WASHINGTON • Scientists have known for several months that the coronavirus can become suspended in micro-droplets expelled by patients when they speak and breathe, but until now there was no proof that these tiny particles are infectious.
A new study by scientists at the University of Nebraska that was uploaded to a medical pre-print site this week has shown for the first time that Sars-CoV-2 taken from micro-droplets, defined as under five microns, can replicate in lab conditions.
This boosts the hypothesis that normal speaking and breathing, not just coughing and sneezing, are responsible for spreading the virus causing Covid-19 - and that infectious doses of the virus can travel distances far greater than the 2m urged by social distancing guidelines.
The results are still considered preliminary and have not yet appeared in a peer-reviewed journal, which would lend more credibility to the scientists' methods. The paper was posted to the medrxiv.org website, where most cutting-edge research during the pandemic has first been made public.
The same team wrote a paper in March showing the virus remains airborne in the rooms of hospitalised Covid-19 patients, and this study will soon be published in a journal, said the lead author.
"It is actually fairly difficult" to collect the samples, Dr Joshua Santarpia, associate professor at University of Nebraska Medical Centre, told Agence France-Presse.
The team used a device the size of a cellphone for the purpose, but "the concentrations are typically very low, your chances of recovering material are small".
The scientists took air samples from five rooms of bedridden patients, at a height of about 30cm over the foot of their beds.
The patients were talking, which produces micro-droplets that become suspended in the air for several hours in what is referred to as an "aerosol", and some were coughing. The team managed to collect micro-droplets as small as one micron in diameter. They then placed these samples into a culture to make them grow, finding that three of the 18 samples tested were able to replicate.
For Dr Santarpia, this represents proof that micro-droplets, which also travel much greater distances than big droplets, are capable of infecting people.
The potential for micro-droplet transmission of the coronavirus was at one stage thought to be improbable. Later, scientists began to change their minds and acknowledge it may be a possibility, which is the rationale for universal masking. The World Health Organisation was among the last to shift its position, doing so on July 7.
"I feel like the debate has become more political than scientific," said Dr Santarpia. "I think most scientists that work on infectious diseases agree that there's likely an airborne component, though we may quibble over how large."