Luke MintzFri, 12 February 2021, 10:46 am
What is the most likely way you will catch coronavirus? Is it through cough droplets, transferred at close range; or tiny particles, known as aerosols, that linger in the air for hours; or is it through touching an infected object?
It seems a simple enough question, and perhaps one that should already have been answered 11 months into a pandemic that at the time of writing has killed 2.4 million people (including 115,000 in the UK). But virology – the study of viruses and how they spread – is a famously tricky science.
The problem, says Paul Hunter, a professor of medicine at the university of East Anglia, and occasional advisor to the World Health Organisation (WHO), is that it is very difficult for scientists to isolate how exactly a patient has picked up a virus – especially once it is as widespread as Sars-Cov-2 (the virus that causes Covid-19), which has now infected 107 million people across the world.
An infected person might indeed have touched the same supermarket checkout screen as another who recently tested positive – but they also went shopping, spoke to a delivery courier, and their children mingled with classmates in school. How can we be sure the checkout screen was to blame?
A year of intense scientific research has given us some valuable clues about how the coronavirus spreads.
There’s little doubt that coronavirus can theoretically spread through commonly-handled physical objects or surfaces that become ‘infected’ with the virus – known by epidemiologists as ‘fomites’. Yet recently some scientists have said too much emphasis has been placed on fomites. In a review of research published in The Lancet in July, Emanuel Goldman, a microbiologist at Rutgers New Jersey Medical School, said some government advice about deep-cleaning surfaces was “thin and probably flawed”. Most of the research at that point was carried out in controlled laboratory settings, using huge amounts of the virus, he said – not necessarily a realistic environment. He thinks the trend of people using harsh chemicals to deep-clean their post was probably unjustified – although he added that hand-washing and sanitiser is still a good idea.
And an editorial by the journal Nature earlier this month advised: “Catching the coronavirus from surfaces is rare. The WHO and national public health agencies need to clarify their advice.”
Hunter says: “It’s an entirely plausible route of infection, but it’s been difficult to prove it beyond any shadow of doubt.” Photographs of health officials in hazmat suits disinfecting streets and bridges, as seen in Spain, Italy, China, and various other countries, “fills me and most of my colleagues with more than a little amusement,” he adds.
Many of the best studies on transmission were carried out at the beginning of the pandemic, because later on the virus became so widespread it became difficult to isolate specific routes of transmission. In a small study carried out last February, for example, researchers took swabs from various surfaces in a Wuhan hospital that was treating a high number of Covid-positive patients. They found virus particles on computer mice, bins, bed handrails, doorknobs and even on the soles of medical employees’ shoes.
Another study published in the New England Journal of Medicine found the virus can survive for up to 72 hours on plastic and stainless steel, eight hours on copper, and four hours on porous surfaces like cardboard.
But a year into the pandemic, epidemiologists are still to find concrete cases in which fomite transmission was clearly to blame. As long ago as July, a WHO report said fomites are a “likely” route of transmission, but found “no specific reports” of somebody catching the virus this way – although the scientists noted it was “difficult to discern” the difference between fomite transmission and other forms of transmission. Since then, no slam-dunk evidence has emerged, says Hunter.
Close contact droplets
Scientists generally agree that respiratory droplets, transferred via close contact between people, are the most likely route of transmission.
When you cough or sneeze, you emit thousands of droplets, like rain. They are essentially tiny drops of saliva (scientists only call them droplets because they are invisible to the naked eye). They can fly about three feet before dropping to the ground, or six feet if the cough is particularly forceful.
These droplets contain particles. The largest and heaviest of the particles will fall straight to the ground, while the smallest, known as aerosols, will continue to float about in the air for some time (see below). It is those larger, heavier particles that cause the bulk of Covid transmission, scientists say. If they land on a wet part of your face, you risk becoming infected.
Singing or speaking loudly is thought to be particularly risky, because it extends your ‘breath range’ – the distance you expel droplets in your breath. In March, US authorities reported an outbreak at a choir in Skagit, Washington; of the 121 members who attended two rehearsals (on March 3 and 10), 53 became ill with Covid and two died (this at a time when Covid was relatively rare in the West Coast state).
Hunter says the main risk from droplet transmission comes from “indoor, crowded environments, where people don’t socially distance. And if they’re singing, then that increases the risk as well.”
Whilst scientists agree that you can easily catch coronavirus from larger cough droplets exchanged at close range with an infected person, a far more controversial question is whether you can catch the virus from tiny, airborne particles, known as aerosols.
The key point about aerosols is that they float about in the air for hours, meaning you could theoretically catch the virus despite having no contact with an infected person – if you walk into a train carriage where an infected person was standing several hours ago, for example. The bacterium that causes tuberculosis and the viruses that cause measles and chickenpox are all commonly spread through aerosols.
Last spring, most of the scientific community was sceptical that airborne transmission played any meaningful role in the spread of Covid. But the pandemic has seen a gradual shift in thinking; last summer, 239 scientists in 32 countries wrote an open letter to the WHO, urging the body to take airborne transmission more seriously.
Now, scientists in the WHO and the Centers for Disease Control, the influential American equivalent of Public Health England, say aerosol transmission is possible – but much less likely.
In study on Covid published in May in the journal Nature Research, researchers set up ‘aerosol traps’ around two hospitals in Wuhan, and found bits of the virus’s genetic material floating around indoor toilets, as well as a room in the hospital where medical staff removed their masks, gowns, and gloves. Researchers said their findings support the idea that Sars-Cov-2 particles might be able to hang around in the air for hours, highlighting the importance of good indoor ventilation. The study did not try to answer whether those virus particles were actually causing infection (they might have been dead or degraded particles).
Another investigation, of a restaurant in Guangzhou, China, found traces of Sars-Cov-2 in the air conditioning system – suggesting that particles of the virus were being blown about the restaurant. The restaurant became the target of investigation after one diner was found to have infected nine others while eating there.
But as with fomite transmission, scientists have found very few concrete cases in which it was clear that aerosols played a role in transmission.
WHO advice published in October said: “Aerosol transmission can occur in specific settings, particularly in indoor, crowded and inadequately ventilated spaces, where infected person(s) spend long periods of time with others, such as restaurants, choir practices, fitness classes, nightclubs, offices and/or places of worship.”
Last year Germany added regular ventilation of rooms to its formula for tackling coronavirus, with opening windows hailed by Angela Merkel as one of the “cheapest and effective ways” of fighting the virus.
Scientists say it may be possible to catch Covid from urine or faeces – but this is much less likely than other forms of transmission, and again there have been no reported cases.