How to Judge COVID Risks and When to Wear a Mask
Scientific American asks experts in medicine, risk assessment and other fields how to balance the risks of COVID with the benefits of visiting public indoor spaces
- By Devabhaktuni Srikrishna on
As COVID cases declined across the U.S. in recent months and mask mandates were lifted, more people returned to restaurants, concert halls and offices maskless. But the novel coronavirus’s Omicron subvariant BA.2—which caused another wave in Europe and China—and related variants threaten to reverse that progress here. Earlier this month dozens of attendees (including high-ranking government officials) tested positive for COVID after attending a dinner in Washington, D.C. The safest option, of course, is to continue avoiding crowded indoor activities. But there remains a lot of interest in safely enjoying bars, cafes and other higher-risk venues that offer the benefits of social interaction.
Scientific American asked experts in epidemiology, medicine, risk assessment and aerosol transmission for advice on how to decide which risks we are willing to take. These decisions are based on assessments of personal risk, community risk and exposure risk—and the steps one can take to take to mitigate them. Personal risk refers to the danger of contracting COVID faced by an individual and the members of their household. Community risk is the current likelihood of encountering COVID among members of one’s community. And exposure risk accounts for the increased chances of catching COVID at a particular venue based on airflow characteristics of the space itself and other people’s behavior.
Here is what experts say about managing these risks while maintaining some of the benefits of public life.
How should a person factor personal risk for severe COVID into their decisions?
The number-one predictor of having a severe case of the disease is age, followed by the presence of comorbidities and immunocompromised status, according to Katelyn Jetelina, an epidemiologist who studies COVID risks at the University of Texas Health Science Center at Houston. Using data from the U.S. Centers for Disease Control and Prevention, she estimates that even vaccine-boosted people ages 50 to 64 are more than 10 times more likely to die from a severe breakthrough case than 18- to 49-year-olds with the same vaccination status. Donald Milton, a physician and clinical researcher who studies respiratory viruses at the University of Maryland, highlights recent research showing that, in households with a person who was infected with the Omicron variant (B.1.1.529) of the COVID-causing virus SARS-CoV-2, 43 to 64 percent of people became infected as well, depending on whether the initially infected person was boosted, fully vaccinated or unvaccinated. Jetelina cautions that we also need to account for the personal risks of the people with whom we live in our own risk assessments.
In general, people should discuss personal COVID risk with their doctor; it depends, in part, on which medications they take. Ethan Craig, a rheumatologist at the University of Pennsylvania, cares for patients who are immunosuppressed because of disease or medication and studies COVID risks in that population. One such immunosuppressive drug, rituximab, “knocks out your ability to make antibodies against new viral exposures and impairs your ability to make a response to a vaccine,” he says. Craig adds that such patients usually take precautions of their own accord, such as wearing high-filtration N95 masks, and “if anything, I end up having to talk people down sometimes and be like ‘Look, it’s okay to go to the grocery store.’” For some people, however, even this amount of exposure could be considered an unacceptable risk.
How does the risk of dying from COVID compare to the risk of dying from other causes linked to common activities?
Jetelina estimates that, for people between the ages of 18 and 49 who are boosted, the risk of dying from COVID is roughly equal to the risk of dying when someone drives about 10,000 miles. COVID risk goes up substantially with age and with being unboosted or unvaccinated. Thanks to vaccines, infection-induced immunity, therapeutics, better care and other factors, the relative risk of dying from COVID if you catch it is now, broadly speaking, comparable to that of seasonal flu, Jetelina says—but importantly, because you are more likely to catch COVID than flu, the absolute risk remains much greater. Jetelina recommends COViD-Taser’s Relative Risk Tool, a resource funded by the National Science Foundation, that she helped to develop. It compares one’s risk of death from the disease to such risk posed by other activities, including driving. Although it is a research tool, Jetelina says she can “really trust the science and mathematics behind it.”
