A study of the survival time of SARS-CoV-2 on common surfaces has found the virus can remain viable far longer than previously thought; up to 28 days in suitable conditions on surfaces such as the glass on smartphones, stainless steel, and paper banknotes. The authors acknowledge UV light kills the virus quickly, and transmission remains far more common through aerosols or droplets than via touching contaminated surfaces. Nevertheless, they say their findings demonstrate the importance of hand-washing and surface cleaning.
Information sheets about Covid-19 transmission usually refer to the virus responsible surviving for 2-3 days on metal or plastic. Although based on scientific studies published early in the pandemic, these relied on specific combinations of conditions that may not be universal. A team at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) have continued exploring different combinations and have now published their findings in Virology.
Shane Riddell and co-authors found viable SARS-CoV-2 after 28 days on glass surfaces at 20ºC (68ºF) and 50 percent humidity. Higher temperatures killed the virus much faster. The virus lasted longer on paper banknotes than Australia's plastic ones – invented by CSIRO – but either way could survive a month in a wallet. The situation is even more complex for a material like cotton, where the team found the virus is much less likely to rub off but may survive a long time, making transfer rates highly uncertain.
Riddell acknowledged to IFLScience that just because the virus is viable enough to infect cultured cells doesn't mean it poses a threat to humans. “The infectious dose is still unknown,” he said. “For the original SARS, it was thought to be 300-400 viral particles.” As Riddell noted, experiments to see how many particles are required for people to catch Covid-19 aren't ethically possible, and the applicability of animal tests to humans is uncertain.
Riddell thinks there could be several reasons the new results are so different from previous ones. “Some others used much lower amounts of starting material,” he told IFLScience. “We were able to show the material we were using was equivalent to what someone at the height of infectiousness might emit, so it was sort of a worst-case scenario.”
It's important to note the CSIRO team acknowledges it used highly stable temperatures and humidity under strict lab conditions, which may not reflect real-world conditions, something pointed out by other scientists.
“[T]he whole approach/optimization of such lab-based viral culture studies/experiments deliberately try to enhance virus survival – whereas our bodies’ natural immune defenses do the opposite," said Dr Julian Tang, Honorary Associate Professor in Respiratory Sciences and Clinical Virologist at the University of Leicester, UK, who was not involved in the study. “So whilst such survival may be possible to demonstrate in the lab, in real-life everyday situations, such long survival periods may not be realistic.
However, if Riddell is right, SARS-CoV-2 is much more persistent, as well as more deadly, than influenza, which lasts at most 17 days under matching conditions.
The authors do not dispute evidence most people who have been infected with Covid-19 caught it through the air. Riddell referred to work suggesting fingers only collect a third of infectious particles on a surface they touch and then pass on a third of those to nose or mouth. Without a high viral load to start off with, the chain might introduce so few viruses into the body that infection is avoided.
Nevertheless, the work does explain some puzzling outbreaks, such as one in New Zealand traced to two people touching a rubbish bin lid, possibly several days apart.
Riddell told IFLScience the work highlighted the importance of frequent handwashing and the use of sanitizer. He also stressed the study was done under dark conditions and suggested people leave items like facemasks in direct sunlight to sterilize them.