Saliva and mucous droplets
Wölfel et al.; Zhang W, Du et al.), showing that saliva or mucous can probably transmit the novel coronavirus. Tests using viral cultures showed that some patients shed viable viruses for 8-9 days and stopped, but some were not tested longer than that and perhaps could have shed longer (Arons et al.; Kujawski et al.; Wölfel et al).
Speaking, sneezing, or coughing spreads droplets of different sizes into the air, that can go in the mouth, nose, or eyes of another person (Anfinrud et al.; Asadi et al.). The larger droplets (>5μm, microns, micrometers) in diameter, usually float less than two meters in distance, and fall down onto a surface. When a person inhales them, they can deposit in and infect the mucous membranes in the nose and bronchial tubes, or the cilia hairs can carry them up and out of the respiratory system (Atkinson et al.; Dhand, Li; Brosseau; Meselson; Tang, Li et al.).
Respiratory droplets and close contact probably cause most of the COVID-19 transmission (World Health Organization-China). Some German medical scientists who examined novel coronavirus RNA from their patients also think that it is spread mostly by droplets in the air (Wölfel et al). Hong Kong scientists found SARS-CoV-2 replicates competently in bronchus and lung cells (Hui et al.) Infected people of all ages have the viruses in their nasopharynx (nose and throat), but small children have more of the viral RNA, indicating (but not completely proving) that they might transmit to others more than older children and adults transmit (Heald-Sargent et al.)
Coughing, talking, and breathing produce turbulent gas clouds with large and small droplets. Those larger than 5 micrometers usually fall in seconds, within 2 meters. But smaller droplets, called airborne aerosols, can float for hours, dry out with viruses still viable in them, and dissipate many meters, even into other rooms (Bourouiba; Bromage; Brosseau; Stadnytskyi et al.). When inhaled, they can float past the cilia and mucous membranes, and go into the lungs and infect the alveoli air sacs (Meselson). Scientists found SARS-CoV-2 in air in many samples near infected people. In a Nebraska hospital, 63% of air samples in COVID-19 patients’ rooms had SARS-CoV-2 RNA, and 67% of air samples in hallways outside their rooms, even near patients who were not coughing. But viral concentrations were so low that viable viruses could not be cultured, showing that it may or may not be possible to get infected from inhaling the air (Kimball et al.).
Initial reports from China indicated that droplets and surfaces were transmitting these viruses (World Health Organization “Modes…”). Later, in Wuhan hospitals scientists found SARS-CoV-2 RNA in air samples in many rooms, with different kinds of patients, especially those having little ventilation (Guo et al.). The rooms where medical staff changed into and out of personal protective equipment (PPE) clothing had the virus RNA in the air. Maybe just moving the clothes and PPE around shook viruses into the air. Scientists found the RNA in the air of bathrooms, which patients used only a few minutes at a time. Perhaps the patients coughed the viruses into the air, and closed the bathroom door, leaving the tiny aerosol droplets to float for hours. Scientists also found the RNA in the air outside a store and in a crowded area, where outpatients and others went, some of whom could be asymptomatic carriers (Liu, Ning, et al.). But the air in critical care unit (CCU), intensive care unit (ICU), ward room, and workstations had no or low concentrations of SARS-CoV-2 RNA, possibly because they exchanged air at high rates or were using negative pressure ventilation (Liu, Ning et al.; Ong et al.).
Other scientists conducted a lab experiment in which they nebulized SARS-CoV-2, took samples from the air three hours later, put these into living cells, and were able to culture viable viruses. These studies proved that airborne transmission is possible, but did not prove that it actually occurred (Brosseau; National Institutes of Allergies and Infectious Diseases; van Doremalen et al.).
Some experiments and epidemiology studies in different environments showed that SARS-CoV-2 lasted longer in cooler, drier air, and away from sunlight, but this might affect transmission less than non-environmental factors. So, warm, sunny weather might decrease transmission a little (Department of Homeland Security; National Academies of Science “SARS-CoV-2 survival…”).
SARS-CoV-2 RNA has been found in patients’ feces and in anal swabs for as long as five weeks after symptom onset. Viable viruses have been cultured from stool (Chen W et al.; Su et al.; Tang A et al.; Wong et al.; World Health Organization-China; Xing et al.; Young et al.; Zhang T, Cui, et al.; Zhang W, Du, et al.). Anal swab testing might reveal more than nasopharyngeal swab testing about the patient’s infection, and for deciding the length of isolation (Xu et al.). Some suspect fecal-oral transmission contributes to the epidemic (Wang Y et al; World Health Organization-China; Wong et al.). But scientists found SARS-CoV-2 RNA in the air, toilet bowls, and surfaces in patients’ bath rooms. So, the viruses may be coming from the patients breathing or coughing in bathrooms, or feces may be going into the air during defecation or from the swirling water while flushing (Liu, Deng et al.; Liu, Ning, et al.; Ong et al.).
To reduce transmission in bath rooms, people should ventilate and disinfect toilets and surfaces (Liu, Ning, et al.; Ong et al.), and flush with the lid down. This potential of fecal-oral transmission underscores the benefit of hand washing after defecation.
Scientists found SARS-CoV-2 RNA in wastewater and in rivers receiving wastewater in Italy, indicating that the virus might be transmitted in water, and these methods might serve to monitor community epidemiology (Quilliam et al.). But they could culture only small amounts of the virus from those samples, showing that water might transmit the virus less than droplets, aerosols, and surfaces (Rimoldi et al.; Thomas). In response to these findings, scientists in Australia, France, Italy, Netherlands, Nigeria, Palestine, and Spain are monitoring wastewater as an indicator of people’s COVID-19 infections, partly using methods from the Global Polio Eradication Initiative. They found SARS-CoV-2 in wastewater collected weeks before patients in those areas developed symptoms and tested positive for SARS-CoV-2 (Baraniuk; Kelland; O’Reilly et al.).
SARS-CoV-2 RNA has been found in blood (World Health Organization-China; Young et al.; Zhang W et al.). But this probably rarely transmits to people, because they rarely spill blood into another person’s broken skin. Since RT-PCR can find almost all those infected, there is low risk of transfusion transmission (Chang et al.; Corman et al.).
SARS-CoV-2 was found in one patient’s urine, and cultured in cells (Sun et al.). It was not detected in any infected people’s urine in several other studies (Lo et al.; To, Tsang, Leung et al.; Wang W, Xu, et al; Young et al.).