Saliva and mucous droplets
[Publication date of latest article cited: May 24, 2021]
Saliva and mucous, and the tissues producing them, are some of the main routes in which SARS-CoV-2 replicates and transmits (Collins; Huang N, Perez, et al.). SARS-CoV-2 replicates in the throat (Wölfel et al.), mouth, tongue, and salivary glands because they have much angiotensin-converting enzyme 2 (ACE2), transmembrane protease serine 2 (TMPRSS2), and furin. Those viruses attach to those molecules to enter cells (Huang N, Pérez, et al.; Salamanna et al.; Salas Orozco et al.; Xu H, Zhong L et al.; Xu J, Li Y et al.). PCR tests found SARS-CoV-2 RNA in saliva samples and oral swabs of patients (Chen W, Lan, et al.; Huang N, Perez, et al.; To KKW, Tsang, Leung et al.; To KKW, 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.; Huang N, Perez, 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). Cases with higher viral loads were more likely to transmit to secondary cases, with shorter incubation periods in the secondary cases (Marks et al.).
Speaking, sneezing, or coughing spreads saliva 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”; Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 Transmission”; Dhand, Li; Jones N, Qureshi et al.; Lerner et al; Meselson; Tang, Li et al.; WHO “Coronavirus disease (COVID-19): How is it transmitted?”).
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.; Jones N, Qureshi 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; Wiersinga 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.). A controlled clinical trial in COVID-19 patients found that ß-cyclodextrin and citrox (bioflavonoids) (CDCM) mouthwashes reduced SARS-CoV-2 viral load modestly compared to a placebo (Carrouel et al.). 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”).
[Publication date of latest article cited: May 25, 2021]
Airborne spread of smaller droplets (called aerosols) is probably the main transmission mode of SARS-CoV-2 (Centers for Disease Control and Prevention ”Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”; “Scientific Brief: SARS-CoV-2 Transmission”; “How COVID-19 Spreads”; Drake; Greenlagh et al.; Khamsi; Lee, Wada, et al.; Lerner et al.; Mandavilli “239 experts”; Molteni; Morawska, Milton; National Academies “Airborne Transmission of SARS-CoV-2”; Tang S, Mao; The Lancet Respiratory Medicine ; Tufekci “Why Did It Take So Long”; WHO “Coronavirus disease (COVID-19): How is it transmitted?”; Zhang R, Li Y, et al.). 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.; Molteni; National Academies “Airborne Transmission of SARS-CoV-2”; WHO “Transmission of SARS-CoV-2”; WHO “Advice on the Use of Masks”). The varied research and calculation methods, and many studies trying but not culturing viruses “prevents firm conclusions over airborne transmission” (Heneghan et al.). But the evidence is sufficient to wear masks, stay physically distant or outdoors, avoid crowding, ventilate, and use air filters, to protect ourselves.
Aerosol transmission occurs when 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; Erath et al.; Jones N, Qureshi et al.; Molteni; 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 infected people 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 them. 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 (Santarpia et al.). In a Singapore hospital, PCR found SARS-CoV-2 RNA in the air of most patients’ rooms, in particles sized >4 µm and 1–4 µm, even though none had aerosol-generating procedures or intranasal oxygen supplementation (Chia et al.). In London hospitals, RT-qPCR found SARS-CoV-2 RNA in the air in patients’ room, staff rooms, and a public area. They could not culture the viruses, perhaps because of low concentrations or long times since viruses were left (Zhou J, Otter, et al.). Even 12-47 days after symptom onset, air samples around patients had SARS-CoV-2 RNA (Feng B, Xu K, 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.). Aerosols exhaled by patients float and spread all round a room (Saw et al.).
Finding SARS-CoV-2 RNA in the air still did not prove that it was actually transmitted through the air. But, an experiment showed it could be transmitted through over a meter of air around corners, from infected ferrets to other ferrets. The large droplets probably did not travel that distance, proving that tiny aerosols probably transmitted the viruses (Kozlov; Kutter et al.).
Aerosol transmission in air flowing around corners probably occurred in an apartment building through vertical air ducts connecting bathrooms. In a large, multi-wing building, only people in rooms sharing the same ducts were infected, and were probably not infected by other sources (Hwang 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 out in society (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 “S&T’s Research”; 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 “S&T’s Research”; National Academies of Science “SARS-CoV-2 survival…”; Science Daily; Ward et al.).
The menu sections under “Risky situations” explain more how people transmitted via aerosols indoors and in airplanes, buses, and cars.
[Publication date of latest article cited: May 11, 2021]
SARS-CoV-2 can replicate in intestines (Qian Q, Fan, et al.). Its RNA has been found in patients’ feces and in anal swabs for weeks (Cevik, Tate, et al.; Cuicchi et al.; Jones DL et al.; Wang S, Tu J, et al.; Wang X, Zhou Y, et al.; Xiao F, Tang M, et al.; Zhang W, Du RH, et al.). ACE2, the receptor chemical that SARS-CoV-2 uses to enter cells, is expressed in intestinal epithelial cells (Hamming et al.; Salamanna et al.; Xiao F, Tang M, et al.), so SARS-CoV-2 replicates in the intestine (Amirian; Lamers et al.). 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; Xiao F, Tang M, et al.; Xing et al.; Young et al.; Zhang T, Cui, et al.; Zhang W, Du, et al.). Even 12-47 days after symptom onset, feces-related air, surface, and water samples from in and around patients’ toilets had SARS-CoV-2 RNA (Feng B, Xu K, et al.). Since many patients’ anal swabs test positive longer than their nasopharyngeal swabs, anal swab testing might reveal more about the patient’s infection, and for deciding the length of isolation (Xiao F, Tang M, et al.; Xu Y, Li X et al.).
