With the peak of hurricane season fast approaching, possible evacuations must be planned to help people dodge the storms and avoid causing uncontrolled outbreaks of COVID-19.
Now, a new mathematical model offers guidance on how to minimize COVID-19 spread during large-scale evacuations: People evacuating from hard-hit counties should be directed to counties with relatively lower rates of viral spread. The burden then falls to these "destination counties" to enforce social distancing and mask wearing, among other countermeasures to reduce COVID-19 transmission. If all counties adequately prepare, additional coronavirus spread can be minimized, according to the research, which has not yet been peer-reviewed.
In the worst-case scenario modeled by the team, more than two million evacuees from high-transmission counties retreated to areas with similarly high viral transmission, and their travel and interactions with others resulted in about 66,000 extra COVID-19 cases. In the best-case scenario, evacuees were systematically divided among low-transmission counties, resulting in only about 9,000 new cases.
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"Those are just hypothetical scenarios," but the overall trends revealed by the model could help local officials plan for large-scale evacuations to come, study author Sen Pei, an associate research scientist in the Mailman School of Public Health at Columbia University in New York, told Live Science. In their report, posted Aug. 11 to the preprint database medRxiv, Pei and his co-authors noted that the ability to minimize viral spread largely lies with the destination counties — namely, "the degree to which counties are prepared to host, isolate and meet the needs of evacuees while also minimizing virus exposure."
"The major factor here is just to limit the contact of evacuees with local populations," Pei said. "But it's challenging when you have to provide accommodation for those people."
The researchers built their model using data from the formidable Hurricane Irma, which made landfall in Florida in September 2017. They found that evacuees from the storm dispersed to 165 different destinations across 26 states; these locations served as the "destination counties" in their hypothetical model. Based on additional historical data from four southeast Florida counties — Palm Beach, Broward, Miami-Dade and Monroe — the team estimated that about 2.3 million evacuees would leave the counties in response to a Category 3 hurricane. The researchers then assigned these evacuees to different destination counties to devise four hypothetical scenarios.
In the "baseline" scenario, evacuees retreated to the same counties they would have for Hurricane Irma, in terms of overall proportions. In two additional scenarios, 90% of the evacuees were either directed to the 82 counties with the highest rates of COVID-19 transmission or the 82 counties with the lowest rates. After noting that movement to low-transmission counties minimized spread, the team designed a fourth scenario that assigned evacuees to low-transmission counties more systematically, to determine precisely how many should be sent where to limit overall case counts.
In addition, the model assumed that the rate of COVID-19 spread would increase by 20% in each origin county as the refugees prepare to leave and again when they finally return home, given that coming back requires travel, restocking on supplies, and potentially cleanup from the storm or sheltering with others if homes have been destroyed.
"Essentially, the model simulates what would happen if we move people from one location to the other and then move them back," Pei said. "We wanted to see how this movement of infections across the country would impact the course of the pandemic."
The model does not account for the behavior of individual evacuees once they reach their destination, Pei noted. For example, rates of COVID-19 spread may change depending on how many evacuees stay with family or friends, rather than in public shelters, and the supplies they have on hand may determine how much they interact with the local community. To capture different degrees of mingling between the hosts and refugees, the team adjusted the rate of transmission in the destination counties, increasing it by either 0%, 10% or 20% when the refugees arrived.
"Those [percentages] are all abstractions of those people's individual behaviors," Pei said. COVID-19 cases go up in the 20% scenario, where refugees mingle with their hosts a fair amount, and especially in counties with already high rates of viral spread. "That makes sense intuitively because you're moving people around more," said Pamela Murray-Tuite, a professor of civil engineering at Clemson University in South Carolina, who was not involved in the study.
However, to fine-tune the model and make it more realistic, the researchers would have to incorporate data about real human behavior, Murray-Tuite said.
"What we don't know yet is whether the evacuees … will behave the same as the people living in that destination [county]," in terms of their compliance with mask wearing, how often they frequent local businesses and whether they stick to social distancing, for instance, Murray-Tuite said. In addition, the amount of time people remain in the destination county would depend on the path of the storm, whether the roads home remain safe to travel on and whether a given evacuee stays in a shelter or with friends, among other factors.
What's more, "I would be surprised if 90% of people would allow you to direct them to a given location … if they don't have friends or family there," she noted. The mathematical model rests on the assumption that the vast majority of evacuees will accept their destination county assignment, but that would likely be an unrealistic expectation, she said.
Murray-Tuite and her research team plan to survey people seeking refuge from natural disasters during the pandemic to see how they behave. The survey data will be combined with information from Tweets, mobility data and traffic data to fit individual behaviors into their greater context. Murray-Tuite said that she expects the individuals' perceptions of risks will drive their behavior and determine how they interact with communities they encounter.
Given the risk of catching COVID-19, a person's "age and medical conditions may play a greater role than they even have in the past," in terms of whether people are willing to evacuate their homes, she added.
"It's one thing to have COVID, but COVID in a hurricane? Now you're dealing with multiplicative risk," said Robert Stein, a professor of political science at Rice University, who was not involved in the study. Evacuees must weigh the relative risks of leaving their homes and potentially exposing themselves to COVID-19, versus staying home and weathering a dangerous storm. To help people resolve these tough decisions, public officials must clearly communicate who should evacuate — and who should stay home, Stein said.
So-called shadow evacuees, or people who evacuate when there is no recommendation to do so, can clog the roads during typical evacuations, but during a pandemic, they also amplify the risk of viral spread, Stein noted. Communicating the risk that shadow evacuees pose to others and getting people to comply with official guidance "requires a level of public trust," he said.
Stein and his research team are studying who should deliver messages about risk and disaster responses to reach the public most effectively. He noted that county-level elected officials and governors, as well as local celebrities and athletes, all hold sway in public discourse and can help communicate clear, trustworthy guidance in times of uncertainty.
"The key thing here is to stay away from partisanship," Stein noted. "The argument that we've used … is that we try to communicate to people that we're all in this together." To get people to not only evacuate but to go to an approved destination, counties need adequate supplies and funding to care for evacuees once they arrive, he said.
Beyond providing food, transportation, accommodations and medical care, ideally, destination counties should be able to test evacuees for COVID-19, isolate those who test positive and perform thorough contact tracing, Stein said. Evacuees should also be reminded to pack their own food, water, medical supplies and masks so they can avoid relying on stores in their destination county to stock up, Murray-Tuite added. (The American Red Cross has further guidance on what to pack in your evacuation kit.)
In short, while Pei's model provides helpful hints for planning this year's hurricane evacuations, the hard work will be in applying those lessons in real life.
"I think what they're raising is the obvious: If we have the COVID virus running around and a hurricane, it's going to be a problem," Stein said. The model hints at one solution, that is, sending evacuees to counties with low COVID-19 transmission rates. Now comes the work of figuring out how that can be done, in practicality, Stein said.
"You've identified a solution, now tell us how we're going to implement this."
Originally published on Live Science.