But Baruch Fischhoff, a professor of engineering and public policy at Carnegie Mellon University and an authority on how to communicate health risks, cautions against using risk-risk comparisons to make choices without fully considering benefits or unquantified risks. Employers may also misuse such comparisons to compel employees to accept certain risks on the job, which is not exactly a choice. Currently, risk calculators provide estimates based on retrospective data and may be unable to reliably weigh long-term complications of COVID.
How should one assess community risk?
There is no perfect way to measure community risk because it would take repeated random testing, so experts use other estimates: daily cases per 100,000 residents, test positivity rates and growth rates. Jetelina recommends using the New York Times’ tracker to look up community transmission for your county. She considers community risk high when there are more than 50 weekly cases per 100,000 residents. When the risk is lower than that, Jetelina—a healthy, young boosted person—feels comfortable taking off her mask indoors. “I will say it’s taken a lot of time for me to be comfortable with that,” she says. “Once transmission rates of those indicators start increasing a bit, I’m putting my mask back on.” Others suggest a slightly higher risk threshold of 10 daily (or 70 weekly) cases per 100,000 residents.*
Daily city or county case counts are often an undercount because not everyone is getting tested and home test results are not always reported. As a work-around, health authorities use the “test positivity rate,” or “percent positive”—the percentage of COVID tests reported to public health authorities that were positive. If that number exceeds 5 percent, it is widely considered high risk for community transmission (provided the amount of testing in that area is adequate). But the community sample used to measure test positivity likely includes many people who seek out testing because they are currently experiencing COVID symptoms. So test positivity is typically higher than the infection rates among the people you might encounter in a cafe or grocery store, most of whom do not have any symptoms but could still be infectious.
Still, Robert M. Wachter, a professor and chair of the department of medicine at the University of California, San Francisco, says there is no test positivity threshold that separates “safe” from “not safe” because it also depends on other factors, such as whether the benefit outweighs the risk to you, personally, the number of people you will be exposed to, and the closeness and duration of exposure.
Because of these large uncertainties in test coverage, Gerardo Chowell, a professor of mathematical epidemiology at Georgia State University, prefers to look at the general trend in daily COVID cases, hospitalizations and deaths, or percent positive. “When the trend is going up, you’re seeing the transmission chains expand,” Chowell says. “That means that the reproduction number”—the expected number of secondary infections from each infected person—“must be greater than one. If it is increasing, that’s probably the time when [one has the] highest risk of acquiring COVID in a social setting without a mask,” he says.” Wachter points out that, where available, wastewater surveillance may also give an early indication of COVID trends.
What is known about exposure risk in different settings, such as bars or movie theaters?
Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and one of the world’s leading experts on airborne transmission of viruses, says COVID risk in indoor spaces exists on a continuum. It is believed that reducing the amount of virus inhaled (i.e., the inhalation dose) makes infections less likely or illness less likely to be severe. Marr says one of the riskiest settings is an aerobic exercise studio: if somebody is infected, they are going to be exhaling more virus, and everyone else will be inhaling at a faster rate, too. Breathing heavily produces up to 10 times more aerosol particles that carry viruses than breathing normally, according to Richard Corsi, an expert on indoor air quality and dean of the College of Engineering at the University of California, Davis.
Marr says that talking in bars expels a similar number of respiratory particles as coughing, “so it’s like everyone’s in there coughing together.” Craig uses smoking as an analogy for aerosols exhaled during breathing and talking. In other words, “if a person was smoking in this place, would I be able to smell it?” he says. In movie theaters, there is risk of exposure from those seated immediately around you, but because of limited talking and, typically, a high ceiling, there is a lot more dilution of the air. So such a theater may be less risky than other crowded indoor venues. By that reasoning, museums, big-box retailers and grocery stores with high ceilings tend to be relatively safer as well.
