How COVID-19 Is Spread

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

Because respiratory droplets are too heavy to remain suspended in the air, direct person-to-person transmission 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 medical setting, 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 family members or between patients and health workers, says David Heymann, an infectious disease epidemiologist at the London School of Hygiene and Tropical Medicine who led the World Health Organization’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 or other secretions from an infected person, notes Elizabeth McGraw, director of the Center for Infectious Disease Dynamics at Penn State University. A paper published in The Journal of Hospital Infection earlier this month by researchers in Germany concluded that, based on previous studies of viruses such as MERS-CoV and SARS-CoV, at least some human coronaviruses could remain infective on materials such as metal, glass, or plastic for up to nine days. Ethanol or hydrogen peroxide solutions disinfected the surfaces 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 long the virus can remain infective outside the body. Researchers who spoke to The Scientist say they expect data on that very question to be published soon.

The facts on face masks to prevent COVID-19 transmission © ISTOCK.COM, RECEP-BG Although the chance of encountering COVID-19 outside China is still very low, global public demand for face masks has soared in recent weeks, with health-care workers warning of shortages and thieves breaking into hospitals to steal supplies. The most commonly worn masks are surgical masks, loose-fitting pieces of cloth that cover the nose and mouth. These are frequently worn by doctors and dentists, and are designed to help protect other people and 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 “to prevent contamination of the surrounding area when a person coughs or sneezes,” according to the agency’s website. A 2019 study of health-care workers exposed to the flu suggested that surgical masks may also provide the wearer with some protection from respiratory illness—probably by reducing the number of times a person touches their face, according to researchers. However, the CDC notes that frequent incorrect usage and the slippage of masks when people breathe or talk make them ineffective as protection from respiratory pathogens. Surgical masks are not the same as N95 respirators, tight-fitting face protection that filters out airborne particles including viruses and bacteria. N95 respirators are worn by health workers at risk of inhaling hazardous particles, need to be professionally fitted, and are not recommended by the CDC for members of the public. Instead, McGraw says, the best protective measures are the ones recommended for the common cold or the flu. “People should be taking the same precautions that they would anyway during flu season,” she says, 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, R₀, to describe how transmissible a disease 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 people seem to transmit disease more easily.

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

R₀ estimates for COVID-19 are currently based on limited data, but most have so far fallen between 2 and 3. That means that a typical infected person is expected to pass the disease to two or three other people, McGraw explains.

The R₀ is, by definition, an average value. “What it misses is the fact that not everybody is average,” says James Lloyd-Smith, an epidemiologist at the University of California, Los Angeles, who researches disease transmission and adaptation. “There is a lot of variation among individuals in terms of how much they transmit.”

For instance, one British man who contracted COVID-19 in Singapore was linked to a further 11 cases after he made a trip to a ski resort in France in late January. Earlier this week, a woman in South Korea was linked to as many as 15 new cases after she attended a church and then visited a hospital. These people have 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 people seem to transmit disease more easily , says Lloyd-Smith, who studied the impact of superspreading during the SARS outbreak. For example, some infected people just make more virus than other people do, he says, “whether that’s something about the genetics, prior immune status, possible cross-immunity from something else. . . . It may have to do with their initial infection—whether they had a high-dose or low-dose exposure.”

There’s also variation in the size of respiratory droplets that people, through no fault of their own, produce as 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 droplets are heavier and fall out of the air faster, for example, but may last longer than smaller droplets before evaporating.

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 will determine whether you’re out moving around in the world, doing all your normal stuff, contacting lots of people, or whether you’re at home feeling crappy, or self-isolating because you’re aware you might be an infection risk.”

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

Partly because of these contextual factors, Heymann says, the term “superspreaders” is misleading and unhelpful. “It’s not the person, it’s the situation.” He adds that, at the moment, there isn’t any evidence to suggest there is variation in how people spread COVID-19. The current understanding is that “it’s a difference 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 term should not be used to stigmatize people linked to more cases than usual. “It’s better to try and extract the idea of this being responsibility of an individual out of that scenario, and think more about [transmission events] 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 a huge outbreak [of SARS] due to an infected individual who lived on the top of an apartment building whose sewage caused a blockage,” says Heymann. The blockage is thought to have subsequently contaminated the 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, or another eye.

—Ian Mackay, University of Queensland

Recent reports of a similar situation with COVID-19, in which people on different floors of an apartment building in Hong Kong were diagnosed with the disease, led to concerns that fecal-oral transmission might be occurring for SARS-CoV-2. A couple of studies from researchers in China also recently documented viral 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 be present without the virus being infective, Heymann says.

A couple weeks ago, a few news organizations also reported concerns about vertical transmission (in which a mother passes the virus to her fetus or newborn) after a woman in Wuhan with COVID-19 gave birth to a baby who was later diagnosed with the disease. Some viruses are transmitted vertically: Zika virus, 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 to be fairly good indications that those infections were acquired at birth, rather than in utero, because there were 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. In a 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 cases suggest that there is currently no evidence for intrauterine infection caused by vertical transmission in women 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 early February. 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 a gel 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 in respiratory droplets. But Mackay says that there is no evidence to suggest that SARS-CoV-2 is spread through airborne transmission. Within 24 hours of the Shanghai official’s comments, the Chinese Center for Disease Control and Prevention had put out a statement emphasizing that there was no indication that SARS-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 of transmission as it evolves, Mackay says. “A change in mode of transmission is a big deal for a virus,” he says. “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’t think the virus really needs to adapt too much further to its ability to transmit from human to human. It’s doing a really good job right now.”

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