A team that included researchers from the DRAGON project found that viral load is lower on face mask filters than on nasopharyngeal swabs
Breathing, talking, singing, and coughing release respiratory droplets of different sizes, and these droplets can transmit SARS-CoV-2. Bigger, coarser aerosols tend to fall quickly to the ground, while smaller, finer ones can carry respiratory viruses through the air over long distances. Face masks and personal respirators can help stop the spread of droplets that carry the virus, and while nasopharyngeal swabs are the most dominant method for the collection of samples for COVID-19 diagnosis, the DRAGON project wanted to know if filters embedded in this personal protective equipment could be used as a non-invasive way to collect samples for, say, at-home testing.
If the amount of virus emitted by symptomatic individuals is lower when wearing a mask, then we can assume that some of the virus particles are retained in the filters. DRAGON conducted a study where they generated inactivated virus-laden aerosols and dispersed them onto filters within face masks. These laboratory-based tests could detect coronaviruses down to a level of 10 copies per filter. However, testing of around 45 clinical samples suggested that the viral load emitted in breath aerosols of most patients with COVID-19 fell below this threshold. Compared to nasopharyngeal swabs, face mask filters showed only 8.5% sensitivity in samples collected from hospitalised COVID-19 patients, but, in most cases, the swabs were collected as much as a week earlier than the filter samples, which limits their direct comparison.
The difference in detection of SARS-CoV-2 between filters and nasopharyngeal swabs suggests that the number of viral particles collected on the face mask filter was below the limit of detection for all patients except those with the highest viral load – which has been shown to peak just before the onset of symptoms. This indicates that face masks are unsuitable for replacing nasopharyngeal swabs in the diagnosis of COVID-19. However, it might yet be suitable for chronic infectious agents where pathogen release is sustained, rather than having a transient period of high emission followed by a rapid resolution.
Future work will focus on other applications of nucleic acid amplification, such as surveying and classifying the airway microbiome present in breath aerosols captured on face mask filters. In addition, the use of face mask filters to detect patients infected with bacteria or fungi such as Mycobacterium tuberculosis, Aspergillus species, or Pseudomonas aeruginosa shows promise and warrants further investigation, according to the authors.