By Carol A. Kemper, MD, FACP

Clinical Associate Professor of Medicine, Stanford University, Division of Infectious Diseases, Santa Clara Valley Medical Center

Dr. Kemper reports no financial relationships relevant to this field of study.

SOURCE: MacIntyre CR, Seale H, Dung TC, et al. A cluster randomised trial of cloth masks compared with medical masks in healthcare workers. BMJ Open 2015;5:e006577.

How effective are homemade fabric masks many are using to protect others from COVID-19 transmission?

MacIntyre et al conducted a randomized clinical trial that evaluated the effectiveness of cloth vs. regular procedural masks in 1,607 healthcare workers (HCWs), recruited for study at 14 different hospitals in Hanoi, Vietnam, in 2011. The HCWs worked full-time in 74 high-risk units of the hospital, including the emergency departments, intensive care units, and infectious disease units. The HCWs were randomized to wear a regular procedural mask, cloth mask, or no mask throughout an eight-hour shift for four weeks. Then, they were followed for an additional one week for signs of respiratory illness.

Either two procedural masks were provided to each worker per shift, or five cloth masks were provided per month, which were to be washed and rotated throughout the four-week study period. HCWs also were asked to wear their masks throughout their shifts, except for tea, meal, and bathroom breaks. Each HCW maintained a diary of the number of patient contacts per shift and their activities, including suctioning, sputum induction, intubation, and bronchoscopy. At the first onset of symptoms, HCWs reported for evaluation and respiratory polymerase chain reaction (PCR) panel testing. HCWs logged an average of 36 patient contacts per day. Those with cloth masks reported they washed their masks 23 of 25 days, either at home (80%), using the hospital laundry (4%), or both (16%).

Despite the fact that workers reported better compliance with cloth masks, an intent to treat analysis showed rates of clinical respiratory illness, influenza-like illness (ILI), and laboratory-confirmed respiratory infection were significantly higher in the cloth mask group vs. the procedural mask group. Laboratory confirmation of respiratory viral infection was detected in 31 of 659 cloth mask users vs. 19 of 580 medical mask users. The relative risk of ILI in the cloth mask group, compared with the other groups, was 13.25. Surprisingly, no significant difference was observed in the rates of infection between those wearing medical masks and those without masks. The reported frequency of hand washing was found to be significantly protective against clinical respiratory illness.

In the laboratory, the penetration of particles through cloth masks was significantly higher (97%) than with either medical masks (44%) or N95s (< 0.01%). In addition to barrier protection, it is conceivable other factors may be responsible for this observed difference in the risk of respiratory illness. Workers may re-adjust cloth masks more often than medical masks. Certain types of cloth used for such masks may be better at “acquiring” viral particles as workers move through their day, or the warm moisture from breathing may allow improved virus survival on masks.