Officials of the World Health Organization have said since February that the new coronavirus that causes Covid-19 spreads, at least in most cases, via virus-containing droplets emitted when an infected person coughs, sneezes or exhales. These can infect another person by falling into an eye, nose or mouth, or by getting stuck to a hand or finger and placed there. With new evidence suggesting the virus may linger in the air, the WHO says it is assessing the ongoing research. Here’s an explanation of the established 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 and are 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. Transmission can also occur indirectly, 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 long as 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

People also emit particles from their noses and mouths that are so small, instead of falling right to the ground, they can float for a time through the air. When a virus is carried by these so-called aerosols, its odds of infecting people are higher because of the potential for them to be inhaled. Since February, the WHO has said this method of transmission “can be envisaged” in health-care facilities conducting procedures such as a tracheotomy on people with Covid-19. Researchers who aerosolized it intentionally found active virus can float in the air for up to three hours. The WHO, in a statement, said the experiment reflected neither normal human coughing nor aerosol-generating medical procedures.

In another study, not yet peer-reviewed, researchers found particles of the virus in the air of rooms where patients were receiving care and adjacent hallways. The particles they identified, however, were not capable of causing infection. In another study, also not yet peer-reviewed, 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.

These risks are 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, and dispose of them carefully. Increasingly, health authorities across the world are recommending members of the public cover their faces when out in public. With medical masks in short supply, many suggest that ordinary people use home-made versions.

Fecal-to-Oral

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.

Food and Water

They may be a source of infection if 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 infected women 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 whose baby 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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