Droplet or aerosol infection spread? Surgical mask or respirator? Asymptomatic transmission or symptomatic only?

These are the defining questions surrounding the person-to-person transmission of the novel coronavirus disease COVID-19 that have yet to be answered—or answered sufficiently—for US experts monitoring the disease. And the answers hold the key to infection control policies that will become necessary as the disease continues to infect tens of thousands in China and hundreds more across the globe.

Yesterday the media reported that a Japanese doctor who was monitoring cruise ship passengers quarantined aboard the Diamond Princess in the port of Yokohama contracted the virus despite wearing gloves and a surgical mask. He joins the more than 500 healthcare workers in Wuhan, China, who have also reportedly contracted the disease—just by the middle of January.

These infections put into question how effective infection control policies are at preventing transmission.

Masks vs respirators—key differences

In its newly published infection prevention and control recommendations for COVID-19, the US Centers for Disease Control and Prevention (CDC) recommends the use of N95 respirators in a healthcare setting with the suspected novel coronavirus, but the World Health Organization (WHO) has recommended surgical masks for general patient care and respirators for aerosol-generating procedures only.

Face masks have become ubiquitous in China, sold on the street, and worn by almost everyone in public. A new survey, the Premise Coronavirus Awareness Survey, showed that in Taiwan 79.9% of people questioned said they were wearing masks in an effort to protect themselves from COVID-19. A similar percentage was noted in the Philippines.

Bruce Ribner, MD, medical director of the Serious Communicable Diseases Unit at Emory University Hospital, said the two masks serve very different functions. A surgical mask, or procedural mask, is meant to protect the environment from the wearer.

"It's meant to keep the surgeon's respiratory issues away from a patient," Ribner explained. A surgical mask does a good job of trapping large droplets, and some aerosol transmission, he said. Many of the masks being worn in China, though, are not designed for medical use or to any standards and so their effectiveness in trapping droplets is unknown.

A respirator, such as an N95, fits tighter to the face and is meant to help protect the wearer from inhaling infectious droplets in the environment.

"We don't really know how the coronavirus is being transmitted from person to person, because no one has done the NIOSH studies that simulate the cough big droplets that land 3 to 6 feet away from a person or the little droplets that can travel long distances and in air handling system," Ribner said. "So we have to use what we know about other coronaviruses and influenza when it comes to this disease."

What we know, Ribner said, is that multiple modes of transmission are likely at play, including large droplets, small droplets (or aerosols), and contaminated hands.

At Emory, Ribner is responsible for the care of patients treated for Ebola, Lassa fever, and other serious pathogen diseases.

Michael Osterholm, PhD, MPH, director of the University of Minnesota's Center for Infectious Disease Research and Policy, which publishes CIDRAP News, says, "The very unsettling data regarding the number of infected healthcare workers in the Wuhan area and the infection of a Japanese doctor working with those quarantined on the cruise ship and who used standard PPE [personal protective equipment]—except for a surgical mask instead of an N95 respirator—should be a real wake-up call to all of us about how do we really protect against this critical occupational risk."

How the virus travels in the air

Donald Milton, MD, a professor of environmental health at the University of Maryland, helped prove via the use of his Gesundheit machine that influenza could be spread via aerosol transmission. He said he is in contact with colleagues in Singapore who are attempting to study the transmission of the COVID-19 viruses, which are often called nCoV, for novel coronavirus.

Though Chinese officials said earlier this week that they believe the coronavirus is transmitted only via droplets, implying they do not believe airborne or contact transmission plays a role, Milton said that statement is likely rooted in fear, not science.

"To me this sounds like someone trying to deal with panic, because people panic when they hear airborne transmission and long-distance transmission," he said. He said there has been scientific evidence of aerosol transmission of MERS-CoV (Middle East respiratory syndrome coronavirus), so it is likely possible for this novel coronavirus, as well.

Milton cautions that the difference between aerosol and droplet transmission is largely in name only. Respiratory droplets, emitted with a sneeze or a cough, are commonly thought to land within 6 feet of patients and are too large to be buoyant on air currents. Respiratory aerosols are droplets too, Milton said, but smaller and light enough to travel farther.

"You cannot tell the difference epidemiologically between something aerosol transmitted by weak sources and large droplet spray," said Milton. "They behave so similar, it's very hard to pick up the difference."

He said he suspects the capability of long-distance transmission with COVID-19 will be connected to source strength, or how symptomatic a person is.

Protection may depend on resources

For Ribner, the issue of correct mask use will linger until the exact route of transmission is documented, and it is established if an asymptomatic person (one having no symptoms) can shed the virus before becoming sick.

"If we have a person who is shedding nCoV in the environment, the best way to stop transmission is to stick a surgical procedure mask on them," Ribner said. "In the best of all worlds, anyone coming into the room with an nCoV patient would wear a respirator."

But if a respirator is not available, because of cost or manufacturing, the next best option is a surgical mask, Ribner said. He said that likely contributes to the WHO's surgical mask recommendation.

"The WHO is sensitive to the fact that not every part of the world has the resources of the US and Western Europe," he said.

Keeping health workers from becoming patients

But Osterholm counters, "Frankly, the WHO recommendation that surgical masks provide adequate protection for healthcare workers caring for COVID-19 patients is not consistent with data we have from similar transmission dynamics we see with influenza virus."

In an op-ed today in the Washington Post, Osterholm and coauthor Mark Olshaker stressed, "In a very real sense, what happens to our health-care workers will be the metric of how we respond to this unfolding crisis. If we don't do all we can to protect them, they will quickly transition from providers to patients, further stressing already overburdened facilities."

They add, "Governments must support private-sector manufacturers in providing N95s and other equipment to front-line health-care providers and other essential workers. In terms of minimizing illness and death, this will be more important than any border closing, airport screening, or quarantine."

See also:

Feb 12 New York Times Japanese cruise ship story

Feb 11 Premise Data news release on COVID-19 survey

Feb 12 CDC guidance on respirator use

Feb 13 Washington Post op-ed