Officials of the World Health Organization say it’s now clear how the new coronavirus that causes Covid-19 spreads, at least in most cases: via virus-containing droplets emitted when an infected person coughs or sneezes. Pinpointing where, when and how the virus transmits is key to formulating the most effective ways to protect people and stop the pandemic. Here’s an explanation of the main route of contagion and other pathways under investigation.

Respiratory Droplets

In its first major report on the Covid-19 outbreak, the WHO said the virus spreads in respiratory droplets -- spatters of liquid that are sometimes visible to the naked eye -- forcefully expelled from an infected person’s cough or sneeze. These are usually heavy enough to fall immediately to the ground or surrounding surfaces. Infection could occur if the droplets reach the mouth, nose or possibly the eye of someone nearby, either directly or via a hand that has touched a so-called fomite -- a contaminated surface or object such as a doorknob or utensil. A study concluded that the coronavirus can survive as many as four hours on copper, 24 hours on cardboard, 48 hours on stainless steel and 72 hours on plastic.

The WHO recommends people wash their hands frequently and keep a distance of least 1 meter (3 feet) from anyone who has a fever, cough or other respiratory symptoms. It advises against shaking hands, hugging and kissing for now. For households with a suspected or confirmed case of infection, the U.S. Centers for Disease Control and Prevention suggests 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.

Tiny Aerosolized Particles

When people sneeze, cough or even breathe, they also emit particles so small that instead of falling right to the ground, these so-called aerosols can float for a time through the air. When a virus is carried by such particulates, its odds of infecting people are higher because of the potential for them to be inhaled. According to the WHO report, this method of transmission has not been reported for the new coronavirus virus, though it “can be envisaged” in health-care facilities conducting procedures such as a tracheotomy. Researchers who aerosolized it intentionally found active virus can float in the air for as many as three hours. This risk is one reason medical workers who deal with suspected or confirmed Covid-19 patients are directed to wear face masks, specifically the more sophisticated type known as respirators.

It’s less clear how helpful masks are for the hordes of regular people rushing out to buy them, provoking shortages. A few small studies have suggested that widespread use of face masks by the public may have reduced transmission in outbreaks of two other respiratory diseases, influenza and severe acute respiratory syndrome, or SARS. However, the evidence is not generally considered conclusive. And with masks in short supply, most public health authorities have argued that everyone is better off if they are reserved for those who actually require them and who can’t avoid exposure to people who are infected.

Fecal-to-Oral

Another possible route of transmission is infected people improperly washing their hands after using the toilet, and then touching surfaces that healthy people then come in contact with. Some patients have been found to have live virus particles in their stool. The WHO said in its report 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.

Food and Water

They may be a source of infection if they have been contaminated by a carrier’s respiratory secretions or feces. The virus was found to have spread readily among food service workers on the Diamond Princess cruise ship, where about one in five people became infected while it was quarantined off Yokohama, Japan. Chinese health officials recommended strengthened sanitation and hygiene measures in epidemic areas. These include drinking boiled water, avoiding eating raw food, washing hands frequently, disinfecting toilets and preventing water and food from being contaminated by patients. According to U.S. officials, the coronavirus has not been detected in municipal drinking water supplies. Conventional treatment methods that use filtration and disinfection should remove or inactivate the virus.

Mother-to-Child

After the infection appeared in babies born to mothers with the disease in China, the possibility was raised that the virus could be passed in utero. Subsequent studies reached different conclusions. One looked at nine pregnant women infected with the coronavirus who gave birth to uninfected babies. No virus was detected in amniotic fluid, cord blood, the babies’ throats or in breast milk, suggesting that transmission from mother to child occurs through respiratory droplets. However, doctors in China reported on the case of a mother with Covid-19 who gave birth to a baby that had elevated antibody levels two hours after birth. That suggests the baby was infected in utero, since the antibodies aren’t transferred to the fetus via the placenta, and usually don’t appear until three to seven days after infection.

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