We still don't know exactly how Covid is transmitted.
It may seem surprising, but more than a year after the coronavirus pandemic began, we still don’t know the relative importance of the different ways of catching covid-19: by touching contaminated surfaces or by breathing it in.
In March, UK government advice was that face coverings weren’t generally needed and that people should focus on frequent handwashing, use of hand sanitiser and wiping down surfaces.
Since then, the evidence has mounted that the virus can spread through the air, not only through larger droplets, but also through smaller ones that linger in the air for hours and spread for many metres – estimates for the cut-off range from 5 to 100 micrometres. Though most coronavirus safety guidelines now recognise the virus can spread through the air, they tend to focus on the risks from catching it via surfaces or large droplets that fall from the air in seconds and rarely spread more than 2 metres.
Studies in places where covid-19 is rare, which helps narrow possible transmission routes, are turning up incidents where aerosols seem to be the culprit. For instance, a person in South Korea was found to have caught the virus from someone sitting more than 6 metres away at an air-conditioned restaurant for five minutes.
A study of a superspreading event during a bus journey in China found that people caught the virus who were sitting 5 metres away.
There have also been cases within apartment blocks where the virus seems to have spread from one flat to others above it through their shared sewage pipes. “When you flush the toilet there’s massive aerosolisation,” says Raina MacIntyre at the University of New South Wales in Sydney.
Some aspects of UK government guidelines now acknowledge a risk of aerosol transmission but not all, and the public acts inconsistently, says Eilir Hughes, a GP in North Wales. While most people know spending time outside is safer than inside and to open windows when indoors with others outside their bubble, they may frequently go to supermarkets and stand within metres of people going maskless, for instance.
Hughes is co-founder of Fresh Air Wales, which campaigns for measures against airborne spread. The group wants the UK’s safety slogan to become “Hands, Face, Space, Replace”, indicating the need for people to refresh their air supply by opening windows. They also urge workplaces to use carbon dioxide monitors to check ventilation rates. The Isle of Anglesey County Council is due to give such monitors to schools, care homes and other public buildings like leisure centres.
Unlike larger droplets, aerosols can float around the face shields and plastic screens used in many supermarkets and restaurants. They can also get around the loose blue face masks widely used by the public.
A recent UK study showed healthcare staff in ICUs are less likely to catch the virus than those working elsewhere in hospitals, even though they carry out the riskiest procedures on covid-19 patients – perhaps because ICU staff use higher-grade masks. This month the British Medical Association asked Public Health England to recommend hospitals give all healthcare staff higher-grade masks.
Aerosol campaigners agree surface transmission also happens, and definitely don’t want people to stop handwashing, although they tend to downplay the benefits of frequent surface cleaning. “A casual touch of a surface is not going to get that much virus off it,” says Julian Tang at the University of Leicester, who is one of those calling for aerosol precautions.
But there is still much resistance to the idea that aerosols are the key transmission pathway. In September, the US Centers for Disease Control and Prevention updated its website to say aerosols are the main way the virus spreads – but later removed the statement, saying it had been posted in error.
A UK government spokesperson said: “Letting fresh air into indoor spaces can greatly reduce the risk of infection from coronavirus.”
