A recent video animation suggests that it is safe to stop and talk to someone at a distance of two metres and to walk or run alongside them at the same distance. But that because of their movement, cough droplets can reach out to a possible distance of four metres behind them.

Physical distancing, also called social distancing, is recommended because the coronavirus is primarily spread by respiratory droplets expelled when a person coughs or sneezes. Study on other respiratory viruses such as influenza has shown that these large droplets fall to the ground within 1 or 2 metres. The coronavirus is very small and there is some evidence that it may also stay in the air as an aerosol for longer and potentially spread further. But, does this or the movement of running mean this advisory distance needs revising?

Probably the distance does not need revising for everyone, but runners should keep well clear of others.

Where did the story come from?

The Sun was among media sources to display the animation of droplet transmission, which they described as a ‘horrific video.’  The video itself was reportedly produced by Ansys, a US-based technology company.

What is the basis for the claim?

The animation shows two runners close to each other. It demonstrates the fall of different sized respiratory droplets when one person coughs or sneezes. The following are said to be ‘safer’ behaviours;

  • Talking at a distance of 1.5 metres– respiratory droplets fall closely around the person
  • Walking briskly or jogging at 4 km/hour alongside but 2 metres apart – droplets fall in a cloud behind the person
  • Walking briskly or jogging (at 4 km/hour) in a staggered formation 2 metres apart – droplets don’t fall in your path.

But staying in the space directly behind someone who is walking briskly, jogging or running is said to be ‘less safe’. The animation shows the spread of different sized droplets, the finest particle size hangs in the air the longest.

It is worth noting that infectivity of different sized droplets has not been modelled and the video is not specific to the coronavirus, but only to expelled droplets after coughing.

Previous research has studied aerosol transmission in influenza. This study explains that large respiratory droplets settle to the ground within 1-2 metres, but smaller particles (diameter less than 5 micrometres) can remain airborne for longer. Overall there is little evidence on the relative contribution of droplet and aerosol transmission and infectivity in respiratory infections, particularly for coronavirus.

Also worth saying that this video was based on a hypothetical runner who was coughing or sneezing, without covering his/her mouth and nose with a hand or elbow as advised.

What do trusted sources say?

The WHO  advises that COVID-19 is spread primarily through ‘small droplets from the nose or mouth, which are expelled when a person with COVID-19 coughs, sneezes or speaks… These droplets are relatively heavy, do not travel far and quickly sink to the ground… This is why it is important to stay at least 1 metre away from others.’

CDC similarly advises that COVID-19 is spread between people who are in close proximity to one another (within about 6 feet). However, they do note that the virus appears to spread more efficiently than influenza but not as efficiently as measles.

WHO say they are continuing to research how COVID-19 is spread and will update accordingly.

Analysis by EIU Healthcare, supported by Reckitt Benckiser

 

Reading list

  1. Cowling BJ, Ip DK, et al. Aerosol transmission is an important mode of influenza A virus spread. Nature communications. 2013 Jun 4;4:1935.
  2. Q&A on coronaviruses (COVID-19).
  3. How COVID-19 spreads.