Saliva and mucous droplets
[Date of latest publication cited: November 21, 2020]
SARS-CoV-2 replicates in the throat (Wölfel et al.), tongue, and salivary glands because they have much angiotensin-converting enzyme 2 (ACE2), to which this virus attaches to enter cells (Xu H, Zhong L et al.; Xu J, Li Y et al.). The PCR tests found SARS-CoV-2 RNA in saliva samples and oral swabs of patients (Chen W, Lan, et al.; To, Tsang, Leung et al.; To, Tsang, Yip et al.; 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). SARS-CoV-2 viral load (the number of viruses) peaks at about the fifth day after start of symptoms, lasting on average 17 days. But infected people did not infect another person more than 9 days after symptom onset (Cevik, Tate, et al.; Coleman).
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, Wexler, et al.). The larger droplets (>5μm, microns, micrometers in diameter) usually float less than 2 meters in distance (6 feet) 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.; Brosseau; Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”; Dhand, Li; Lerner et al; Meselson; Tang, Li et al.).
The more loudly and forcefully a person talks or sings, the more aerosol particles they produce. Singing produces more aerosols than talking (Alsved et al.; Asadi, Wexler, et al.).
Respiratory droplets and close contact probably cause much of the COVID-19 transmission (Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission; Weirsinga et al.; 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.).
Using mouthwashes could damage SARS-CoV-2 lipid envelopes, and thus prevent some people from getting infected or infecting others with COVID-19 (Huzar, Flynn; O’Donnell et al.; Stone). Scientists tested common over-the-counter mouthwashes in artificial in vitro laboratory conditions that mimic the nasopharyngeal secretions people’s mouths. They found that mouthwashes containing cetylpyridinium chloride, ethanol, methyl, thymol, povidine iodine, dequalinium chloride, and benzalkonium chloride inactivated almost all the SARS-CoV-2 (Chiu “What those studies on mouthwash”; Meister et al.; Statkute et al.). Another study showed that some over-the-counter nasal rinses and mouthwashes inactivated the coronaviruses that cause common colds (Meyers et al.). Next, scientists should do more research on people using these methods. But people should not over-interpret this to mean that these products will protect or cure most people from COVID-19 (Wu “No, Mouthwash will not save you”).
[Date of latest publication cited: November 19, 2020]
Airborne spread of smaller droplets (called aerosols) is probably one of the main transmission modes of SARS-CoV-2 (Centers for Disease Control and Prevention ”Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”; Drake; Khamsi; Lee, Wada, et al.; Lerner et al.; Mandavilli “239 experts”; Morawska, Milton; National Academies “Airborne Transmission of SARS-CoV-2”; Tang S, Mao; The Lancet Respiratory Medicine). But it cannot be exactly proven because in most transmission situations have a mixture of routes: some droplets are flying and becoming aerosols, or falling on fomite surfaces, which dry into dust that floats up from surfaces into air (Klompas et al.; National Academies “Airborne Transmission of SARS-CoV-2”; WHO “Transmission of SARS-CoV-2”). 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. But this varies with air conditions; larger droplets sometimes float, and small aerosols can drop (Bourouiba; Bromage; Brosseau; National Academies “Airborne Transmission of SARS-CoV-2”; 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).
COVID-19 patients exhale millions of SARS-CoV-2 viruses just by ordinary breathing, especially during early stages (Ma et al.). SARS-CoV-2 was found in the air near those patients. Likewise, in a Florida hospital, viable SARS-CoV-2 viruses were collected from air 7 feet and 16 feet from COVID-19 patients, and cultured in cells, showing they could potentially infect other people (Lednicky et al.; Mandavilli ‘A Smoking Gun’). 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 air in those rooms (Kimball et al.).
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.). 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.).
Finding SARS-CoV-2 RNA in the air still did not prove that it was actually transmitted through the air to other ferrets. But, an experiment showed that infected ferrets could transmit it through over a meter of air, around corners. The large droplets probably did not travel that distance, proving that tiny aerosols probably transmitted the viruses (Kozlov; Kutter et al.).
SARS-CoV-2 could float through aerosols, land on fomite surfaces, and stay there. For example, from an apartment with COVID-19 infected people, SARS-CoV-2 was found on surfaces in the unoccupied room upstairs. The only apparent connection was through a bathroom drain pipe (Laguipo; Tang S, Mao et al.).
SARS-CoV-2 can also float from surfaces up into the air (National Academies “Airborne Transmission of SARS-CoV-2”). In Wuhan hospitals, 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(Liu, Ning et al.) .
Scientists conducted lab experiments demonstrating that SARS-CoV-2 can float in air for hours and remain infectious. In one, they nebulized SARS-CoV-2 into aerosols, took samples from the air three hours later, put these into living cells, and could culture viable viruses (van Doremalen et al.). In another, they nebulized SARS-CoV-2 for up to 16 hours, and samples from that air could still infect rabbits and rodents (Fears et al.). These lab 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.).
