BOSTON — They are the two words that have come to define the Covid-19 pandemic: social distancing.
But staying six feet apart may not be enough in certain indoor spaces, according to more than 200 scientists from around the globe, who petitioned the World Health Organization (WHO) to recognize the importance of aerosolized transmission of the virus.
"All the evidence is adding up more and more to support this," said Lisa Brosseau, ScD, a part-time research consultant with the Center for Infectious Disease Research and Policy at the University of Minnesota. "The problem I see is that a lot of infectious disease -- especially infection prevention folks -- aren't really familiar with the literature, the recent literature which shows that people generate particles and that these particles stay airborne and that they're infectious -- they contain virus."
That may be why, despite the full-court press by the scientific community on aerosol transmission, the WHO initially resisted the claim -- before finally endorsing it last week.
Brosseau, a former professor at the University of Illinois-Chicago School of Public Health, said those particles are emitted when talking or even breathing.
“Some are larger, some are smaller,” Brosseau said. “Those smaller ones stay airborne and they can be carried on air currents so they can be distributed all throughout a room, over time... so it can be well beyond six feet.”
The implications for states attempting to reopen are potentially huge. It could mean, for example, that indoor restaurant seating six feet apart may not be enough to offer protection from infection.
Brosseau said risk increases when the space is small, crowded and has poor ventilation.
“I do think we can reopen some places safely,” said Brosseau. “We do have to think more carefully than just wearing a face covering.”
"What I don't hear a lot of people talking about is ventilation and air currents." Brosseau said.
In that regard, what would help is more frequent exchanges of room air through the day and a decrease in recirculations.
“That’s called dilution ventilation, and the problem with that is we’re not thinking about the concentration of the air at a source,” said Brosseau.
In other words, if you're sitting near someone generating a lot of particles, an HVAC system alone is not going to take care of lessening the exposure. Brosseau said you have to think, instead, about the configuration of a space and its resultant air currents.
One local example of a likely aerosol transmission event was last winter's Biogen conference in Boston, considered by public health officials as the trigger for the pandemic in Massachusetts. At that 'superspreading' event, 99 people were infected with Covid-19
So far, infection rates in Massachusetts have remained low -- with about 2.6% of Covid-19 tests coming back positive in the latest 7-day period. The rate has not varied much in recent weeks, despite the opening of limited indoor dining late in Phase 2 and the early Phase 3 opening of gyms.
But one Covid-19 forecasting model suggests that loosening up pandemic restrictions in Massachusetts will result in a sharp rise in deaths and infections.
The Institute for Health Metrics and Evaluation projects nearly 14,000 deaths in Massachusetts by November 1, 2020 and nearly 24,000 infections if restrictions are loosened. Wearing masks significantly reduces each of those numbers, the model projects.
Not everyone agrees, by the way, that Covid-19 is spread as an aerosol.
“I think there are a number who would say they’re surprised and a number who will say they disagree and I think WHO is pretty lukewarm about whether it agrees there is aerosol transmission,” Brosseau said. “But it isn’t a surprise to most of us who work with and understand aerosols and infectious aerosols.”
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