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Understanding COVID-19 risks on planes and public transit

By Jeff Levin-Scherz, MD, MBA | April 27, 2022

When a federal judge threw out the CDC’s mask mandate for planes and other transit, questions about safety ensued. Also news about a new antibody treatment and a look back on the Clean Air Act’s success.
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About the series

Dr. Jeff Levin-Scherz provides regular updates on the latest COVID-19 developments with a focus on the implications for employers and guidance on how they can tackle pandemic-related challenges to keep their workplaces safe. Explore the series.

Last week a federal judge threw out the public transportation mask mandate initiated by the Centers for Disease Control and Prevention (CDC) early in the pandemic. All major airlines immediately removed the mask mandate, as did Uber, Lyft and many other transit organizations. The federal government announced that it will appeal this ruling without asking for a stay, possibly to preserve the ability of the CDC to implement such mandates to combat future health emergencies.

I’ve heard a few noteworthy questions surrounding this ruling.

Can people get COVID-19 on planes despite their excellent ventilation and filtration?

Yes. There are multiple papers from the National Library of Medicine and JAMA Network documenting outbreaks of COVID-19 associated with air travel. Here is a good summary of this issue.

Planes usually exchange air over six times an hour and have excellent filtration, which reduces the risk of transmission from airborne viruses. But those sitting within two rows of people with infectious COVID-19 are at high risk of getting infected from droplets before they can be filtered out. Even though air exchange and filtration are good while in flight, many airlines turn off their air handling systems while sitting at a gate, so passengers boarding a plane go through congested jetways that have little air circulation.

Do masks really reduce the risk of infection while in transit?

Yes. Most of the large outbreaks associated with planes were early in the pandemic before masks were common or required, and lack of masks was identified as an independent risk factor for infection. Of course, the type of mask matters too. Single-layer cloth masks are much less effective at protecting the wearer than surgical masks. High filtration, well-fitted masks like KN95 and N95 masks provide the most protection. Masks that fasten behind the head are more protective than those with ear loops.

Has the risk of transmission increased with the removal of the mask mandate on planes and subways?

Not as much as you might think. The mask mandate was issued before we fully understood transmission and didn’t require using a high quality mask. Further, in recent weeks many passengers were unmasked or using masks as “chin diapers.”

Here's data showing the average amount of time it takes to transmit an infectious dose of the coronavirus if two people are six feet apart.

COVID-19 transmission and mask usage

Source: American College of Government Industrial Hygienists. See also Wall Street Journal
Person not infected is wearing:
Person infected is wearing: Nothing Cloth mask Surgical mask N95 respirator mask
Nothing 15 mins 20 mins 30 mins 2.5 hours
Cloth mask 20 mins 27 mins 40 mins 3.3 hours
Surgical mask 30 mins 40 mins 1 hour 5 hours
N95 respirator mask 2.5 hours 3.3 hours 5 hours 25 hours

You’ll notice that the difference in time to infection in the first row of the table (infected person without a mask) and the second row (infected person wearing a cloth mask) is modest. Surgical masks or respirators provide more protection. Individuals who want to protect themselves from COVID-19 infection would be safer wearing N95 masks with others being maskless than wearing cloth masks or surgical masks if others were doing the same.

Despite the movement on mandates, there is still ample reason to mask when in crowded indoor places and the amount of community spread is high. Those who are at higher risk or more cautious can protect themselves effectively with an N95 or a KN95 mask, and both are now readily available.

Will this ruling encourage employers to remove mask mandates in workplaces not affected by this ruling?

I don’t think so. WTW’s mid-March Emerging Trends Survey showed that most employers had already removed mask requirements, and more than two-thirds of employers who maintained a mask mandate were likely to remove it before the end of the year. Many companies that continue to report having mask mandates are healthcare facilities, which often provide care to those with compromised immune systems.

WTW found 62% of employers have already removed mask requirements. Only 14% have mask requirements in place with no end date determined.
Employer onsite mask mandate

Source: WTW 2022 Emerging Trends in Healthcare Survey. Note: Numbers do not add to 100% due to rounding

Should you wear a mask on your next flight?

Many people should continue to wear masks, and everyone should feel comfortable masking. Masks are especially important for the immunocompromised, for the elderly and for pregnant women. I will continue to wear a comfortable N95 mask when I’m in transit, since there is still substantial community transmission of COVID-19, and there are many points of a journey where the risk of infection could be high.

