Do masks work?


Yes. Despite a recent high-profile review, high quality masks DO reduce transmission of COVID-19 (and other respiratory viruses).

A recent Cochrane review grabbed headlines for concluding there was “insufficient” evidence that masks are effective in reducing respiratory illness.

Several good articles have outlined the limitations of this review. Read the links below for detailed critiques, but we sum up a few key points:

➡️ Apples & Oranges. The study combined different types of masks (surgical vs. N95), continuous vs. infrequent usage of masks, and different settings (health care, communities, schools).

🌬️ COVID is primarily spread through aerosols–small particles that can linger in the air like smoke. Medical or surgical masks are designed to block droplets (especially the spittle that might come out of a surgeon’s mouth into an open incision). These masks leak air around the sides and are much less effective than N95s in preventing transmission by aerosols.

Combining studies of surgical masks and N95 respirators is combining apples and oranges.

As stated in the Conversation piece below, “If apples work but oranges don’t, combining all studies in a single average figure may lead to the conclusion that apples do not work.”

Similarly, it’s not surprising the using masks occasionally would show little effect, since it only takes one exposure to become infected. Healthcare settings with many sick people and opportunities for transmission are also very different from communities where virus circulation may be high or low during the study. When transmission is low, it’s very hard to show an effect of an intervention.

➡️ Almost all the studies reviewed were conducted pre-COVID (76 of 78 studies). COVID is MUCH more transmissible than influenza, meaning it’s harder to detect effects of prevention for the flu.

➡️ The study ignored other types of evidence that masks physically block aerosol transmission.

Randomized control trials (RCTs) are a great way to build scientific evidence for many questions. But they are not our only source of evidence.

🪂 The benefits of seatbelts, airbags, and even parachutes are trusted based on basic engineering and controlled laboratory experiments (those poor crash test dummies!).

We hope there is no imminent call for an RCT of seatbelts with placebo controls.

Similarly, we have lots of direct evidence of the ability of masks to block both exhalation and inhalation of aerosol particles that can carry viruses. (For an example, see the article at this link.)

Academic “meta-analyses” that combine results from many studies such as this Cochrane review are subject to many challenges. Measures and contexts that are not comparable can make these types of studies not very informative.

Masks (and the people wearing them) are not perfect, which makes the benefits difficult to measure in real world settings. Scientists look at multiple levels of evidence, and the physical evidence of well fitted masks blocking aerosols that carry viruses is strong.

Healthcare personnel caring for COVID patients are very mindful of proper respiratory protection, because the stakes are high, and it works. You can use the same technology to protect yourself and others.


Despite noise to the contrary, there is good evidence that masks are effective in reducing the risk of transmission and infection of COVID-19.


Those Nerdy Girls


Additional Links and Resources:

The Conversation: “Yes, masks reduce the risk of spreading COVID, despite a review saying they don’t”

“Wear a respirator, not a cloth or surgical mask, to protect against respiratory viruses” (includes critique of Cochrane paper)

“Don’t believe those who claim science proves masks don’t work”

What’s the best MASK to protect me from the Omicron variant?

Does my mask protect me if no one else is wearing one?

Science Brief: Community Use of Masks to Control the Spread of SARS-CoV-2

Link to original Those Nerdy Girls Facebook post