5 min read.Updated: 14 Oct 2020, 03:50 PM ISTJason Gale, Bloomberg
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) to get infected with SARS-CoV-2, the virus that causes Covid-19
In most cases, it’s impossible to pinpoint precisely how a person got infected with SARS-CoV-2, the virus that causes Covid-19. It’s just too small. 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.
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. 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 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 does 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.
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. 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.
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. 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 crammed together in a confined space have spawned clusters of cases. Cold and stale air in meat-packing plants, and intensive-care units where patients may be undergoing certain procedures and treatments including inserting and removing airway tubes, are also hazardous. 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. The virus can be highly stable in favorable environments, lingering for weeks in cool temperatures on solid surfaces, such as glass and banknotes, and other coronaviruses and respiratory viruses can spread this way. Transmission of SARS-CoV-2 through feces or urine hasn’t been proven, though it may be relevant in areas with poor sanitation.
Likewise, transmission via contaminated food or food packaging hasn’t been proven. Some scientists have said it may pose some risk in the context of meat-processing or preparing ready-made meals, where Covid-19 outbreaks have occurred. Experiments with salmon indicate the virus may survive up to eight days in the cold-chain conditions used to transport fish internationally, scientists in China said in a report ahead of publication and peer review. The findings, which match laboratory experiments conducted in Singapore, support the Chinese government’s rationale for testing imported meat, packaging and containers, and halting goods deemed potentially infectious. The feasibility of this “non-traditional" transmission mechanism is debated. The WHO has said it’s “highly unlikely," but recommends people who handle food practice good hygiene, including frequently cleaning and disinfecting work surfaces, to minimize the risk of contamination.
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