Viruses didn’t become ubiquitous by being wimps: From the rhinoviruses that cause the common cold to the new coronavirus that has spread across the world, they are able to survive on surfaces far away from the living cells that they need in order to reproduce.
How long they can lurk before a living organism comes along to infect depends on the kind of surface and the properties of the virus: The Covid-19 virus, according to a new study, sticks around on plastic surfaces for up to three days, but for a shorter period on metals.
Rhinoviruses can survive on human skin for hours, which is why shaking hands with someone who has a cold is a good way to catch it. Influenza viruses remain infectious for up to 48 hours after landing on nonporous surfaces such as stainless steel or plastic such as that in computer keyboards, but that seems like the outer limit: A 2011 study found that the H1N1 flu virus that caused the 2009 pandemic could be recovered from glass, stainless steel, plastic, and aluminum for up to 48 hours, but most were gone after nine hours. Both cold and flu viruses survive for much shorter times on porous surfaces such as cloth, paper, or tissue, with very little infectious virus remaining after four hours.
Viruses covered in “envelopes” have the most trouble surviving outside a living cell. On surfaces, the surrounding light, heat, and dryness break down the envelope, killing the virus. (Porous surfaces pull moisture away from viruses that land on them, accelerating the destruction of the envelope.) Most rhinoviruses have such envelopes; so do some influenza viruses. Norovirus doesn’t, enabling it to last longer in the environment.
Then there’s the new coronavirus. Its survival on surfaces is similar to that of the SARS virus, to which it’s related. On plastic, after eight hours only 10% of what researchers deposited was still there, according to a study published on Tuesday in the New England Journal of Medicine. But the virus didn’t become undetectable until after 72 hours. On stainless steel, the numbers began plummeting after just four hours, becoming undetectable by about 48 hours. On copper and cardboard, the virus was undetectable by eight hours and 48 hours, respectively.
The fewer the virus particles on a surface, the lower the chances that someone touching it will become infected. “You have to get a certain level of virus exposure to be infected,” said Ross McKinney, chief scientific officer of the American Association of Medical Colleges. And infection cannot happen through the skin: to “self inoculate,” one must transfer the virus from, say, the fingers to the nose or eyes, where it can enter the body via mucus membranes.
The first section of the new paper focuses on how long coronaviruses can survive on inanimate surfaces, such as tables and door handles. The authors show that, depending on the material and the conditions, human coronaviruses can remain infectious from 2 hours to 9 days.
At temperatures of around 4°C or 39.2oF, certain versions of the coronavirus could remain viable for up to 28 days. At temperatures of 30–40°C (86–104°F), coronaviruses tended to persist for a shorter time.
At room temperature, a coronavirus responsible for the common cold (HCV-229E) persisted significantly longer in 50% humidity than 30% humidity. Overall, the authors conclude:
“Human coronaviruses can remain infectious on inanimate surfaces at room temperature for up to 9 days. At a temperature of 30°C [86°F] or more, the duration of persistence is shorter. Veterinary coronaviruses have been shown to persist even longer for 28 d[ays].”
When the scientists delved into the literature on the persistence of coronaviruses on different surfaces, the results were variable. For instance, the MERS virus persisted for 48 hours on a steel surface at 20°C (68°F). However, on a similar surface and at the same temperature, TGEV survived for up to 28 days.
Similarly, two studies investigated the survival of two strains of SARS coronavirus on a paper surface. One survived 4–5 days, the other for just 3 hours.
In the next section of their paper, the authors address the best way to inactivate coronaviruses.
They conclude that agents, including hydrogen peroxide, ethanol, and sodium hypochlorite (a chemical in bleach), quickly and successfully inactivate coronaviruses.
For instance, the authors write that “[h]ydrogen peroxide was effective with a concentration of 0.5% and an incubation time of 1 minute.”
No Virus Is an Island
Viruses don’t have the right enzymes to create the chemical reactions necessary for reproduction. Instead, viruses need a host cell, which can be bacteria, fungi, a plant or an animal, including a human. With help from the host, viruses are then able to multiply. That’s good for the virus but generally bad for the host.
Without the host cell, a virus cannot survive long term; however, it does have a short window of time during which it can function in hopes of attaching to (aka infecting) a new host.
Outside its host, a virus can be divided into two categories — either it can be intact and remain infectious or it is simply identifiable, which means it has enough genetic material to be identified but is no longer capable of attaching to host cells, Julia Griffin and Nsikan Akpan wrote in an article for PBS News Hour. At the point that a virus on a surface is only identifiable, it won’t be able to cause harm.