A two-week countdown has begun to see whether the proximity of demonstrators at widespread protests in cities across the US could have sped up a spike in Covid-19 cases.
After thousands of people joined protests against police racism and brutality — some of them in cities and states still under coronavirus lockdowns — the US Centers for Disease Control and Prevention, or CDC, said it was monitoring the demonstrations, as the lack of social distancing might put people at risk.
But public health experts warn there is no way to predict the degree to which the disease has spread until the test results have been delivered. Kathleen Sebelius, former US health secretary, said she was worried that there would be spikes in cases of the disease — but does not know yet whether it could be a massive outbreak.
“It could be very devastating, community by community,” she said, adding that demonstrators had a responsibility to get tested so they did not infect family and friends. The mayors of Atlanta and Washington have also advised protesters to get tests after joining marches.
In New York, Governor Andrew Cuomo said he was concerned that the protests could trigger further outbreaks before New York City begins to reopen as early as next week.
“You see these mass gatherings that could probably be infecting hundreds and hundreds and hundreds of people after everything we’ve done. You have to ask yourself: what are we doing?” he said this week.
Many disease modellers have not yet changed their forecasts for Covid-19 in the US as they struggle to weigh the complexities of how it spreads. Smartphone data will soon give them an idea of how much more people are moving around, but they will not know who was wearing masks or how close they were to other people.
Axel Gandy, professor of mathematics at Imperial College London, which has been monitoring and modelling the spread of coronavirus across different US states, said it was too early to model what the protests might do to infection numbers.
But he added: “The epidemic in the US is at a critical stage right now. In many states the epidemic is not receding, it is hovering in a steady state.”
According to Imperial’s models, there are probably about 6,000 infectious people in Washington at the moment and 60,000 in New York.
There have been some potentially mitigating factors: the protests are outdoors, where the risk is lower; many marchers have worn masks; and some peaceful demonstrations gave attendees space to socially distance.
But if the protests turn violent, it is harder for people to stay 6ft apart. Crowds become cramped, people run breathing heavily, and shouting sprays droplets further than quiet conversation. Some demonstrators have been manhandled by police — or worse — bringing them in proximity.
While protesters may be younger and less vulnerable, they are also more likely to be black, in a country where African Americans are almost twice as likely to die from Covid-19, according to the Covid Racial Data Tracker.
Like many public health experts, Ashish Jha, professor of global health at Harvard, was hesitant to admonish protesters, saying their cause is much more “compelling” than Memorial Day pool parties.
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But virologists fear a situation like a parade in Philadelphia in 1918 that set off a surge in cases of the Spanish flu. “There was this feeling that everything was fine and yet literally within a few days, all the hospital beds in the city were filled up,” said Jason Kindrachuk, assistant professor in microbiology at University of Manitoba.
Even though it was a different era of medicine, the story illustrates how unpredictable viruses can be.
A recent study in China, which has not yet been peer reviewed, showed that of more than 7,000 coronavirus patients, only two became infected outdoors.
Linsey Marr, an engineering professor at Virginia Tech who specialises in how viruses spread through the air, compared them to smoke, which also gets diluted more quickly outside. Transmission depends on how long protesters are near each other, with estimates varying of needing between 15 and 30 minutes of close contact.
“Walking by someone is probably not enough,” she said.
Chanting and yelling, however, are risk factors, since the virus sits in the upper respiratory tract. At an indoor choir practice in Washington state in March, one symptomatic singer infected 87 per cent of the other members, according to the CDC.
“Bringing noise makers or drums or something that can make a lot of noise would be lower risk,” said Tom Inglesby, director of the Center for Health Security at Johns Hopkins University.
If a protester is arrested, their chances of being infected could rise, warned Dr Jha. “I’ve definitely been worried about people getting arrested, being put in police vans that are very enclosed with lots of potential arrestees, and going to jail cells with no social distancing and not wearing masks,” he said.
And as many states move to a “test and trace” model using armies of contract tracers, it could be hard for demonstrators to remember — or know — with whom they came into close contact. Authorities may need to do outreach on social media to grab the attention of anyone who attended the march.
The real wild card is whether a “superspreader” was in a crowd. Recent studies suggest as few as 10-20 per cent of people could be responsible for infecting 80 per cent of other Covid-19 cases.
“My message to everybody, whether you are a superspreader or not, is assume that you are,” said Ali Mokdad, professor of health metrics sciences at the University of Washington.
Additional reporting by Dave Lee in San Francisco