How are you most likely to catch Covid? New research suggests that the risk of surface transfer is low
What’s the Most Likely Way to Get Coronavirus? Is it transmitted through cough drops, at close range; or tiny particles called aerosols that linger in the air for hours; or is it by touching an infected object? It seems like a fairly simple question, and perhaps one that should have been answered just 11 months after a pandemic that killed 2.4 million people at the time of writing (115,000 of them in the UK). But virology – the study of viruses and their spread – is a known tricky science. The problem, says Paul Hunter, a professor of medicine at the University of East Anglia and occasional advisor to the World Health Organization (WHO), is that it is very difficult for scientists to isolate exactly how a patient ingested a virus – especially if so is widespread like Sars-Cov-2 (the virus that causes Covid-19) which has now infected 107 million people around the world. One infected person may have touched the same supermarket checkout screen as someone else who recently tested positive – but they also went shopping, spoke to a delivery courier, and their children mingled with classmates at school. How can we be sure that the checkout screen was to blame? A year of intensive scientific research has given us some valuable clues as to the spread of the coronavirus. Surfaces (fomites) There is little doubt that the coronavirus can theoretically spread through frequently treated physical objects or surfaces that are “infected” with the virus – referred to as “fomites” by epidemiologists. Recently, however, some scientists have said that too much emphasis has been placed on fomites. Emanuel Goldman, a microbiologist at Rutgers New Jersey Medical School, said in a study published in The Lancet in July that some government recommendations on deep cleaning of surfaces were “thin and likely flawed.” Most of the research at the time was done in controlled laboratory settings using large amounts of the virus, he said – not necessarily in a realistic setting. He believes the trend of people using harsh chemicals to thoroughly clean their post was likely unjustified – although he added that hand washing and disinfectants are still a good idea. An editorial in Nature magazine earlier this month noted: “Intercepting coronavirus from surfaces is rare. WHO and national health authorities need to clarify their advice. “Hunter says,” It’s a perfectly plausible route of infection, but it has been difficult to prove beyond doubt. ” Photos of health officials in hazardous suits disinfecting roads and bridges, as seen in Spain, Italy, China and various other countries, “fill me and most of my colleagues with more than a little amusement,” he adds. Many of the best studies of transmission were done at the beginning of the pandemic, as the virus later became so widespread that it became difficult to isolate certain routes of transmission. For example, in a small study conducted last February, researchers took swabs from various surfaces in a Wuhan hospital that was treating a large number of Covid-positive patients. They found virus particles on computer mice, containers, handrails, doorknobs, and even on the soles of medical workers’ 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 such as cardboard. But a year after the pandemic began, epidemiologists have yet to find specific cases that clearly indicate the transmission of fomites. Back in July, a WHO report stated that fomites were a “likely” route of transmission, but found “no specific reports” of someone intercepting the virus this way – although the scientists found it “difficult” to do Identify the difference between Fomite transmission and other forms of transmission. No slam dunk evidence has surfaced since then, says Hunter. Close-Contact Droplets Scientists generally agree that respiratory droplets, transmitted through close contact between people, are the most likely route of transmission. When you cough or sneeze, you give off thousands of droplets, like rain. They are essentially tiny droplets of saliva (scientists only call them droplets because they are invisible to the naked eye). They can fly about three feet before falling to the ground, or six feet if the cough is particularly strong. These droplets contain particles. The largest and heaviest of the particles falls directly to the ground, while the smallest, so-called aerosols, remain suspended in the air for some time (see below). It is these larger, heavier particles that cause most of the covid transmission, scientists say. If they land on a damp part of your face, you are at risk of becoming infected. Singing or speaking loudly is considered to be particularly risky because it increases your breathing range – the distance over which you expel droplets in your breath. In March, US authorities reported an outbreak in a choir in Skagit, Washington. Of the 121 members who took part in two samples (March 3 and 10), 53 contracted Covid and two died (at a time when Covid was relatively rare in the West Coast state). According to Hunter, the main risk of droplet transmission is in “crowded indoor spaces, where people do not socialize. And when they sing, it increases the risk too. Airborne Transmission While scientists believe 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 get the virus from tiny airborne particles so-called aerosols. The key point about aerosols is that they float in the air for hours, which means that in theory you can intercept the virus even though you are not in contact with an infected person – if you get into a train wagon that had an infected one a few hours ago Person stood, for example. The bacteria that cause tuberculosis and the viruses that cause measles and chickenpox are commonly spread through aerosols. Last spring, most of the scientific community was skeptical that airborne transmission was playing a significant role in the spread of Covid. But the pandemic has seen a gradual change in thinking; Last summer, 239 scientists in 32 countries wrote an open letter to the WHO asking the body to take airborne transmission seriously. Now scientists at 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 a study on Covid published in Nature Research in May, researchers set aerosol traps around two hospitals in Wuhan and found bits of genetic material from the virus floating in indoor toilets and a room in the hospital where medical staff removed their masks, dresses and gloves. The researchers said their results support the idea that Sars-Cov-2 particles could potentially hang around in the air for hours, highlighting the importance of good indoor ventilation. The study did not attempt to answer whether these virus particles actually caused an infection (it could have been dead or degraded particles). Another investigation into a restaurant in Guangzhou, China found traces of Sars-Cov-2 in the air conditioning system – suggesting particles of the virus had been blown over the restaurant. The restaurant became a target of investigation after it was discovered that a diner infected nine others while eating. But as with Fomite transmission, scientists have found very few specific cases where it was clear that aerosols played a role in transmission. The WHO advice published in October read: “Aerosol transmission can occur in certain environments, especially indoor, crowded and poorly ventilated rooms where infected people spend long periods of time with other people, e.g. B. in restaurants, choir practices, fitness classes. Night clubs, offices and / or places of worship. “Last year, Germany expanded its formula for combating the coronavirus to include regular ventilation of the rooms. Opening windows has been described by Angela Merkel as one of the “cheapest and most effective methods” of fighting the virus. Other forms Scientists say it might be possible to catch Covid from urine or feces – but this is much less likely than other forms of transmission, and again no cases have been reported.