Coronavirus: Infection takes ‘mere minutes’ in the presence of a virus cougher

People may be at risk of contracting coronavirus if they spend just a few minutes in a small room with an infected person with a high viral load who is coughing.

Shot of illness young woman coughing in the street.

People may be at risk of contracting coronavirus if they spend just a few minutes in a small room with an infected person with a high viral load who is coughing.

A paper by researchers from the Swiss Centre for Occupational and Environmental Health found that even individuals with asymptomatic or mild COVID-19 can have large amounts of virus in their sputum, and if those individuals are breathing and coughing in small, poorly ventilated rooms, there is a high risk of transmission of the disease even after a few ­minutes.

The study may have impli­cations for contact-tracing guidelines, which generally specify that close contacts of an infected person are deemed to be those who have spent 15 minutes or more in a room with the COVID-19 case.

Australia’s COVIDSafe app also works on the basis that a contact is traced when an individual has been in proximity to an infected person for 15 minutes or more.

The Swiss study used mathematical modelling to estimate the concentration of virus in a room generated by someone with COVID-19 coughing or breathing.

The researchers found that while there was a low risk of infection from a person with typical viral load breathing normally, if that person had a high viral load and was breathing and coughing, they could release large numbers of viruses, ranging from thousands to millions of viruses per cubic metre, and someone in the same room could catch the virus within minutes.

The study noted that a series of community transmissions had been reported in international research that were associated with individuals who had no apparent symptoms of COVID-19.

Super-spreading events have been documented in situations where many people engaged in loud-voice activities while in closed rooms for a prolonged time, such as at restaurants, in a call centre or at choir rehearsals. In superspreader events, up to 75 per cent of the people in a room with an infected person caught the virus.

The study said those in a small room with a superspreader with a high viral load could catch the virus in a minutes even when the amount of virus inhaled was small.

“A person spending time in a room with an individual emitting (breath) at a typical rate and breathing normally has the chance of inhaling only a few ­copies of the virus when keeping distance from that person,” the study said. “However, the risk may be higher in the presence of an individual with a high emitting rate and if the individual is coughing.

“A review of a wide range of respiratory viruses suggests the infective dose is often low. Sometimes as few as a few hundred units of active virus seem sufficient to cause disease.

“Our modelling suggests there is a risk of infection for a person spending an extended period in a small room with an individual with COVID-19 who has an elevated viral load, even if the distance is too large for direct transmission. The risk may be higher if the individual is coughing.”

The results suggest viral load in the air can reach critical concentrations in small and poorly ventilated rooms, especially when the individual is a superspreader.

“People who are high emitters are not very common in the population and our findings suggest only few people with very high viral load pose an infection risk in poorly ventilated environments,” the researchers said.

“Nevertheless, strict respiratory protection is recommended whenever there is a chance to be in the same room with such an ­individual.”