Wearing masks at home appeared to help prevent presymptomatic COVID-19 cases from infecting family members, researchers in China found in a retrospective cohort study.
When at least one household member wore a face mask — which could be the infected person or an uninfected family member — prior to anyone showing symptoms, odds of within-family transmission were cut 79% (OR 0.21, 95% CI 0.06-0.79), reported Peng Yang, PhD, of Beijing Center for Disease Prevention and Control, and colleagues.
Other factors associated with reduced risk of transmission were frequent chlorine or ethanol-based disinfectant for house cleaning, while factors associated with increased risk included close contact at home with primary cases and diarrhea in the infected individual, the authors wrote in BMJ Global Health.
“This study is the first to confirm the effectiveness of mask use prior to symptom onset by family members, daily household disinfection and social distancing in the home,” Yang and colleagues wrote, noting this may be applicable to families at high risk of transmission, such as those under quarantine/isolation or even families of healthcare workers.
The CDC currently recommends everyone in the U.S. wear a cloth face mask in public, but has not made any recommendations about home use.
Yang’s group examined data from COVID-19 patients who had lived with primary cases in a house 4 days before and more than 24 hours after illness developed. All laboratory-confirmed COVID-19 cases reported in Beijing as of Feb. 21 were included in the study and then followed up with a three-part structured questionnaire via telephone.
Secondary transmission was defined as some or all family members becoming infected within 2 weeks of symptom onset of the primary case. “Close contact” was defined as within 3 feet of the primary case.
Of 124 families, secondary transmission occurred in 41 families over 2 weeks of follow-up, resulting in 77 secondary cases. However, 83 families reported no secondary transmission. The secondary attack rate in families was 23%.
Not surprisingly, adults had a higher attack rate than children under age 18 (69.6% vs 36.1%, respectively). The median age of the secondary child cases was 3, and 12 of 13 were asymptomatic. About 83% of secondary adult cases were mild, 11% were severe, 2% were critical, and 5% were asymptomatic.
In addition to family members wearing masks, frequent use of chlorine or ethanol-based disinfectants was significantly associated with reduced transmission (adjusted OR 0.23, 95% CI 0.07-0.84). However, close contact with the primary case increased risk of transmission by 18 times, and the case having diarrhea quadrupled risk of transmission, though both had a wide confidence interval.
Following symptom onset in the primary case, risk of secondary transmission was still markedly lower (OR 0.20, 95% CI 0.07-0.60) when all household members wore masks, relative to homes in which no one did. No significant reduction in risk was seen in households in which some but not all members were masked.
Limitations to the data include that telephone interviews could introduce recall bias, and while “evaluation results of mask wearing were reliable,” there were no data on the concentration of disinfectant used by families. Nor did the study compare the apparent preventive benefit of masks when worn by the primary case versus uninfected family members.
“[Non-pharmaceutical interventions] are effective at preventing transmission, even in homes that are crowded and small,” Yang and colleagues wrote. “[Universal face mask use] is a low-risk intervention with potential public health benefits.”
This work was supported by Beijing Science and Technology Planning Project.
The authors disclosed no conflicts of interest.