A new study has shown that the airborne transmission of the coronavirus is highly random and suggests that the two-metre rule was a number chosen from a risk "continuum", rather than any concrete measurement of safety.
A team of engineers from the University of Cambridge used computer modelling to show how droplets spread when people cough.
They found that without masks, a person with Covid-19 can infect another two metres away, even outdoors.
The team also found that coughs vary widely, and that the "safe" distance could have been set at anywhere between one and three metres, or even more, depending on the risk tolerance of the public health authority concerned.
The results, published in the journal Physics of Fluids, suggest that social distancing is not an effective measure on its own.
They emphasise the continued importance of vaccination, ventilation and masks as the northern hemisphere heads into winter.
Despite the focus on washing hands and cleaning surfaces in the early days of the pandemic, it has been clear for nearly two years that Covid-19 spreads through airborne transmission.
Infected people can spread the virus through coughing, speaking or even breathing, when they expel larger droplets that eventually settle or smaller aerosols that may float in the air.
Prof Epaminondas Mastorakos from the University of Cambridge’s department of engineering, who led the research, is an expert in fluid mechanics – the way that fluids, including those in exhaled breath, behave in different environments.
Over the course of the pandemic, he and his colleagues have developed various models for how Covid-19 spreads.
“One part of the way that this disease spreads is virology: how much virus you have in your body, how many viral particles you expel when you speak or cough,” said lead author Dr Shrey Trivedi, also from Cambridge's engineering department.
“But another part of it is fluid mechanics: what happens to the droplets once they’re expelled, which is where we come in.
"As fluid mechanics specialists, we’re like the bridge from virology of the emitter to the virology of the receiver and we can help with risk assessment."
In the current study, the researchers set out to "measure" this bridge through simulations.
For example, if a person coughed and emitted 1,000 droplets, how many would reach another person in the same room, and how large would these droplets be, as a function of time and space?
The simulations used refined computational models solving the equations for turbulent flow, together with detailed descriptions of droplet motion and evaporation.
The researchers found that there is not a sharp cut-off once the droplets spread beyond two metres.
When a person coughs and is not wearing a mask, most of the larger droplets will fall on nearby surfaces.
But smaller droplets, suspended in the air, can quickly and easily spread well beyond two metres. How far and how quickly these aerosols spread will depend on the quality of ventilation in the room.
In addition to the variables of ventilation and wearing masks, there is also a high degree of variability in individual coughs.
“Each time we cough we may emit a different amount of liquid, so if a person is infected with Covid-19, they could be emitting lots of virus particles or very few, and because of the turbulence they spread differently for every cough,” said Dr Trivedi.
Prof Mastorakos said: “Even if I expel the same number of droplets every time I cough, because the flow is turbulent, there are fluctuations.
“If I’m coughing, fluctuations in velocity, temperature and humidity mean that the amount someone gets at the two-metre mark can be very different each time."
The researchers say that while the two-metre rule is an effective and easy-to-remember message for the public, it isn’t a mark of safety, given the large number of variables associated with an airborne virus.
Vaccination, ventilation and masks, while not 100 per cent effective, are crucial for containing the virus.
“We’re all desperate to see the back of this pandemic, but we strongly recommend that people keep wearing masks in indoor spaces such as offices, classrooms and shops,” Prof Mastorakos said.
“There’s no good reason to expose yourself to this risk as long as the virus is with us.”
The research team is continuing this research with similar simulations for spaces such as lecture rooms as people spend more time indoors.