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Ever since the coronavirus epidemic started, we've been bombarded with messages on how important washing hands is:

Respiratory viruses like coronavirus disease (COVID-19) spread when mucus or droplets containing the virus get into your body through your eyes, nose or throat. Most often, this happens through your hands. Hands are also one of the most common ways that the virus spreads from one person to the next.

During a global pandemic, one of the cheapest, easiest, and most important ways to prevent the spread of a virus is to wash your hands frequently with soap and water.

But is it actually proven that washing hands significantly decreases one's odds of becoming infected? Are there any known cases where a person who didn't interact with a COVID-19 patient (including indirect contact via air) was infected after touching a contaminated surface?

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    This is a somewhat dubious question as conducting such an experiment with Covid-19 would be quite unethical in the present circumstances. And any epidemiological data would have (a lot of) uncertainties as to what the subjects actually did. The question I answered here on masks has some experiments with other respiratory virus outbreaks that show that combined use of masks and hand washing is beneficial. Either in separation (much) less so. – Fizz Apr 1 at 5:13
  • We know that the Covid-19 virus remains viable on non-porous surfaces up to 3 days. That should be a good enough indicator, but I don't know it satisfies your level of skepticism (i.e. you ask for proof someone was actually infected like that). – Fizz Apr 1 at 5:23
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    @Fizz someone getting infected with a different respiratory disease could be good enough too – JonathanReez Apr 1 at 5:29
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    I thought that one issue was that coronaviruses and many other viruses have a lipid envelope when between host cells, making them sensitive to detergents. So washing with soap and water is likely to be substantially more effective than merely rinsing with water. – Henry Apr 1 at 9:42
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    You're asking the wrong question. Your risk of becoming infected with COVID19 doesn't decrease (much) by washing your hands (unless you're a compulsory face-toucher/finger-licker); Your risk of becoming infected is significantly decreased when other people wash their hands. So wash your hands. – Shadur Apr 1 at 11:48
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Direct evidence of actual viral disease transmission via fomites (the medical term for inanimate surfaces) is unfortunately notoriously difficult to produce, according to a 2007 review:

direct experimental evidence of viral transmission via fomite has been very difficult to generate due to a variety of uncontrollable variables and the unpredictability of human infection (7, 66). An example of the difficulty in producing illness in the host after exposure was indicated in the Gwaltney study using rhinovirus. Over a 10-year period, Gwaltney intranasally challenged 343 adults without rhinovirus antibodies and infected 95% of the participants (28). However, only 30% of the individuals who became infected displayed disease symptoms (28). Generally, the majority of laboratory and clinical evidence is considered indirect; however, fomite transmission data are supported by both epidemiological studies and intervention studies.

Epidemiological data indicating transmission via fomite are also difficult to evaluate (19). This difficulty stems from problems in distinguishing between different routes of transmission, such as person-to-person transmission or autoinoculation (19). Currently, laboratory studies, epidemiological evidence, and disinfection intervention studies have generated strong indirect and circumstantial evidence that supports the involvement of fomites as a vehicle in respiratory and enteric virus transmission. Studies from a variety of disciplines investigating viruses clearly support the following: (i) most respiratory and enteric viruses can survive on fomites and hands for varying lengths of time; (ii) fomites and hands can become contaminated with viruses from both natural and laboratory sources; (iii) viral transfer from fomites to hands is possible; (iv) hands come in contact with portals of entry for viral infection; and (v) disinfection of fomites and hands interrupts viral transmission (7, 24, 66).

[...]

Studies have proven that RSV, HPIV, influenza virus, coronavirus, and rhinovirus can remain viable on fomites for several hours to several days (Tables 1 and 3) (5, 7, 9, 51). Avian influenza virus was detected on several surfaces for over 6 days (73). Studies have demonstrated that RSV, influenza virus, parainfluenza virus, and rhinovirus can survive on hands for significant periods of time and that these viruses can be transferred from hands and fingers to fomites and back again (Tables 1 and 2) (5, 7, 33, 51). After a 10-second exposure, 70% of rhinovirus was transferred from donor to recipient hands in the 1978 study by Gwaltney et al. (30). Also, Gwaltney et al. demonstrated that subjects with cold symptoms had rhinovirus on their hands, and the virus was recovered from 43% of the plastic tiles they touched (30). Contaminated hands frequently come into contact with portals of entry, and so the potential for viral infection from contaminated fomites and hands exists. A study by Hendley et al. (36) found that 1 in 2.7 hospital grand round attendees rubbed their eyes and 33% picked their nose within a 1-hour observation period (36). Indirect evidence from clinical and laboratory studies clearly supports the involvement of fomites in respiratory virus infection. However, direct evidence supporting respiratory virus transmission or infection is still scarce. A study by Gwaltney et al. (29) observed that 50% of subjects developed infections after handling a coffee cup contaminated with rhinovirus. The study also demonstrated that rhinovirus self-inoculation can result from rubbing the nasal mucosa with contaminated fingers and could lead to infection (29).

