With the two RCTs that directly tested source control providing essentially no support for the claim that wearing a mask benefits others, what about RCTs that test the combination of source control and wearer protection? By dividing participants into a hand-hygiene group, a hand-hygiene group that also wore masks, and a control group, three RCTs allow us to see whether the addition of masks (worn both by the sick person and others) provided any benefit over hand hygiene alone.

A 2010 study by Larson, et al. in New York found that those in the hand-hygiene group were less likely to develop any symptoms of an upper respiratory infection (42 percent experienced symptoms) than those in the mask-plus-hand-hygiene group (61 percent). This statistically significant finding suggests that wearing a mask actually undermines the benefits of hand hygiene.

A multivariable analysis of this same study found a significant difference in secondary attack rates (the rate of transmission to others) between the mask-plus-hands group and the control group. On this basis, the authors maintain that mask-wearing “should be encouraged during outbreak situations.” However, this multivariable analysis also found significantly lower rates in crowded homes—“i.e., more crowded households had less transmission”—which tested at a higher confidence level. Thus, to the extent that this multivariable analysis provided any support for masks, it provided at least as much support for crowding.

Two other studies found no statistically significant differences between their mask-plus-hands and hands-only groups.