“Laboratory experiments provide good reason to believe that masks, especially N95s, can reduce the risk that someone will be infected or infect other people. But those experiments are conducted in idealized conditions that may not resemble the real world, where people often choose low-quality cloth masks and do not necessarily wear masks properly or consistently. Observational studies, which look at infection rates among voluntary mask wearers or people subject to mask mandates, can provide additional evidence that general mask wearing reduces infection. But such studies do not fully account for confounding variables.
If people who voluntarily wear masks or live in jurisdictions that require them to do so differ from the comparison groups in ways that independently affect disease transmission, the estimates derived from observational studies will be misleading. Those studies can also be subject to other pitfalls, such as skewed sampling and recall bias, that make it difficult to reach firm conclusions.
Despite those uncertainties, the CDC touted an observational study that supposedly proved “wearing a mask lowered the odds of testing positive” by as much as 83 percent. It said even cloth masks reduced infection risk by 56 percent, although that result was not statistically significant and the study’s basic design, combined with grave methodological weaknesses, made it impossible to draw causal inferences.”
“If wearing a mask had the dramatic impact that the CDC claimed, you would expect to see some evidence of that in RCTs. Yet the Cochrane review found essentially no relationship between mask wearing and disease rates, whether measured by reported symptoms or by laboratory tests. Nor did it confirm the expectation that N95s would prove superior to surgical masks in the field. The existing RCT evidence, the authors said, “demonstrates no differences in clinical effectiveness.””
“Does the Cochrane review prove that masks are worthless in protecting people from COVID-19? No. But it does show that the Centers for Disease Control and Prevention (CDC) misled the public about the strength of the evidence supporting mask mandates”