An interesting article published by Lelieveld and others in the International Journal of Environmental Research and Public Health looked at this vexing issue.
Their results substantiated that airborne transmission in the indoor environment is an important factor.
They assessed the risk reduction from the wearing of facemasks and concluded that improved quality masks, optimal fitting and hygienic discipline greatly increased the efficiency of a mask.
They were confident that the relative reductions predicted for different mitigation measures such as active ventilation with outside air, the ubiquitous wearing a facemasks of intermediate and high quality, and air filtering were robust. They did not look at surface contact and droplet transmission.
The role of aerosolised SARS-CoV-2 viruses in airborne transmission of COVID-19 has been debated.
The aerosols are transmitted through breathing and vocalisation by infectious subjects.
Some authors have stated that this represents the dominant route of spreading, whilst others dismiss the option.
The study presented an adjustable algorithm and estimated the infection risk for different indoor environments, constrained by published data of human aerosol emissions, SARS-CoV-2 viral loads, effective dose and other parameters.
They evaluated the typical indoor setting such as an office, a classroom, choir practice and a reception/party.
Their results suggested that aerosols from highly infective subjects can effectively transmit COVID-19 in indoor environments.
This highly infective category represented approximately 20% of the patients who tested positive for SARS-CoV-2.
They found that these super infective subjects, representing the top 5-10% of subjects with a positive test plus an unknown fraction of less, but still highly infective, high aerosol emitting subjects – may cause COVID-19 clusters.
However, in general, active room ventilation and the ubiquitous wearing of facemasks by all subjects reduced the individual infection risk by a factor of 5-10, similar to high volume, high efficiency particular air filtering HEPA. A particular effective mitigation measure is the use of high quality masks which can drastically reduce the indoor infection risk through aerosols.
The London General Practice has always expressed the opinion that measures such as the wearing of face masks, social distancing and hand sanitisation are essential in reducing the transmission of SARS-CoV-2.
Dr Paul Ettlinger
London General Practice