Masking Yields Small Reduction in COVID Risk, Review Concludes

Use of masks in both community settings and healthcare settings may slightly reduce risk of COVID-19, but the evidence base leaves much to be desired, the final installment of a living, rapid review of multiple studies found.

"The strength of evidence remained low for reduced risk for SARS-CoV-2 infection with surgical masks versus no mask on the basis of two prior randomized controlled trials (adjusted prevalence ratio 0.89, 95% CI 0.78-0.997 and OR 0.82, 95% CI 0.52-1.23) and two observational studies and insufficient for N95 respirators versus no mask or cloth mask versus no mask," wrote Roger Chou, MD, and Tracy Dana, MLS, both of Oregon Health & Science University in Portland, in the Annals of Internal Medicine.

"There were no new studies and insufficient evidence for surgical versus cloth masks and N95 versus surgical masks," they added.

"Despite focusing on higher-quality studies, the evidence base continues to have important limitations," Chou and Dana noted. "Randomized controlled trials (RCTs) were few and had some imprecision and methodological shortcomings. In addition, RCTs evaluated interventions to promote or encourage mask use and were designed pragmatically, improving applicability but potentially attenuating estimated effects due to suboptimal adherence and crossover."

Ninth in a Series

This article was the ninth in a series of reviews; the first eight looked at a total of two RCTs and 10 observational studies, Chou and Dana explained. "While conducting Update 8, we were aware of a completed RCT of N95 versus surgical masks and planned a final update after its publication. The purpose of this update is to incorporate this RCT and other new studies."

In all, three RCTs and 21 observational studies were included.

The authors were seeking answers to two questions: What is the effectiveness and comparative effectiveness of respirators (N95 or equivalent), face masks (surgical), and cloth masks in addition to standard precautions in community and healthcare (high- or non-high-risk) settings for prevention of SARS-CoV-2 infection? And what is the evidence for extended or reuse of N95 respirators for prevention of SARS-CoV-2 infection?

To get at the answer, they searched PubMed, MEDLINE, and Elsevier EMBASE for relevant studies; they also searched medRxiv and reviewed reference lists of relevant articles. Searches for the last update were done from June 2022 to January 2023.

Although they found no new studies of mask use versus non-use, one good-quality Danish RCT (n=6,024) found that a recommendation for mask use was associated with a small but statistically insignificant reduction in COVID risk on the basis of antibody testing, polymerase chain reaction (PCR) testing, or hospital diagnosis at 1 month (1.8% vs 2.1%; OR 0.82, 95% CI 0.54-1.23).

There were no differences in mask effects based on age (≤48 vs >48 years), sex, or daily time outside the home (≤4.5 vs >4.5 hours), Chou and Dana reported. "In addition, mask adherence was suboptimal (46% as recommended, 47% predominantly as recommended), and high implementation of other infection control measures could have attenuated benefits."

Benefits of Masking

In the studies related to community settings, "observational studies of masks versus no masks consistently found masks associated with decreased risk for SARS-CoV-2 infection but had methodological limitations and some imprecision," the authors wrote. "The evidence on surgical versus cloth masks or more versus less consistent mask use remained insufficient."

In the healthcare setting, "a new RCT found that effects of instruction to use surgical masks were noninferior to instruction to use N95 respirators for routine patient care," they added. "However, noninferiority was defined as less than a doubling of risk, with the CI consistent with up to a 70% increase in risk. Due to a single trial with imprecision, the strength of evidence was low. In addition, the RCT could have reported attenuated benefits of N95 respirators in the healthcare setting due to infections acquired in the community or home."

The researchers listed several limitations to their review. In addition to not attempting meta-analysis, "we did not formally assess for publication bias due to heterogeneity and few studies for most comparisons," Chou and Dana wrote. "We restricted inclusion to English-language articles and excluded ecological studies and studies on mask policies that did not provide information on individual mask use, which may provide complementary information."

"Additional research would further clarify the comparative effectiveness of masks for prevention of SARS-CoV-2 infection," they added. "Future studies should have adequate statistical power for primary as well as stratified analyses. Assessing masks as source control represent a challenge, requiring evaluation of SARS-CoV-2 infections in communities of masked and unmasked persons. Studies should use appropriate methods for diagnosing SARS-CoV-2 infection, describe key mask characteristics, evaluate adherence, and assess harms as well as benefits."

In an accompanying editorial, Annals editor-in-chief Christine Laine, MD, MPH, and deputy editor Stephanie Chang, MD, MPH, said that many questions about masks remain unanswered.

"There is evidence that masks could work to prevent COVID-19, but substantial gaps remain about whether they do work and under what conditions," they wrote. "In the face of future viral outbreaks, we must move quickly to fill the gaps with timely studies that avoid the limitations of studies done to date. We may never reach definitive truth about when the benefit of masking interventions outweighs the harms, but it is imperative that we design studies that fill information gaps, interpret the evidence accurately, and are honest about what we do and do not know with certainty when making decisions and setting policy about masking."

Masking in Health Settings Urged

In an accompanying opinion piece, Tara Palmore, MD, of George Washington University in Washington, D.C., and David Henderson, MD, of the NIH, said it would not be wise to discontinue masking in healthcare settings.

"Real-world experience shows the effectiveness of mask wearing in clinical settings," they noted. "Thanks largely to universal masking and use of other personal protective equipment, healthcare personnel have been at far greater risk for acquiring COVID-19 from community than occupational exposures. Transmission from patient to staff and staff to patient when both are masked does occur but is uncommon."

"Presenteeism" -- coming to work even when ill -- is a well-known problem among clinicians, they added. In an unpublished study done at the NIH that involved contact tracing interviews and subsequent COVID testing among staff who tested positive, "more than 50% subsequently acknowledged having had some symptoms characteristic of COVID-19 at the time of testing," Palmore and Henderson wrote. "These data underscore the problem of presenteeism and emphasize the appropriateness of masking."

However, even though these clinicians tested positive, "no transmission to patients was identified" at the institution, which mandates masking for clinicians and patients, they added.

"In our enthusiasm to return to the appearance and feeling of normalcy, and as institutions decide which mitigation strategies to discontinue, we strongly advocate not discarding this important lesson learned for the sake of our patients' safety," they concluded.