How COVID-19 Is Spread | The Scientist Magazine®

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The global outbreak of COVID-19, the disease caused by the coronavirus SARS-CoV-2, is approaching theend of its second month amid widespread confusion among members of the public about how the virus istransmitted.

“There’s a massive amount of education that clearly hasn’t reached the public about this stuff,” says IanMackay, a public health virologist at the University of Queensland who helped develop diagnostics forCOVID-19 in Australia. As is the case for many aspects of COVID-19 biology, “there are a lot of knowledgegaps out there in the community.”

With researchers around the world working to understand the pathology of the disease and slow itsspread, The Scientist rounded up the latest on what is and isn’t known about how the virus is transmittedfrom person to person.

See “Follow the Coronavirus Outbreak”

The main route of transmission for COVID-19

Like the flu, COVID-19 is spread primarily via respiratory droplets—little blobs of liquid released assomeone coughs, sneezes, or talks. Viruses contained in these droplets can infect other people via theeyes, nose, or mouth—either when they land directly on somebody’s face or when they’re transferred thereby people touching their face with contaminated hands.

Because respiratory droplets are too heavy to remain suspended in the air, direct person-to-persontransmission normally only happens when people are in close contact—within about six feet of each other,according to the US Centers for Disease Control and Prevention (CDC). It could also occur in a medicalsetting, if someone has to handle respiratory secretions such as saliva or mucus from an infected person.

People should be taking the same precautions that they would anyway during flu season.

—Elizabeth McGraw, Penn State University

Initial reports from China state that the majority of transmissions have occurred either among familymembers or between patients and health workers, says David Heymann, an infectious diseaseepidemiologist at the London School of Hygiene and Tropical Medicine who led the World HealthOrganization’s (WHO) global response to the SARS outbreak in 2003.

It might also be possible for the virus to be transferred via surfaces contaminated by respiratory droplets orother secretions from an infected person, notes Elizabeth McGraw, director of the Center for InfectiousDisease Dynamics at Penn State University. A paper published in The Journal of Hospital Infection earlierthis month by researchers in Germany concluded that, based on previous studies of viruses such asMERS-CoV and SARS-CoV, at least some human coronaviruses could remain infective on materials suchas metal, glass, or plastic for up to nine days. Ethanol or hydrogen peroxide solutions disinfected thesurfaces within one minute, the researchers note in their paper.

See “Scientists Compare Novel Coronavirus to SARS and MERS viruses”

McGraw emphasizes that it’s not clear whether SARS-CoV-2 is being transmitted in this way, nor how longthe virus can remain infective outside the body. Researchers who spoke to The Scientist say they expectdata on that very question to be published soon.

Although the chance of encountering COVID-19 outside China is still very low, global public demand forface masks has soared in recent weeks, with health-care workers warning of shortages and thievesbreaking into hospitals to steal supplies.

The most commonly worn masks are surgical masks, loose-fitting pieces of cloth that cover the nose andmouth. These are frequently worn by doctors and dentists, and are designed to help protect other peopleand the environment from the mask-wearer by trapping respiratory droplets emitted from the mouth or

nose.

The CDC only recommends these masks for people who are already infected with SARS-CoV-2, so as “toprevent contamination of the surrounding area when a person coughs or sneezes,” according to theagency’s website.

A 2019 study of health-care workers exposed to the flu suggested that surgical masks may also provide thewearer with some protection from respiratory illness—probably by reducing the number of times a persontouches their face, according to researchers. However, the CDC notes that frequent incorrect usage andthe slippage of masks when people breathe or talk make them ineffective as protection from respiratorypathogens.

Surgical masks are not the same as N95 respirators, tight-fitting face protection that filters out airborneparticles including viruses and bacteria. N95 respirators are worn by health workers at risk of inhalinghazardous particles, need to be professionally fitted, and are not recommended by the CDC for membersof the public.

Instead, McGraw says, the best protective measures are the ones recommended for the common cold orthe flu. “People should be taking the same precautions that they would anyway during flu season,” shesays, including washing hands frequently and avoiding touching their faces. And “if you’re not well, self-isolate, don’t go to work.”

Transmissibility and the mislabeled “superspreader”

Researchers use what’s known as the basic reproduction number, R , to describe how transmissible adisease is in the absence of any special quarantining or social distancing measures.

Research on other respiratory viruses suggests that there may be biological reasons that some peopleseem to transmit disease more easily.

—James Lloyd-Smith, University of California, Los Angeles

R estimates for COVID-19 are currently based on limited data, but most have so far fallen between 2 and3. That means that a typical infected person is expected to pass the disease to two or three other people,McGraw explains.

The R is, by definition, an average value. “What it misses is the fact that not everybody is average,” saysJames Lloyd-Smith, an epidemiologist at the University of California, Los Angeles, who researches diseasetransmission and adaptation. “There is a lot of variation among individuals in terms of how much theytransmit.”

For instance, one British man who contracted COVID-19 in Singapore was linked to a further 11 cases after

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he made a trip to a ski resort in France in late January. Earlier this week, a woman in South Korea waslinked to as many as 15 new cases after she attended a church and then visited a hospital. These peoplehave been referred to as “superspreaders” by some epidemiologists and media outlets.

Research on other respiratory viruses suggests that there may be biological reasons that some peopleseem to transmit disease more easily , says Lloyd-Smith, who studied the impact of superspreading duringthe SARS outbreak. For example, some infected people just make more virus than other people do, hesays, “whether that’s something about the genetics, prior immune status, possible cross-immunity fromsomething else. . . . It may have to do with their initial infection—whether they had a high-dose or low-doseexposure.”

