In most cases, it’s impossible to pinpoint precisely how a person got infected with SARS-CoV-2, the virus that causes Covid-19. But months of research has enabled scientists to identify and rank the most likely ways (such as spending more than 15 minutes close to an infected person) as well as highly unlikely ones (such as handling frozen fish at the supermarket). Understanding these risks is crucial for figuring out how best to stem the pandemic’s spread.

In a Nutshell

Person-to-person close contact is widely considered the dominant mode of infection, because those infected expel droplets carrying virus particles when they speak, sing or just breathe normally. More than half of cases are transmitted by infected people who aren’t showing symptoms, according to U.S. modeling. Hence the advice to keep your distance. There are other pathways which are considered less common. Scientists have implicated smaller particles called aerosols, which can float farther and for longer, adding to evidence that suggests good ventilation and face masks -- especially those of medical-grade -- reduce the risk of infection. All such particles can contaminate surfaces when they land, creating “fomites.” People who touch the fomite can transfer virus particles to their own nose, mouth or eye and become infected. Frequent and thorough hand washing and cleaning of high-touch items like doorknobs mitigate that risk. Standard disinfectants kill the virus, as do sunlight and other sources of ultraviolet light.

Some scientists use a “Swiss cheese” model to illustrate how several layers or defenses are needed to minimize opportunities for the virus to find a new host.

Here’s a more detailed look at these individual pathways.

Close Contact

Spending at least 15 minutes near an infected person, or even briefer periods with someone who is coughing or sneezing, is risky because they are spewing virus-laden particles. Living with other people increases the prospect of prolonged, high-intensity interaction, and that makes households and other residential settings, such as nursing homes and prisons, among the most likely places for SARS-CoV-2 infections to occur.

The U.S. Centers for Disease Control and Prevention recommends a distance of at least 6 feet, which is roughly how far such droplets usually can fly. The World Health Organization says at least 1 meter (3 feet, 4 inches) and an “even greater distance” indoors. Public health authorities also recommend people wash their hands frequently and forgo shaking hands, hugging and kissing. Infected people may be most contagious a day or two before starting to feel sick, and they may continue to be infectious as long as 15 days after symptoms emerge. People who experience very mild or no symptoms may also transmit SARS-CoV-2, which is why testing has been promoted as a way of detecting hidden carriers, who need to be isolated until they are no longer contagious, and monitored for symptoms.

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 people are crowded together, such as on buses and subways, health authorities across the world are telling them to cover their faces. If medical masks are in short supply, many suggest using home-made versions.

While humans are the biggest source of SARS-CoV-2, some pets, farm animals, zoo exhibits and bats are susceptible to the infection. Health authorities say there’s no evidence that animals play a significant role in spreading the virus, however, and consider the risk of them doing so to be low.

Inhaled Particles

While the biggest respiratory droplets fall to the ground fast, aerosols can be carried farther and stay aloft longer, especially if they attach to other fine particles such as dust and pollution. Their small size increases the odds of them being inhaled deep into the lungs of a bystander, which can cause a more severe infection. A cough can disperse virus particles 4-to-5 meters and a sneeze can project them as far as 8 meters away, depending on humidity, temperature and airflow. The degree of someone’s SARS-CoV-2 infection, body mass, and age can influence their propensity to spread the virus in respiratory particles, researchers at Harvard University and Tulane University School of Medicine found. Speaking loudly is also associated with increased particle emissions.

Indoor environments without adequate ventilation or air filtration exacerbate the spread of virus-laden particles. Nightclubs, bars, restaurants, exercise classes, call centers and choir practice -- where people are close together in a confined space -- have spawned clusters of cases. Cold and stale air in meat-packing plants can also be hazardous, as can intensive-care units where patients may be undergoing certain procedures and treatments including inserting and removing airway tubes. The use of face masks, especially the more protective type known as N95 respirators, and ventilation systems that change the air at least 12 times an hour have been found to mitigate airborne transmission.

Infectious particles on surfaces, clothing, and other objects may also become airborne if they are agitated, making walking on a contaminated floor or removing contaminated clothing a potential risk. Since the virus can invade the gastrointestinal track and be shed in fecal matter, there is also potential risk that the virus might spread via aerosols generated by toilet-flushing and trapped air in sewage pipes and drain traps. In Guangzhou, China, viral traces were detected in the bathroom of a vacant apartment directly above one where five people had been confirmed as having Covid-19, and airflow and dispersal experiments found the pathogen may have wafted upward through drain pipes after a toilet was flushed.


Although there are no specific reports directly demonstrating fomite transmission, the WHO considers this a “likely” pathway, given consistent findings about environmental contamination in the vicinity of cases of infection. A study published in October found a person was infected by touching an elevator button that had been contaminated by virus-laden snot on the same day. The virus can be highly stable in favorable environments, lingering for weeks in cool temperatures on solid surfaces, such as glass and banknotes. Some scientists have hypothesized that that characteristic makes SARS-CoV-2 more difficult to control in the winter than summer. That’s a feature of other coronaviruses and respiratory viruses. Transmission of SARS-CoV-2 through feces or urine hasn’t been proven, though it may be relevant in areas with poor sanitation.

Covid-19 outbreaks have occurred in meat-processing plants or those preparing ready-made meals, but virus transmission via contaminated food or packaging is contested. The Chinese Center for Disease Control and Prevention in October detected potentially infectious, or “active,” virus on a package of frozen cod that had been handled by two port workers in Qingdao who tested positive five days later. The finding suggested they may have gotten infected from the package, although that wasn’t confirmed. Experiments with salmon indicate the virus can survive up to eight days in the cold-chain conditions used to export fish, scientists in China said in a report ahead of publication and peer review. Those results, which match laboratory experiments conducted in Singapore, support the Chinese government’s rationale for testing imported meat, packaging and containers and blocking goods deemed potentially infectious. No “live” virus had been isolated out of almost 3 million samples tested until the Qingdao case. Only 22 came back positive as of mid-September, but none was suspected of posing an infection risk. The WHO has said transmission by such means is “highly unlikely,” but recommends people who handle food practice good hygiene, including frequently cleaning and disinfecting work surfaces, to minimize the risk of contamination. Dale Fisher, an infectious-diseases physician in Singapore who has studied the durability of the SARS-CoV-2 virus, has said that, although infections from contaminated food and packaging may be “freakish” events, the scale of the global food trade increases the chances of them occurring in settings where large volumes of potentially contaminated goods are handled. That’s in contrast to supermarket shelves, which don’t represent an infectious hazard.

(Adds research on risk factors for emitting infectious droplets in section on inhaled particles.)

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