When will we start dropping masks?
CLINICAL MATTERS
One contentious point of discussion throughout the whole pandemic has been the issue of masks. At the start of the pandemic, the prevailing scientific understanding of what was then called nCoV (short for novel coronavirus, now formally named SARS-CoV-2), was that people were only contagious when they had symptoms. This was in line with the scientific understanding of the behavior of its sibling virus SARS-CoV, the causative agent of Severe Acute Respiratory Syndrome (SARS). Many papers were written on the possible role of asymptomatic transmission, with no clear consensus at the outset. It was also thought that the major route of transmission for SARS-CoV-2 was short respiratory droplets and not airborne. It was acknowledged, however, that airborne transmission could occur, especially at hospitals when aerosol-generating procedures such as nebulization were done.
Due to the uncertainty of asymptomatic transmission as well as the tenuous supply of masks and other forms of personal protective equipment, the advice from the World Health Organization was that only symptomatic people should wear masks. This advice was changed later when it became apparent that asymptomatic transmission was occurring. The WHO however maintained that most transmission occurs at close range, within three to six feet, and that fabric or surgical masks were suitable for general public use. Only healthcare workers needed to use N95 respirators when taking care of COVID-19 patients. Things became contentious when a group of aerosol scientists pronounced that COVID-19 was airborne and that the general public should also be wearing N95 or KN95 respirators instead.
As more data emerged, guidance changed, especially as some variants of concern became more contagious. One major source of misunderstanding is that specialized terminologies significantly differ between infectious diseases clinicians and aerosol scientists. This discrepancy further contributed to the confusion. Without a consensus on what constitutes “droplet” and “airborne,” there was bound to be significant disagreement. Anti-maskers were quick to exploit this and used the uncertainty to question the effectiveness of masks in general.
There are two ways that masks protect people. The first is that it prevents outward movement of infectious particles from the source, also known as the “emitter.” The second way it protects is by preventing deposition and inhalation of those infectious particles by the healthy person, or “receiver.” When only symptomatic people are releasing infectious particles, it makes sense that only those “emitters” should wear a mask.
Historically, surgical masks were always meant to prevent infection from the source. Its utility in preventing deposition or inhalation of infectious particles has been less certain compared to medical grade PPE such as purpose-fitted N95 respirators. There wasn’t strong evidence that surgical masks had a significant effect on protecting the receiver vis-à-vis its known role in preventing emitters from discharging infectious particles.
One difference that further contributed to the confusion is that aerosol scientists define anything that is inhaled as “airborne,” and inhaled droplets as aerosols. Any large particles that emanate from coughing or sneezing are called “ballistic” droplets and are only considered infectious when these are deposited directly onto the eyes, nose, or mouth.
In the realm of infection control, however, there are only three categories of infectious diseases precautions:
- Contact precautions – refers to measures against infections that can be spread by touch. This means wearing gloves and gowns when interacting with a patient.
- Droplet precautions – refers to measures against infections that can be transmitted by respiratory droplets within three to six feet. Unlike in the realm of aerosol scientists, short-range respiratory droplets can either be inhaled or “ballistic.” This is a major point of disagreement because aerosol scientists assert that even short-range aerosol particles are airborne while infection control still considers droplet precautions as adequate protection for these. A surgical mask plus contact precautions is used when interacting with these patients within three to six feet.
- Airborne precautions – refers to measures against infections that can be transmitted by long-range aerosols beyond six feet. An N95 or other high efficiency respirator is needed as protection. There are traditionally only three diseases that fit this category in infectious diseases: measles, chicken pox, and tuberculosis. N95s have been used for COVID-19 patients especially in the hospital because there are many aerosol-generating procedures done on COVID-19 patients, including nebulization and the use of high flow nasal oxygen.
Cognizant of these disparate definitions, the WHO and the US Centers for Disease Control and Prevention (CDC) have updated their definitions to reflect the presence of short-range and long-range aerosols. The precautions to prevent transmission as defined by infection control principles, however, have not reflected these changes. The CDC continues to assert that the infection control measures as currently defined remain valid in protecting wearers within the distances that were originally proposed. Healthcare workers are advised to use N95 respirators when caring for known or suspected COVID-19 patients because of potential aerosol generation in the hospital. For the general public, the advice is to wear the best mask you can tolerate, recognizing that there are cost and comfort implications when wearing masks, which can affect their efficacy.
A recent paper from the CDC looked at real-world efficacy of masks versus N95 respirators while indoors (https://www.cdc.gov/mmwr/volumes/71/wr/mm7106e1.htm). The study authors found that masks, whether surgical masks or N95s, can reduce the risk of getting COVID-19 indoors by more than half compared to not wearing a mask. A medical grade mask (but not a cloth or fabric mask) decreased the risk of getting COVID-19 by 56 percent when used all the time indoors.
While there was a trend toward better protection by N95 over surgical masks, this effect is dependent on wearing the mask correctly and consistently. Due to variability in the use of masks among different people, the study reported a 95 percent confidence interval. This is a range of values where if the study was repeated 100 times, 95 percent of the point estimates (known as odds ratios) would fall. This confidence interval is useful to gauge the strength of the association between masking and prevention of disease.
The 95 percent confidence intervals of the point estimates for the N95 and surgical masks overlap. This means that a properly used surgical mask can equal or exceed the efficacy of an N95 if it is worn correctly, while a poorly worn N95 may be no better or even worse than a properly worn surgical mask. In general, a N95 has the potential to better protect against COVID-19 than a surgical mask (83 percent versus 66 percent), but this is not statistically significant because their confidence levels cross (N95 95 percent confidence interval is 0.05–0.64 versus surgical mask 95 percent confidence interval is 0.13–0.90). A comfortable surgical mask that is always worn may be just as or more effective than an N95 that is ill-fitting, uncomfortable, and removed constantly.
Proper use and comfort of the mask is a major factor in protection, hence the CDC recommendation to wear the best mask that you can tolerate and will wear consistently. There have been many studies that show long-term use (a full eight-hour shift) of N95 respirators is difficult to tolerate among a majority of healthcare workers. This can significantly affect its protective effect. In the CDC study, protection was only significant for “always” wearing the mask, while for even “most of the time” and “some of the time,” the protection was no longer statistically significant.
To summarize, during times of high community transmission, the general public should wear the mask that is most protective that one can tolerate and afford. This should be done, especially indoors and in high-risk settings. Healthcare workers in hospitals should still wear N95s when caring for COVID-19 patients. We need to find solutions for better protection of our healthcare workers, however, if they cannot tolerate N95s for long periods of time. These include the use of powered air purifying respirators and other technological innovations.
In the meantime, many countries are starting to ease mask mandates as they transition from a pandemic to an endemic mindset. In the Philippines, the near-term transition period will likely still include masks to give our healthcare system an additional safety net if cases spike as we continue to relax restrictions. This is not forever, however, and should eventually be eased if cases remain manageable. The pandemic has taught us that masks are important, and these lessons will be applied moving forward especially in healthcare settings. Having said that, I can’t wait to remove mine once this is over.