They found that the virus dies much faster on surfaces at warmer temperatures and can survive on several non-porous surfaces for up to four weeks – much longer than previous studies have indicated.
Overwhelmingly, the evidence has shown that the primary way COVID-19 spreads is through droplets and through sharing air with others, but that hasn’t stopped the fear of surface transmission. Hand washing is still one of the most important prevention methods touted by health officials.
Previous studies have looked at how long SARS-CoV-2, the virus that causes COVID-19, remains infectious on different surfaces, with some studies concluding that it is only a matter of hours, and other than it might take days.
In this study, surface researchers tested the virus on included Australian banknotes – which, like Canadian banknotes, are made of polymer – paper, glass, vinyl, stainless steel and cotton banknotes.
The researchers noted that they wanted to include money because it is an object that frequently travels between different people. Stainless steel, vinyl, and glass are materials found in most public spaces, and cotton is often found in clothing and bedding.
When a virus enters a surface, it is often through a sneeze or droplets expelled from the mouth. Researchers diluted SARS-CoV-2 “in a defined organic matrix […] designed to mimic the composition of bodily secretions’ before placing it on materials to measure longevity.
They noted in the article that the concentration of virus in each sample was high, this “always represents a plausible amount of virus that can be deposited on a surface.”
Samples of each virus-carrying material were placed in a “humidified climatic chamber” to maintain a fixed humidity of 50% relative humidity while the samples were tested at different temperatures and time periods.
The samples were tested at 20, 30 and 40 degrees Celsius, and were inspected 1 hour, 3 days, 7 days, 14 days, 21 days and 28 days after the introduction of the virus into the material.
The researchers found that at 20 degrees Celsius, the virus could survive for at least 28 days on all materials except cotton, the most porous of the materials tested.
SARS-CoV-2 could not be detected on cotton after 14 days.
“The majority of the reduction in the virus on cotton occurred very soon after virus application, suggesting an immediate absorption effect,” the report says.
Does this mean that every banknote in our wallets could infect us? According to Colin Furness, an infection control epidemiologist at the University of Toronto, we shouldn’t be alarmed.
“What we are finding empirically, clinically, with contact tracing is that COVID does not spread strongly through touch,” he said.
It is possible to contract the virus through surfaces, he said, “but it doesn’t happen very often. ”
He said that at the start of the pandemic, when we had a looser understanding of the virus, there was a greater fear of things like groceries or the mail in terms of surface transmission. But at this point, we have a better understanding of how COVID-19 primarily spreads.
“It’s shared airspace,” Furness said. “It’s droplets and aerosols and shared air with poor ventilation and prolonged contact. This is how you get sick. That’s the thing I’m scared of, that’s why I’m very, very worried about meals inside. And it’s not because you could touch contaminated cutlery. This is because you are in this room with a lot of other people and you are not wearing a mask or sharing the air.
This study conducted its experiments in a laboratory at the Australian Center for Disease Preparedness, with the samples in total darkness “to negate the effects of UV light” – just one way the conditions of the experiments differed from real life. .
« [This study] tells you what can happen under lab conditions, ”Furness said.
A banknote in your pocket or wallet rubs against other things, he explained, undisturbed to measure the longevity of a virus. If the surfaces are also exposed to sunlight, it can help any virus break down faster on the surface.
These studies are the first step, he said, and then the researchers “have to test in the real world. What is the real meaning of this?
“And those numbers are usually quite different.”
The raw numbers from the study don’t give the complete picture either. Although the virus is still detectable on most surfaces after 28 days, its concentration has dropped much faster than that.
“Viruses are not alive,” Furness said. “They can’t regenerate, metabolize, or protect themselves as soon as they leave your body. As soon as you breathe out a virus, the virus begins to die. ”
The half-life of the virus (the time it takes to reduce by 50%) on a 20 degree Celsius paper banknote was 2.74 days, which shows that the viral load is dropping much faster than 28 days would suggest it. After 9.13 days, 90% of the virus was gone.
On cotton, at 20 degrees Celsius, the half-life was 1.68 days and it took 5.57 days for a 90% reduction in the virus.
Five to nine days is still a long time for a virus to remain infectious on a surface, even if it is still not clear how too low the viral load would be to make a person sick.
The researchers said in the article that the extended half-life in this study compared to others could be due to the controlled conditions they created for the experiment.
While this study doesn’t mean we should panic about surface transmission, which remains one of the rarest ways of transmitting the virus, it does provide some insight into how temperature interacts with survivability. virus.
The researchers did not measure any of the virus samples below 20 degrees Celsius, but they observed how much the rate of decline of the virus accelerated when the temperature rose from 30 to 40 degrees Celsius. Extrapolating from this, they postulate that if the temperature drops significantly from 20 degrees Celsius, the lifespan of the virus on various surfaces could increase.
“These data could therefore provide a reasonable explanation for COVID-19 outbreaks around meat processing and cold storage facilities,” they theorize.
Furness said temperature is a huge factor when it comes to the survival of a virus.
“In winter, in freezing weather, COVID will last [longer] on surfaces, ”he says.
“So if you go to a playground in the winter it can be quite worrying. I wonder if we’re going to see COVID spreading more to touch in the winter. I can’t say it is, but it is quite possible that it is.
He said the concept of temperature had not been emphasized enough as Canada began to tackle its second wave.
“It’s not just the numbers that are increasing,” he says. “The numbers are rising, while the temperatures are falling.”
Best thing to do?
“We have to continue to wash our hands and be vigilant,” Furness said. “In fact, during COVID, I would say the best result of handwashing is actually not catching other colds that you might be afraid of having COVID.