Today, moderator Dr. Bob Wachter kicked off Grand Rounds with two questions that for months have preoccupied everyone from the world’s top epidemiologists to the humble grocery store clerk: How does the coronavirus spread, and what are the most effective ways of stopping it from spreading? 

To answer these questions about a virus that “continues its rampage through towns and cities” across the world, Wachter recruited a team of experts to talk about three critical topics: aerosols vs. droplets, the importance of masks, and the effectiveness of face shields.

Aerosols vs. droplets

The difference between the two transmission modes of respiratory viruses, it turns out, is nothing to sneeze at.  Dr. Don Milton, professor of Environmental & Occupational Health at the University of Maryland School of Public Health, noted the three ways in which the virus can spread: contact (touching one’s eyes, nose or mouth after coming in contact with the virus), splash and spray (droplets landing on the eye, nose, or mouth) and inhalation of particles varying in size.

Different viruses bind to and infect different sites in the body. Moreover, particle size becomes important, because smaller particles travel farther into the respiratory tract. Aerosols are much smaller, travel farther, and remain in the air much longer. 

What Milton said next was chilling, at least in Wachter’s eyes. With indoor fans or air conditioning,  “droplets can travel much further than two meters.” Though no one has cultured SARS-CoV-2, the virus that causes COVID-19, from the air, Milton discussed one case analysis of a restaurant outbreak that infected nine people at various tables. The study by infectious disease experts from Hong Kong University indicated that airborne transmission can occur between people sitting up to five meters apart.

“Don, you’ve got me scared that we blow the virus around and instead of it only traveling three or four feet, it’s now going to travel like a football field,” Wachter said. “So is ventilation good or bad, or is it both?” 

Milton answered that it “cuts both ways.” Blowing unfiltered air around a room with a fan won’t protect from the virus; there must be ventilation to allow airflow out of the building. 

Milton segued nicely into the next presentation on masks by giving some much-needed hope, noting that one study in Hong Kong of masks and COVID-19 seemed to indicate that “what works with flu is working here with coronaviruses.” One of the key roles of masks, Milton continued, “is that it reduces the amount of respirable aerosols and pretty much eliminates the larger aerosols and splash and spray generation from infected persons.” 

Masks and asymptomatic patients

Dr. Monica Gandhi, UCSF professor of Medicine and director of the UCSF Center for AIDS Research, was up next to give listeners the lowdown on mask use, especially regarding asymptomatic spread. 

One of the big surprises of Gandhi’s presentation is the fact that 40 percent of those who have COVID-19 are likely asymptomatic, according to “really well-designed studies,” such as the mass testing in the Mission District. This is important, because Gandhi contends that COVID-19 may be dose-dependent, meaning that the dose of virus you are exposed to determines how sick you become. 

She cited a study done with hamsters infected with COVID-19 that simulated masking using surgical mask barriers. The study found that “masked” hamsters were less likely to acquire COVID-19 and that those who did become infected experienced a mild form of the virus. 

Though ethical reasons rule out formalized experiments with humans to test that claim, Gandhi argued that cruise ships that experienced outbreaks are “nice” substitutes. On an Argentinian cruise, for example, all passengers wore surgical masks and all crew members wore N95s. Though 128 out of all 217 passengers got sick, 81 percent of the infected people were asymptomatic. Researchers observed a similar phenomenon in outbreaks in meat processing plants in Oregon and Missouri that had major outbreaks with 95 percent of infected workers (all of whom wore masks to work) remaining asymptomatic. 

The universal masking policy has been in place in San Francisco since April 17. A Bay Area lab processed 30,000 tests from last week and found that most people who tested positive were asymptomatic or had very few symptoms. Gandhi contends that universal masking may contribute to milder disease, and also noted that there has only been one additional death in San Francisco since June 27, although there have spikes in the number of cases.

Gandhi also said that study-based support for masking grew even larger within the last few days. Recent studies bolstered the strength of claims that masking reduces infection rates, including one study of healthcare professionals in Boston that was published in the Journal of the American Medical Association. The Centers for Disease Control’s director also said if everyone wore masks, the pandemic could be under control in a matter of four to eight weeks.

Ending the pandemic could ultimately come down to behavioral change, Gandhi said. Just like closing down bathhouses in San Francisco and telling people to be abstinent did not stop the AIDS epidemic, Gandhi noted that lockdowns are a “blunt instrument” to stop the spread of the COVID-19. “The behavior change was something that was an individual decision that needed to be made,”  she said. In the 1980s, it was encouraging people to wear condoms. Today, it is telling people to wear masks. 

There is one more kind of PPE that individuals have been wearing to protect their faces, and Associate Chief Medical Officer and professor at University of Iowa Dr. Michael Edmond zoomed in to talk all about it: face shields. 

Face shields

Edmond said that “before March 1 of this year, I probably had not thought about face shields for more than 30 seconds.” But as the coronavirus situation became more serious and mask supply lines were disrupted, Edmond and the University of Iowa hospital that he worked at began collecting face shields for staff to wear, along with masks, as they interacted with patients. By March 18, all hospital employees had face shields. 

These devices are “really simple,” in Edmond’s words, with only three parts: a clear visor to block the face, the frame this is attached to, and suspensions to adjust the fit. Edmond described the advantages of face shields, which are often more comfortable than masks, less hot or claustrophobic and easy to disinfect and breathe in. They are also helpful for people who are hearing impaired and need to see others’ mouths to lip-read, and they prevent the wearer from touching their face. 

After his hospital adopted a face shield policy, Edmond started to think about expanding face shield use in the community, hypothesizing in a JAMA paper that universal adoption of face shields combined with hand washing, testing, and contact tracing could drive R0, pronounced “R naught,” below 1. The R0 value is the average number of people who will contract a disease from someone with the disease. It’s currently 2.5 for COVID-19, according to Edmond. (The SF Health Director, Grant Colfax, said Wednesday that San Francisco’s reproductive number was 1.3.)  

Edmond cited some simulation experiments that found a face shield resulted in a 96 percent reduction in the spread of flu virus at 18 inches with 8.5 micron aerosol. For comparison, a micron is 0.00004 inches. He also said that the primary driver of COVID appears to be droplet transmission and that this would support face shield use. 

“This whole controversy about whether it’s airborne or whether it’s droplet is somewhat driven by the framework through which you look at the issue,” Edmond said: an occupational health framework or a public health framework.

Occupational healthcare providers will be looking at efficacy to provide ideal protection from the virus, while public health officials tolerate more risk with the ultimate goal of stopping the outbreak, said Edmond. Recommendations for shields versus masks hospitals and in the community will be different for these reasons. 

Edmond ended his presentation with a salient answer to Wachter’s original question with how to stop COVID-19’s spread: “Particularly since we have 25 states now that still have no mask mandate, I think the best face covering is the one that people will wear.”