Coronavirus can survive on banknotes for 28 days, but what does that mean?
A new Australian study measuring the survival rates of SARS-CoV-2 on various surfaces has found the virus may be able to survive up to 28 days on glass, stainless steel and even banknotes. However, experts urge caution over how these results are interpreted as they do not translate to conditions influencing real-world transmission of COVID-19.
The new research, from a team at the Australian Centre for Disease Preparedness (ACDP), focused very specifically on the effect of temperature on the lifespan of SARS-CoV-2, the virus that causes COVID-19. In order to eliminate other variables influencing survival time for the virus, the research was conducted under strictly controlled laboratory conditions.
The virus was suspended in an artificial mucus solution, designed to mimic the composition of body secretions. The experiments were conducted in darkness, to eliminate the damaging effects of ultraviolet light on the virus. And, the environmental humidity was permanently locked at 50 percent for the duration of all the tests.
The virus’ survival rate was then tested on several different surfaces at three different temperatures: 20 °C, 30 °C, 40 °C. The core findings from the study reveal the virus does survive longer at lower temperatures. It also survives longer on non-porous and smooth surfaces such as glass and stainless steel.
“At 20 degrees Celsius, which is about room temperature, we found that the virus was extremely robust, surviving for 28 days on smooth surfaces such as glass found on mobile phone screens and plastic banknotes,” explains Debbie Eagles, an author on the new study and deputy director of ACDP.
While the findings are robust and certainly of scientific value, adding to our understanding of this novel coronavirus, the general public's interpretation of media reports on this research are in many cases only focusing on the sensational finding that the virus can live on banknotes and phone screens for a month, without any qualification regarding real-world transmission.
Hassan Vally, an epidemiologist from La Trobe University, stresses the limitations of applying this kind of research finding to real-world public health conditions.
“The fact that the virus can survive for a longer time than previously thought under the conditions of this study, which are not generalizable to the real world, is of limited value in understanding the relative importance of contact with surfaces as a transmission route for the spread of COVID-19,” Vally explains.
Timothy Newsome, an expert in virology from the University of Sydney, backs up Vally’s concern over the value of the research influencing an individual’s own risk perception. He suggests studying virus persistence in isolation does not account for many real-world variables that influence viral transmission risk.
Newsome notes that “infectious dose, the effects of saliva and mucus on longevity, social behaviors, and authentic environmental stressors such as sunlight,” are all relevant when accounting for how transmissible the virus may be in real world conditions.
However, Newsome does believe the new study may have some utility, especially in defining what is possible. And understanding possible, albeit potentially uncommon, transmission routes can inform more effective broad public health recommendations.
“The findings that under certain, albeit artificial circumstances (in the dark), the longevity of the virus is greater than previous thought informs contact tracing and risk management,” says Newsome. “For example, fomite [surface] transmission cannot be excluded when transmission between individuals is days apart, particularly in a cold environment lacking sunlight. Defining virus persistence can also inform decontamination procedures.”
The Australian team behind the new research make it clear that the striking 28-day survival time is not necessarily a realistic indication of how the virus behaves in the real world. But, Eagles does affirm the value of the finding as crucial for authorities developing risk mitigation strategies.
“While the precise role of surface transmission, the degree of surface contact and the amount of virus required for infection is yet to be determined, establishing how long this virus remains viable on surfaces is critical for developing risk mitigation strategies in high contact areas,” says Eagles.
Some experts have suggested the new study is so narrow its results border on irrelevant. Speaking to BBC, Ron Eccles from Cardiff University questions the mucus solution used in the study. He points out human mucus is a complex combination of compounds and suggests it is unlikely the virus would persist for days on surfaces.
"Viruses are spread on surfaces from mucus in coughs and sneezes and dirty fingers and this study did not use fresh human mucus as a vehicle to spread the virus," says Eccles. "Fresh mucus is a hostile environment for viruses as it contains lots of white cells that produce enzymes to destroy viruses and can also contain antibodies and other chemicals to neutralise viruses.”
Faheem Younus, an infectious disease specialist from the University of Maryland, called the new study “nonsense” on Twitter. He suggests the artificial lab conditions used in the experiments are not at all generalizable to the real-world and claims banknotes and cell phones are not a COVID-19 risk. He even joked that those concerned should send him their money and phone if they are still afraid.
Myth: Covid survives on phone screens/money for 28 days— Faheem Younus, MD (@FaheemYounus) October 12, 2020
Such studies are conducted in artificial lab conditions: dark, damp without body’s defenses
Cell phones, currency isn’t a risk for Covid. Send me your phone/money if still afraid:) https://t.co/nxLtNDQ5sf
Underpinning these concerns regarding the new study’s findings is the growing belief among scientists that the vast majority of COVID-19 cases are due to respiratory transmission and not surface transmission. Nearly a year into this pandemic, cluster case studies suggest surface transmission of the virus is rare.
Monica Gandhi, from UC San Francisco, notes this shift from concerns over surface, or fomite, transmission to more aerosol forms of transmissions is one of the key things we have learned over the past six months.
“There was a lot of fear at the beginning of the pandemic about fomite transmission,” says Gandhi in a recent interview with Nautilus. “We now know the root of the spread is not from touching surfaces and touching your eye. It’s from being close to someone spewing virus from their nose and mouth, without in most cases knowing they are doing so.”
So although this new study offers valuable new insights into how SARS-CoV-2 can survive on surfaces under particular environmental conditions, the findings certainly do not imply there is a high risk for transmission from viral particles sitting dormant for weeks on banknotes or cell phones.
The new study was published in the journal Virology.