So apparently that 1982 study is the sum of direct evidence. So let's glorify its abstract here:

Transfer of experimental rhinovirus infection by an intermediary environmental surface was examined in healthy young adults, in four studies done in 1980--1981, by having recipients handle surfaces previously contaminated by infected donors. Recipients touched their nasal and conjunctival mucosa after touching the surfaces. Five (50%) of 10 recipients developed infection after exposure to virus-contaminated coffee cup handles and nine (56%) of 16 became infected after exposure to contaminated plastic tiles. Spraying of contaminated tiles with a commercially available phenol/alcohol disinfectant reduced (p = 0.003) the rate of recovery of virus from the tiles from 42% (20/47) to 8% (2/26). Similarly, the rate of detection of virus on fingers touching the tiles was reduced (p = 0.001) from 61% (28/46) with unsprayed tiles to 21% (11/53) with sprayed tiles. Fifty-six per cent (9/16) of the recipients exposed on three consecutive days to untreated tiles became infected while 35% (7/20) touching only sprayed tiles became infected with rhinovirus (p = 0.3). These studies indicate that experimental rhinovirus colds can be spread by way of contaminated environmental surfaces and suggest that disinfectant treatment of such surfaces may reduce risk of viral transmission by this route.

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  • Fantastic! The coffee cup example is exactly what I was looking for. – JonathanReez Apr 1 at 9:43
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COVID-19 is, in this respect, no different than any of a million other pathogens - and hand-washing isn't a new thing.

The Global Handwashing Partnership has been working for years on promoting hand hygiene to reduce or eliminate the spread of a wide variety of bacteria and viruses.

Soap and water reduces viral load on skin. Given hands as the likely vector, regular handwashing is overwhelmingly likely to reduce infection.

As to your final question, this is anecdotal and I cannot find the CBC article, but there was an infection pattern uncovered in British Columbia where an individual who later tested positive was in a room which subsequently was visited by others who tested positive. Given everything reported about the virus, there's no reason to doubt that contact with contaminated surfaces can spread it.

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  • But being in the same room is different, no? Presuming the virus can stay in the air, they could've breathed it in after the other person left the room. A true example would be someone ordering some groceries, picking them up from their porch without seeing the other person and then getting infected. – JonathanReez Mar 31 at 23:40
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    What do you mean by "We know that COVID-19 isn't strictly-speaking airborne"?? Everything I've read says that it has to be inhaled (or maybe rubbed on the eyeball), and it's hard to inhale if it's not airborne. – Daniel R Hicks Apr 1 at 2:01
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    @DanielRHicks Everything I've read says it is primarily spread by respiratory droplets, which are exhaled when coughing etc, but tend to fall pretty due to size. They then need to enter your body somehow. en.wikipedia.org/wiki/Respiratory_droplet – Jack Apr 1 at 4:04
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    Please provide evidence for the claim that "We know that COVID-19 isn't strictly-speaking airborne". That seems to be the central claim that's here in dispute, so I don't think an answer that makes it without sourcing is up to the sites standards. – Christian Apr 1 at 6:30
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    @DanielRHicks Covid-19 does travel on droplets such as those made by people (inadvertently or otherwise) spitting, sneezing and coughing. It has been 'aerosolised' in operating theatres and such during Aerosol Generating Procedures (AGP) - dental hygienists would also be at risk as their work involves AGP. But that doesn't mean it's 'airborne' in the technical language used by medics and others. Try wired.com/story/… – Lag Apr 1 at 12:27
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Yes, washing hands prtoect you from many desiases including flu and covid-19. Not 100% but it's reduces chance to put nasty bugs in your mouth or nose. That is proven practice since thousand years by evolution of Homo Sapiens.

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    This answer does not meet the standards for Skeptics. You must cite a "notable source" -- eg, a published research paper. – Daniel R Hicks 9 hours ago
  • Answers here need sources, or they are deleted. Click edit st the bottom to add them in. – fredsbend 8 hours ago

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