Some suspect fecal-oral transmission contributes to the epidemic (Cuicchi et al.; de Oliveira GLV, Oliveira, et al; Gwenzi; Jones DL et al.; Mohan et al.; Qian Q, Fan; Wang Y et al; World Health Organization-China; Wong et al.; Xiao F, Tang M, et al.). But this is unproven. Almost all people defecate in toilets, so it is unlikely SARS-CoV-2 went from feces to foods. But in low income countries it could go in drinking water, aquatic foods, vegetable production, and via vectors (Gwenzi). So most patients probably received SARS-CoV-2 via their respiratory system, then circulatory, then digestive system (Amirian; Anelich et al.; Wadman et al.).
Some evidence shows that SARS-CoV-2 may be transmitting orally. People who took proton pump inhibitor (PPI) medicines (which reduce stomach acidity) were more likely to test positive for COVID-19 (Charpiat et al.), or rotaviruses, influenza, norovirus, and Middle East Respiratory Virus (MERS) (Almario et al.), than those who did not take PPI. This indirectly implies that stomach acid is protecting many people from COVID-19 and those other diseases in the gastrointestinal system. But another study did not find a correlation with PPI use (Miyake et al.). SARS-CoV-2 has a rigid outer shell and low shell disorder, which could enable fecal-oral transmission (Gwenzi).
Whether this happens depends on if stomach acid inactivates SARS-CoV-2. The related SARS-CoV-1 is inactivated at acidic pH 1 – 2 (Darnell et al.; Scheller et al.). SARS-CoV-2 was stable at moderate pH 3 – 10, but that study did not test it at more acidic or basic pH (Chin AWH, Chu JTS, et al.). Since these two viruses are mostly similar, SARS-CoV-2 is probably inactivated by acids. So, it probably cannot pass through the stomach gastric acid (Pressman et al.), when it is usually pH 1.5 – 3.0 (Cole, Kramer). But a heavy protein meal can neutralize stomach contents to pH 6, which might allow SARS-CoV-2 to pass (Burch; Konturek et al.). Scientists should conduct more experiments to find if SARS-CoV-2 is inactivated by pH<3, and by human gastric acid at its full range, 1.5 to 6. Then they could make more definite statements about whether SARS-CoV-2 can pass through the stomach and cause fecal-oral transmission.
SARS-CoV-2 probably transmitted from feces to air to people’s respiratory systems. Scientists found SARS-CoV-2 RNA in the air, toilet bowls, and surfaces in patients’ bath rooms. Some found probable evidence of fecal-aerosol transmission along vertical drains through apartment bathrooms (Kang M, Wei, et al.). 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 Y, Deng et al.; Liu Y, Ning, et al.; Ong et al.).
To reduce transmission in bath rooms, people should ventilate and disinfect toilets and surfaces (Liu Y, 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 (Mohan et al.; Panchal et al.). 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 could 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). SARS-CoV-2 can survive for days in wastewater, depending on temperature and solid contents (de Oliveira et al.). In response to these findings, scientists in many nations 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, Berger, et al.; Gwenzi; Kelland “How Sewer Science”; Kitamura et al.; O’Reilly et al.; Panchal et al.; Pleitgen; University of California at Merced; Wannigama et al.; Wurtzer S, Marechal, et al.; Wurtzer S, Waldmann, 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). Wastewater programs are inactivating any SARS-CoV-2 in their systems using chemicals and ultraviolet light (Mohan et al.).
[Publication date of latest article cited: March 10, 2021]
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.). ACE2 is in blood vessel linings, which SARS-CoV-2 uses to enter those cells and tissues (Salamanna et al.). In the circulatory system, SARS-CoV-2 goes to many organs and infects them, causing a wide range of symptoms (Wadman 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.). Blood banks and organ transplantation services recognized these potential risks, and screened cells, tissues, and organs before using them (Gaussen 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”).
[Publication date of latest article cited: April 26, 2021]
COVID-19 might be transmitting via people’s urine. ACE2 is expressed in kidneys (Salamanna et al.), especially in proximal tubule (PT) cells (Lin W, Fan J, et al.). Many COVID-19 patients develop acute kidney injury (Hansrivijit et al.). SARS-CoV-2 was found in some people’s urine (de Mattos et al.; Jones DL. et al.; Peng et al.), and was cultured in cells (Jeong et al; Sun et al.) , and could infect ferrets (Jeong et al). It was not detected in any infected people’s urine in several other studies (Lo et al.; To, Tsang KKW, Leung et al.; Wang W, Xu, et al; Young et al.). Two cases may have been infected from a public toilet (Global Times). 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 disinfect toilets and urinals (Abney et al.), and wear masks in public bathrooms (Robinson).
SARS-CoV-2 in urine could also be aerosolized during laboratory procedures, such as centrifuging. So, laboratory staff should add more safety methods, including wearing face masks, face shields, and using containment devices (de Mattos et al.), or should avoid doing microscopy of urine (Hansrivijit et al.).