Places with rapid rates of ventilation and filtration—such as some subways—are also much lower risk. The Bay Area Rapid Transport (BART) system in San Francisco Bay, for example, filters the air more than 50 times an hour with “virus-trapping MERV-14 air filters” inside each car. An Italian study of schools found that classrooms with ventilation systems that exchanged air six times per hour reduced infections by more than 80 percent, but many classrooms in the U.S. fail to meet this standard. Corsi characterized current public health recommendations of four to six air exchanges per hour as “a little bit anemic … we can do better.” He recommends owners or managers of crowded indoor spaces, such as classrooms, offices and bars, aim to filter or ventilate with fresh air at rates approaching 12 air exchanges per hour to reduce risksdown to the level of an airborne isolation room in a hospital. Not all venues have the resources to do this, but the benefits increase with greater filtration rates, so the closer to this ideal, the better. In places with inadequate ventilation, consider bringing a portable high-efficiency particulate air (HEPA) purifier—or building your own using box fans and high-quality HVAC (heating, ventilating and air-conditioning) filters—to run nearby.
Although the virus is thought to be transmitted primarily through the air, there have been a few documented cases of surface transmission, so it remains a good idea to wash your hands frequently, Marr says.
How can one further reduce the risk of getting COVID from everyday activities?
Getting vaccinated and boosted protects against death, hospitalization and, to a lesser extent, catching and spreading the virus. To avoid infection, Wachter recommends wearing an N95 mask. He has observed that the risk of U.C.S.F. health care workers—himself included—getting infected from their patients while wearing a well-fitting N95 is extraordinarily low. These respirators get close to filtering all of the virus, but they do not filter 100 percent. And if an N95 does not form an airtight seal with your face, it may allow unfiltered air into your lungs. So it is essential to try out and select N95 models that fit and seal to your face without gaps.
What is the risk of taking your mask off in a restaurant or bar to take a sip or bite?
In the 1990s medical researcher Stanley Wiener, then at the University of Illinois College of Medicine, proposed that a person could use respirators to survive aerosolized biological attacks, taking it off briefly to consume food and drink. During the pandemic, many places have allowed masks (or N95 respirators) to be removed while actively eating and drinking. Removing an N95 momentarily for a bite or sip carries “some risk, but I think it’s pretty tiny if you’re exposed for three seconds,” Corsi says, unless an infected person is “right in your face ... and shedding a lot [of virus].” Provided community risk is low or trending downward, Chowell, too, feels comfortable briefly removing his respirator to eat or drink at a party.
What do we know so far about the risk of “long COVID”?
Ranu Dhillon, a physician at Brigham and Women’s Hospital in Boston, who advises governments on infectious disease outbreaks, says he is seeing some patients with “a constellation of different types of symptoms after acute COVID infection,” including young, boosted and relatively healthy people. Wachter cautions that some fraction of vaccinated individuals who get infected—which one study estimates to be around 5 percent and possibly higher—may continue to feel short of breath or fatigued or think less clearly than before. COVID may increase the risks of heart attack, stroke, brain abnormalities or the onset of diabetes. While there have been preliminary studies of the rates of long COVID, including risks of developing cardiovascular complications, Wachter says many of these involved unvaccinated people or infections with variants prior to Omicron. Provisionally, he likens these risks to 20 years of untreated high blood pressure or smoking and points out that one cannot know the risk of long COVID among vaccinated and boosted individuals until long-term studies have concluded, which will take years.
How can we balance these risks with the benefits of socializing and being with others?
According to Wachter, one of the most important factors in overall COVID risk is whether “the person next to me has it.” He acknowledges that if someone is both vaccinated and boosted, it is not irrational for that person to decide that the mental energy and angst of calculating risks and taking precautions is high enough—and the risks of getting sick or dying from COVID are low enough—that they will go back to “living like it’s 2019”—as people in many parts of the country already have. He still worries about the risk of long COVID, though. Milton says that many people “don’t want to wear masks forever” and that we should work to make our built environments better at stopping aerosol transmission. He says people also have to decide whether to wear a high-quality mask when they are around those at higher risk, such as the elderly or immunocompromised, or around other people in general, such as at a party. When community transmission is low, Chowell says he may feel comfortable removing his N95 at parties in some situations, such as to have a drink. “Then you find a way to still interact with people, and they smile back once in a while,” he adds.
*Editor’s Note (4/19/22): This paragraph was edited after posting to correct the description of the threshold of COVID transmission that Katelyn Jetelina considers a high community risk.