How Covid-19 spreads: finally conclusive proof about aerosol transmission?

A fierce scientific debate has taken place in recent months about the primary cause of Covid-19 transmission. The idea that the virus is primarily transmitted through droplets when people cough or sneeze now seems conclusively refuted by a study explaining the role of aerosols – tiny particles suspended in the air. This new insight may lead to a reorientation of government measures.

Lindsey Marr has been among the scientists who have insisted for several months that the corona virus can be spread – possibly even primarily – by tiny particles suspended in the air, the so-called aerosols. Early on in the Covid-19 crisis, she therefore argued that the scientific community should not be so quick to dismiss airborne transmissions as a possible route for the virus to spread.

Instead, the WHO and others have focused for a long time on transmission via direct contact or objects. It was only this month that the WHO finally revised its position, acknowledging the important role of aerosols. In an interview with The Cut, Marr does not seem surprised, explaining that she saw there was a scientific void in the area of transmissions of viruses through the air because she knew there is only a very small number of people who work on this in the world. A recent article in The Lancet by Kevin Fennelly, Marr notes, almost brought tears to her eyes because of its correct analysis and interpretation of aerosol transmission.

Marr has conveniently summarized the study’s findings. The authors of the study start by describing the current guidelines, namely that social distancing is key in preventing the spread of the virus, based on the idea that the virus is transmitted through droplets when people cough or sneeze. The article refutes this position, for example by noting that “there is no evidence to support the concept that most respiratory infections are associated with primarily large droplet transmission. In fact, small particle aerosols are the rule, rather than the exception, contrary to current guidelines”. Also, “the logic that transmission within close proximity defines respiratory droplet spread is fallacious, as small particle aerosols are in the highest concentration close to patients and dissipate with distance”. Other key findings point at the importance of environmental factors such as air movement, temperature and humidity – factors that have generally been neglected so far.

Although it now appears clear that aerosols are the most important cause of transmission, this is not to say that we cannot take a range of measures in addition to proper ventilation. On twitter, Marr tries to distill the new findings down. Her practical advise based on the scientific research is that we should be placing at least as much emphasis on distancing, masks and ventilation as we do on handwashing. Thus, we are advised to do the following: distancing, wear a mask, avoid crowds, avoid closed, poorly ventilated rooms, work to improve ventilation with outdoor air and/or good filtration in public buildings, and hygiene.