The Risks of Coronavirus Aerosol Transmission

What are the risks of Covid-19’s aerosol transmission?

As three families lunched at neighboring tables in a stuffy restaurant in southern China in January, they partook of more than a New Year’s meal. They shared an invisible pathogen apparently carried aloft in an air conditioner’s breeze.

Ten diners later came down with Covid-19, but none of the waiters or other 73 patrons in the room contracted the disease. A video recording and a simulation of airflow dynamics support what scientists had feared, namely that the virus could linger in turbulent air long enough to cause multiple infections.

Almost six months later, the New Year’s feast has become the touchstone in a global debate about how the coronavirus can spread in poorly ventilated spaces — and the extra measures that may be required to halt that. At the core of the controversy remain lingering questions about how often such airborne contamination occurs. After all, the Guangzhou incident shows a majority of diners remained healthy.

The World Health Organization on Thursday said it’s evaluating the role of airborne transmission, especially in indoor venues with poor ventilation. The Geneva-based agency faces pressure from scientists led by Lidia Morawska, director of the International Laboratory for Air Quality and Health at Queensland University of Technology, who argue that hand-washing and physical distancing alone aren’t enough to stem infections.

“This problem is especially acute in indoor or enclosed environments, particularly those that are crowded and have inadequate ventilation,” Morawska and a colleague wrote in an open letter published this week and backed by 239 scientists. Officials must endorse other precautions such as increasing ventilation and avoiding recirculating potentially virus-laden air in buildings such as hospitals and schools, they said.

At stake isn’t what happens when an infected person coughs or sneezes globs of virus-laden liquid — a long-established mode of infection — but whether tiny particles known as micro-droplets and aerosols stay afloat long enough to be inhaled and cause infection deep in the lungs.

“Viruses can spread in this way and we need to be aware of that,” said Benjamin Cowling, head of epidemiology and biostatistics at the University of Hong Kong, who assisted China in the early study of transmission dynamics. “But that’s not to scare everybody.”

Morawska and others are calling on the WHO and similar bodies to recognize airborne transmission as a factor contributing to the spread of Covid-19, which has now sickened more than 11 million people.

Some scientists point to the fact that the new pathogen hasn’t spread explosively across hospitals as evidence that it isn’t as contagious by air as flu or measles. The WHO says its existing recommendations, such as avoiding closed settings and ventilating indoor environments, do take into consideration the virus’s potential airborne nature. More research is needed on the subject, it said Thursday in a scientific brief.

“Ventilation can be particularly important,” said the University of Hong Kong’s Cowling, who reviewed the letter. “Spending time outdoors can be a really good thing to do to avoid the risk of transmission of Covid-19.”

The WHO needs to concede that the new coronavirus can spread via the airborne route, according to Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“We are long overdue for the WHO to confront the blind spot it has had for accepting the critical importance of airborne transmission of respiratory pathogens,” he said Monday in his center’s daily news bulletin.

Morawska, 67, started looking at airborne transmission of infectious disease after the SARS epidemic. During the outbreak, she was asked by the WHO to join a team of aerosol scientists to investigate contagion at Hong Kong’s Amoy Gardens housing estate, where a virus-laden plume emanating from a patient with diarrhea was implicated in hundreds of cases.

“This makes me even more determined to get something done about this,” Morawska said. “Once this pandemic is over — which probably won’t happen that quickly — and these issues are not recognized and not taken care of, then we will be in the same situation during the next pandemic.”

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