families in Beijing. This practice was 79% effective at curbing coronavirus transmission before symptoms emerged in the first person infected, but it was not protective once symptoms had developed, the study shows.
“The study confirms the highest risk of transmission before symptom onset and provides the first evidence of the effectiveness of mask use, disinfection and social distancing in preventing Covid-19. This can inform guidelines for community prevention in settings of intense Covid-19 epidemics,” says the research team in their findings published in BMJ Global Health. It includes experts from the Beijing Research Center for Preventive medicine, China; Beijing Normal University, China; University of Nevada, Las Vegas; and University of New South Wales, Sydney.
The experts say that in China, over 70% of human-to-human transmission of SARS-CoV-2, the virus that causes Covid-19, occurred in families. But it was not clear if the wearing of face masks might help curb household transmission rates, along with social distancing and disinfection. “Given epidemic growth is dominated by household transmission, studying the use of NPIs such as face masks, social distancing and disinfection in the household setting may inform community epidemic control and prevent transmission of Covid-19 in households,” they say.
Accordingly, to explore whether face masks might make a difference, the researchers questioned 460 people from 124 families in Beijing, China, on their household hygiene and behaviors during the pandemic. Each family had at least one laboratory-confirmed case of Covid-19 between late February and late March 2020. 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. The average family size was four but ranged from 2 to 9, and usually comprised three generations, says the report.
The research team wanted to know what factors might heighten or lessen the risk of subsequently catching the virus within the incubation period, that is, 14 days from the start of that person's symptoms. 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% or around 1 in 4. The analysis shows that around a third of the study's children caught the virus (36%; 13 out of 36) compared with more than two-thirds of the adults (just over 69.5%; 64 out of 92). Twelve of the children had mild symptoms; one had none. Most (83%) of the adults had mild symptoms; in around 1 in 10, symptoms were severe, and one person became critically ill.
Frequent use of bleach or disinfectants for household cleaning, wearing a face mask at home before symptoms emerged, including by the first person to have them, was associated with a reduced risk of viral transmission. Most significantly, a face mask worn before symptoms started was 79% effective, and the daily use of chlorine or ethanol based disinfectant in households was 77% effective at stopping the virus from being passed on. These practices along with window opening, and keeping at least 1 meter apart were associated with a lower risk of passing on the virus, even in more crowded households.
“Almost a quarter of family members became infected, and the findings suggest that the risk was highest either before symptom onset or early in the clinical illness, as most primary cases were hospitalized after diagnosis, and interventions were not effective if applied after symptom onset,” says the study.
However, daily contact and the number of family members wearing a face mask after the start of symptoms in the first person to develop them were associated with a heightened risk. Of all the behavioral and hygiene factors, some were significantly associated with the secondary transmission of the virus. The risk of household transmission was 18 times higher with frequent daily close contact with the primary case - within 1 meter or 3 feet, such as eating around a table or sitting together watching TV together - and four times higher if the primary case had diarrhea.
The research team says that their findings back universal face mask use, not just in public spaces, but also at home. “The results suggest that community face mask use is likely to be the most effective inside the household during severe epidemics. This supports universal face mask use, and also provides guidance on risk reduction for families living with someone in quarantine or isolation, and families of health workers, who may face an ongoing risk of infection,” says the research team. The authors acknowledge some limitations to their study: telephone interviews are subject to recall and the strength of household disinfectants and bleach used was not recorded.
Dr Antonio Lazzarino, from the department of epidemiology and Public Health at UCL, who was involved in the study, said that the study alone is not enough to make recommendations of any kind. “This study is not robust science, as it has several limitations. The main limitation is that it was designed at the family level, rather than at the individual level. This is a questionnaire-based retrospective study, therefore potentially affected by recall bias. Moreover, there was no distinction between an N95 mask, disposable surgical mask, or just a common mask including cloth mask,” he said in a statement.
However, some others believe that the findings are significant. “This is an important paper because it comes at a time when – as lockdown is eased – the risk of a person entering the home who has become infected (for example whilst on public transport or in the workplace) but is unaware that this is so, is increasing. The study will be very useful for persuading people that social distancing and strict hygiene in their homes can work to reduce the risk of infection transmission to other household members but I think it could be difficult to persuade families to wear masks in their own homes,” says Professor Sally Bloomfield, honorary professor, London School of Hygiene and Tropical Medicine.
According to Trish Greenhalgh, professor of primary healthcare services at the University of Oxford, the mask-wearing is “perhaps the most interesting” as it is something that few people currently do in their own homes, especially when not symptomatic. “Worryingly, whereas people who wore a mask before they became symptomatic with Covid-19 were significantly less likely to pass it on to others in the household, those who only started wearing the mask after they became symptomatic weren’t able to protect their family members. In localities where the incidence of Covid-19 is high, people might feel this measure is worth the inconvenience, especially if there is a vulnerable family member,” said Greenhalgh.