There’s also variation in the size of respiratory droplets that people, through no fault of their own, produceas they breathe or talk, says Lloyd-Smith. Size can help determine how a droplet moves through the air,how likely it is to reach another person, and whether it makes it to that person’s airways. Larger dropletsare heavier and fall out of the air faster, for example, but may last longer than smaller droplets beforeevaporating.

Additionally, for many illnesses, “there’s a lot of difference in the severity of symptoms people show,” Lloyd-Smith says. “This has a direct impact on transmission, because how sick versus well you feel willdetermine whether you’re out moving around in the world, doing all your normal stuff, contacting lots ofpeople, or whether you’re at home feeling crappy, or self-isolating because you’re aware you might be aninfection risk.”

There are also many non-biological factors that influence the probability a disease will spread, from thenumber of people at a particular gathering, to their susceptibility of catching the disease, to the types ofinteractions those people are having. For instance, during the West African Ebola epidemic, which claimedmore than 11,000 lives between 2013 and 2016, at least some new chains of transmission are thought tohave started at unsafe burials, in which lots of people came into close contact with the body of an infectedperson and with one another.

Partly because of these contextual factors, Heymann says, the term “superspreaders” is misleading andunhelpful. “It’s not the person, it’s the situation.” He adds that, at the moment, there isn’t any evidence tosuggest there is variation in how people spread COVID-19. The current understanding is that “it’s adifference in who is exposed, and what numbers are exposed to a person who’s transmitting.”

McGraw agrees that “superspreaders” fails to capture the complexity of transmission, adding that the termshould not be used to stigmatize people linked to more cases than usual. “It’s better to try and extract theidea of this being responsibility of an individual out of that scenario, and think more about [transmissionevents] as being context-dependent,” she says. “Because if we think about them as very contextualized,we have a better chance of trying to shut down transmission.”

Are there other ways COVID-19 could spread?

One proposed alternative route SARS-CoV-2 may take to reach a new host is fecal-oral transmission,which is thought to have played a role in the spread of SARS. In Hong Kong, for example, “there was ahuge outbreak [of SARS] due to an infected individual who lived on the top of an apartment building whosesewage caused a blockage,” says Heymann. The blockage is thought to have subsequently contaminatedthe bathing areas of people living on the floors below, leading to many new cases.

A change in mode of transmission is a big deal for a virus. It’s a bit like growing an extra arm for us, oranother eye.

—Ian Mackay, University of Queensland

Recent reports of a similar situation with COVID-19, in which people on different floors of an apartmentbuilding in Hong Kong were diagnosed with the disease, led to concerns that fecal-oral transmission mightbe occurring for SARS-CoV-2. A couple of studies from researchers in China also recently documentedviral RNA in the feces of infected people.

On their own, these observations don’t show that COVID-19 is spread via feces. Viral RNA can often bepresent without the virus being infective, Heymann says.

A couple weeks ago, a few news organizations also reported concerns about vertical transmission (inwhich a mother passes the virus to her fetus or newborn) after a woman in Wuhan with COVID-19 gavebirth to a baby who was later diagnosed with the disease. Some viruses are transmitted vertically: Zikavirus, for example, can infect a fetus via the placenta, while HIV can be passed through breastmilk.

Newborns diagnosed with COVID-19 are more likely to have caught the illness through the usual means—that is, close contact and exchange of virus-carrying respiratory droplets, says Mackay. “There do seem tobe fairly good indications that those infections were acquired at birth, rather than in utero, because therewere infected people such as a mother or a nanny who were in close proximity.”

One recent study of nine pregnant women with COVID-19 failed to find evidence of vertical transmission. Ina paper published in The Lancet, the researchers reported that, in all nine cases, amniotic fluid, cord blood,breastmilk, and the newborn babies tested negative for the virus. “Findings from this small group of casessuggest that there is currently no evidence for intrauterine infection caused by vertical transmission inwomen who develop COVID-19 pneumonia in late pregnancy,” the authors conclude in their paper.

Another potential mode of viral spread, airborne transmission, was discussed by a Shanghai official in earlyFebruary. When airborne, infective virus can drift through the air as an aerosol. In this form of transmission,“very small droplets that come out of our mouth very quickly evaporate the water off, and we’re left with agel kind of material . . . that forms a bit of a protective environment for those virions to survive for longer,”Mackay explains. This is distinct from droplet-based spread of the virus.

Viruses such as measles that do show airborne transmission can spread further than viruses transmitted inrespiratory droplets. But Mackay says that there is no evidence to suggest that SARS-CoV-2 is spreadthrough airborne transmission. Within 24 hours of the Shanghai official’s comments, the Chinese Center forDisease Control and Prevention had put out a statement emphasizing that there was no indication thatSARS-CoV-2 is spread in this way.

It’s possible, though not very likely for the time being, that SARS-CoV-2 will adopt a new mode oftransmission as it evolves, Mackay says. “A change in mode of transmission is a big deal for a virus,” hesays. “It’s a bit like growing an extra arm for us, or another eye.”

Given the ease with which it’s currently spreading via respiratory droplets, he adds, “at the moment, I don’tthink the virus really needs to adapt too much further to its ability to transmit from human to human. It’sdoing a really good job right now.”

Catherine Offord is an associate editor at The Scientist. Email her at cofford@the-scientist.com.