Air conditions affect how long SARS-CoV-2 lasts. Experiments found that sunlight inactivates SARS-CoV-2 in minutes (Department of Homeland Security; Schuit et al.). Other experiments and epidemiology studies in different environments showed that SARS-CoV-2 lasted longer in cooler, drier air (Cooney et al.). But this might affect transmission less than non-environmental factors. So, warm, humid weather might decrease transmission a little (Ahlawat et al.; Department of Homeland Security; National Academies of Science “SARS-CoV-2 survival…”; Science Daily; Ward et al.).
It would be not-very-ethical to experiment on whether humans can get COVID-19 infection from aerosols in enclosed spaces, such as airplanes and buses. But people created several natural experiments that scientists studied. The Korean Centers for Disease Control studied whether people got infected on air flights evacuating Koreans from Italy during the epidemic there. They found that two probably previously uninfected passengers were infected during the flights. But it is uncertain if they received the SARS-CoV-2 viruses from fomites or aerosols (Bae, Shin, et al.). These natural experiments show that aerosols probably cause much of the transmission.
People transmitted on buses also. Early in the pandemic, hundreds of people traveled to an outdoor Buddhist ceremony in China. One person had just started symptoms, and traveled on a bus to and from the ceremony. The only people subsequently infected either rode the same enclosed bus with recycling air conditioning, or were close to the infected person at the ceremony (Shen et al.; Rabin). One infected person probably transmitted to several others on a bus and a minivan, mainly to people who were not wearing masks and were downwind of the internal air flow (Luo et al. ).
A review of all published, peer-reviewed reports of COVID-19 risk and transmission on airplanes from January 24 to September 21 2020 showed many cases of secondary transmission occurred on flights when few people wore masks, and few or no transmission cases when almost all people wore masks (Duocleff; Freedman, Wilder-Smith.). Transmission probably occurred to several people during long waits in international airports and a flight to Ireland, when some infected people and some uninfected people wore masks and some did not. The infected people then spread COVID-19 to others across Ireland (McMahon; Murphy et al).
Scientists also experimented with measuring floating aerosol particles, and falling particles deposited on surfaces in commercial passenger airplanes. They found low densities of particles in the air, probably “…due to both airframes’ high air exchange rates, downward ventilation design, and HEPA-filtered recirculation.“ They found particles on horizontal surfaces, but less on vertical surfaces (Mitchell; Silcott et al.; Wade). For these reasons, there may be less risk of getting COVID-19 in a passenger plane than in most buildings (Pombal, et al.).
Airlines and public health agencies experimented with methods of decreasing transmission in airplanes and airports. For example, many airlines and airports required mask wearing at all times except when eating, which employees enforce (Harvard “Aviation Public Health Initiative”; Harvard “Assessment of Risks”; United). The US Centers for Disease Control and Prevention tried screening people’s symptoms and temperatures at airports, but found this ineffective for the amount of work finding small numbers of cases (Christensen; Dollard et al.).
COVID-19 risks on airplanes, trains, buses, and taxis vary according to the air circulation, air filters, seat locations, surface cleaning, and trip time (Bushwick et al.).
[Date of latest publication cited: November 19, 2020]
SARS-CoV-2 can replicate in intestines (Qian Q, Fan, et al.). Its RNA has been found in patients’ feces and in anal swabs an average of 17 days after the start of symptoms (Cevik, Tate, et al.), and 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 Y, Li X et al.). Some suspect fecal-oral transmission contributes to the epidemic (Qian Q, Fan; 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 have been testing and monitoring SARS-CoV-2 RNA in wastewater to predict and prevent outbreaks. Early in the pandemic, scientists found it 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, Finland, France, Italy, Netherlands, Nigeria, Palestine, Spain, and Switzerland 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; Foley; Harries et al.; Kelland; O’Reilly et al.; Pleitgen; Wurtzer et al.; Yle). Universities and schools used this method to find and stop outbreaks in dormitories and communities (Conover; Hassard et al.; Jensen; Nadworny; Peccia et al.; Office of the Chancellor; Sanders). Several nations and companies are expanding these services world-wide (IDEXX; Munro, Colangelo).
[Date of latest publication cited: November 19, 2020]
SARS-CoV-2 RNA has been found in blood (World Health Organization-China; Young et al.; Zhang W et al.) for an average of 16.6 days after the start of symptoms (Cevik, Tate, 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 L, Zhao et al.; Corman et al.). The presence of SARS-CoV-2 in blood and other body fluids enables the humoral (antibodies) and cellular (white blood cells) immune responses to attack this virus there (Allegra et al.; Ledford; Robbiani et al.; Yong “Immunology”).
[Date of latest publication cited: August 20, 2020]
COVID-19 might be transmitting via people’s urine. Two cases may have been infected from a public toilet (Global Times). SARS-CoV-2 was found in one patient’s urine, and was 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.). Experiments found that flushing urinals (Wang JX, Li) and toilets (Li YY, Wang, et al.) sprays microscopic liquid particles into the air. Perhaps some people could get infected from breathing in restrooms. So, people should wear masks in public bathrooms (Robinson).