Evusheld provides impressive protection to the immunocompromised

Evusheld, a combination of the monoclonal antibodies tixagevimab and cilgavimab, proved both safe and effective in a randomized trial of over 5,000 immunocompromised adults published recently by the New England Journal of Medicine.

Those who received Evusheld had a 0.2% chance of getting COVID-19 over a six-month period, whereas those who received the placebo had a 1% chance of getting COVID-19. All severe cases and both deaths in this trial were in the placebo group. There was no difference in side effects between Evusheld and placebo.

This is good news for employers as they continue to work on ways to foster a safe workplace and protect employees with impaired immune systems, even if others are not wearing masks. Those who are immunocompromised should:

  • Still be vaccinated
  • Wear masks in close quarters, such as airplanes
  • Get tested if they have symptoms and be treated promptly with the antiviral Paxlovid to further reduce their risks

Employers should note that while Evusheld and other COVID-19 treatments are now paid for by the federal government, in the future they will be covered by employer-sponsored health insurance. The cost per dose of Evusheld is not yet known.

Insights from wastewater surveillance

We know that published case counts of COVID-19 at this point are far less reliable, as many are diagnosed through home testing, which is not reported to state authorities. A reliable measure of community risk is to sample wastewater, which through previous waves of infections has closely tracked community infection rate. Indeed some universities and medical centers are calibrating their pandemic response based on wastewater monitoring. Other countries have used wastewater surveillance to monitor for infectious diseases like polio for decades.

The CDC now reports on virus detection in wastewater, although it is currently reporting only the change from the previous time period, which is less valuable than knowing the actual amount of virus detected. The CDC has also provided many local sewer systems with funding to begin wastewater surveillance reporting, although much of the country is still not covered.

Wastewater allows genetic analysis and confirms the dominance of the Omicron strain BA.2 in every region. We should expect a new wave if wastewater viral count levels increase rapidly as they did in December when the Omicron wave began.

Regional data from BioBot, one of the larger companies performing wastewater surveillance, shows a substantial increase in viral counts over the past few weeks, although the Northeast recently stabilized. Viral counts of near 500 copies/ml are still well below the January peak of about 6,500 copies/ml.

While viral counts continue on an upward trajectory in the Midwest, South and West, the Northeast’s counts seem to have leveled off.
Wastewater surveillance by region, last six weeks

Source: BioBot April 2022

Wastewater surveillance is by no means perfect. Additionally, it is expensive especially for small rural systems, and with small populations there could be more random variation. But wastewater surveillance is another important tool for employers to use to monitor for new clusters and outbreaks.

Celebrating Earth Day with cleaner air

Last week was the 52nd Earth Day. While it sometimes seems like environmental maladies keep piling up, we can celebrate how much cleaner our air is than it was in April 1970, and how much this has improved our health and lowered healthcare costs.

A colleague who lives in Los Angeles told me that he was surprised when he moved back to Southern California in the early 2000s that he could see mountains from the city. They were previously obscured by smog. All of us who run, jog, walk or cycle outdoors benefit from lower levels of air pollution.

The Clean Air Act was projected to avert 230,000 premature deaths a year in 2020 – largely by preventing heart attacks, asthma attacks, chronic obstructive lung disease and prematurity, which leads to a high amount of mortality and a substantial number of years of potential life lost. The EPA estimated that the 1990 Clean Air Act Amendments prevented 17 million lost workdays annually in 2020.

Despite the successes of the Clean Air Act, there is plenty more work to do. Air pollution is worse in poorer neighborhoods and in communities of color. The World Economic Forum reports that air pollution continues to cost $820 billion in extra medical costs annually in the U.S. alone.

Previous research showed that life expectancy is cut short by four months in the U.S. by air pollution, far less than the loss of 1.9 years of life expectancy from air pollution in Egypt. The work that employers are doing globally to reduce energy consumption and decrease carbon emissions will continue to make our air cleaner, improve our health and decrease medical costs.


Population Health Leader, Health and Benefits, North America

Jeff is an internal medicine physician and has led WTW’s clinical response to COVID-19 and other health-related topics. He has served in leadership roles in provider organizations and a health plan and is an Assistant Professor at Harvard Chan School of Public Health.

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