How Can People Catch Covid-19? The Evidence on Virus Transmission


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The modus operandi is becoming clearer. For the most part, SARS-CoV-2, the coronavirus that causes Covid-19, spreads by close personal contact via tiny particles emitted when an infected person coughs, sneezes, speaks, sings -- or even just breathes normally. These can infect another person by falling into an eye, nose or mouth, by being inhaled or getting stuck on a hand and transferred to one of these entry sites. Here’s an explanation of the established route of contagion and other pathways under investigation.

Respiratory Droplets
These spatters of virus-laden liquid of varying sizes, expelled from an infected person in a turbulent gas cloud, are thought to be the main route. The warm, moist atmosphere within the gas cloud delays evaporation, while airflow helps propel the payload of pathogen-bearing droplets further than if they were outside a cloud. A cough can disperse virus particles 4-to-5 meters (13-16 feet) and a sneeze can project them as far as 8 meters away, depending on humidity and temperature. Infection could occur if the droplets drift into the nose, mouth or eye of someone nearby.

Droplets can contaminate surfaces when they settle, creating what’s called a “fomite.” Although less likely, transmission could occur when a hand touches a fomite, such as a doorknob or utensil, and then comes in contact with the mouth, nose or eyes. The virus can be highly stable in favorable environments, lingering for weeks in near-freezing temperatures. At room temperature, it can survive as long as 24 hours on cardboard, 48 hours on stainless steel, and 72 hours on plastic, one study found. Standard disinfection kills it though.

Public health authorities recommend people wash their hands frequently, avoid close contact (within 1 to 2 meters) for prolonged periods with those outside their household, and forgo shaking hands, hugging and kissing for now. For households with a suspected or confirmed case of infection, doctors suggest keeping that person separated from others as much as possible and cleaning and disinfecting “high-touch surfaces” in common areas -- such as switches, tables and remotes -- daily. Where members of the public are crowded together, such as on buses and subways, numerous health authorities across the world recommend or mandate that people cover their faces. If medical masks are in short supply, many suggest using home-made versions.

Tiny Aerosolized Particles
The tiniest particles emitted from an infected person may be carried aloft in gas clouds tens of meters from where they started. Indoor and closed environments without adequate ventilation and air-filtration enable these microdroplets to float longer, while their small size increases their odds of being inhaled, causing a potentially more severe infection. Evidence for airborne transmission is still emerging, however, and incomplete.

Researchers who aerosolized SARS-CoV-2 intentionally found active virus can float in the air for as long as 3 hours. The World Health Organization said the experiment didn’t reflect normal coughing or clinical settings.
Intensive care units are especially hazardous because of procedures known to generate aerosols. These include tracheotomies, inserting and removing airway tubes used for mechanical ventilation, and blowing air into patients’ noses using a nasal cannula. The risk of infection is reduced when patients are treated in a negative pressure room in which the air is changed at least 12 times an hour.

Other studies found particles of the virus in the air and on surfaces of rooms where patients were receiving care and in adjacent hallways. In some cases, such particles weren’t capable of causing an infection. In one study, researchers who examined air samples in two hospitals in China raised the theoretical concern that aerosols could arise from surfaces contaminated by droplets, for instance when hospital workers shed their masks and gowns, or when floors were cleaned.

Another possible route of transmission arises when infected people improperly wash their hands after using the toilet, and then touch surfaces that others come in contact with. Some patients have been found to have live virus particles in their stool. The WHO has said that the fecal-to-oral route does not appear to be a significant pathway for the novel coronavirus. To the extent it is a means of transmission, sanitizing bathrooms, food-preparation and serving areas could help slow the virus, in addition to regular hand-washing.

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