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How Hurricane Evacuation Can Spread the Coronavirus

Where people shelter after a hurricane will make a difference in exacerbating Covid-19 spread. Photographer: John Taggart/The Washington Post via Getty Images


Differences in emergency planning can mean tens of thousands of new Covid-19 cases, a new study finds. 

By Linda Poon for Bloomberg – A duo of storms this week may force tens of thousands of residents in the Gulf region to evacuate, a movement of people that could turn even more dangerous because of the Covid-19 pandemic.

new study on the impact such evacuations could have on disease transmission finds that tens of thousands more people could be infected by the coronavirus depending on how emergency planning is handled.

If evacuations followed the actual routes of Hurricane Irma in 2017, one of the largest hurricane evacuations in U.S. history, new coronavirus cases could spike by 61,000 as those forced to leave their homes shelter with friends and family or in public shelters. Sending the majority of evacuees to counties with lower transmission rates could significantly lower that rate, though there would still be an increase of 6,400, according to the study’s rough estimates.

The paper, prepared by climate researchers and epidemiologists at the Union of Concerned Scientists and Columbia University, was released in early August, weeks before tropical storms Marco and Laura formed in the Gulf. Louisiana, Mississippi and Texas were largely spared by Marco, which weakened as it made landfall on Monday. But Laura is expected to strengthen into a Category 3 hurricane later this week, forcing some areas in coastal Texas and Louisiana to issue evacuation orders. Climate experts are warning that the coming months will be the peak of what they predict to be an “extremely active” hurricane season in the Atlantic.

Researchers built a predictive model combining evacuation patterns from Hurricane Irma — which forced some 7 million people to flee their homes as it moved up the coast of Florida and toward Georgia in September 2017 — and recent county-level coronavirus transmission rates. They then simulated various scenarios of how the movement of 2.3 million evacuees from four coronavirus hotspots might increase Covid-19 cases in both the origin and destination counties. The goal was to determine how emergency managers might better plan evacuations that would limit the spread of the disease.

“We found that we can minimize the number of new Covid-19 cases by directing evacuees to counties with low transmission rates rather than allowing evacuations to unfold along the observed historic pattern,” says Kristina Dahl, a senior climate scientist at UCS and a co-author of the report, which is awaiting peer review. 

The researchers looked at evacuation routes from Hurricane Irma, identifying 165 destination counties across 26 states. They then found Covid-19 transmission rates for each county, including the four origin counties of Palm Beach, Broward, Miami-Dade and Monroe. The researchers simulated four evacuation scenarios, each with varying assumptions on how the influx of evacuees would increase transmission rates in the receiving counties. 

In the worst-case scenario, coronavirus cases could spike by more than 66,000. And while sending people to counties with the lowest transmission rates could minimize the case increase to 6,400, the study considered the possibilities that some people wouldn’t comply with evacuation orders and that some host counties would not be able to take that many people. Factoring in those two limitations, the lowest transmission number was somewhat higher, at 9,069 more cases.

The study assumes that the evacuation process takes 13 days, from the time people prepare to evacuate to when they return home. During this time, Covid can spread as evacuees stock up on supplies, interact with other evacuees in public shelters or with their host families, and as they begin recovery, but the simulations do not consider how these factors individually might contribute to spread.

These numbers illustrate the effect that choosing particular counties could have on pandemic spread. But they are only rough estimates and don’t account for a number of other potential factors. Dahl stresses that their study simplifies a very complicated challenge involving a lot of uncertainties that the team could not capture, including who evacuates, where they stay in the host counties (in hotels or with family and friends, for example) and for how long. It also doesn’t take into account the distance evacuees might have to travel to get to such a wide range of counties.

“The things we’re modeling are very complex and varied,” Dahl says. “The reality is that there is just an incredible amount of variability in terms of the public health guidance counties are giving, and how much compliance and enforcement you have.” 

But the paper remains significant as a preview of what to expect as the U.S. faces an unprecedented duo of emergencies, says David Abramson, a professor at New York University who studies risk communication and the health consequences of disasters like hurricanes, who was not involved in the study. 

It raises crucial questions that state and local emergency officials need to consider, like how much interaction evacuees will have with current residents in the host counties, he says. That in part depends on what accommodation officials set up for the incoming population, and whether they can acquire adequate testing capabilities to monitor the infection rate among evacuees. It also depends on how long that population intends to stay.  The study assumes that evacuees spend seven days in the host counties, but some disasters go on much longer. 

The Superdome in New Orleans became a shelter of last resort for thousands during Hurricane Katrina.
The Superdome in New Orleans became a shelter of last resort for thousands during Hurricane Katrina.

“If it’s a very short period of time, then the odds of them mixing with the population there is much more limited,” he says. “When you’re looking at really long-term displacement events like Katrina, that’s a whole other ball game.” Hurricane Katrina hit 15 years ago this month, displacing 400,000 residents from New Orleans — some for months, and others permanently.

Compliance rates for voluntary evacuations are another big variable. It can range anywhere from 20% to 80%, with the average at about 40% to 50% of a population, Abramson says. And the pandemic throws a wrench into those behavior patterns. A recent consumer survey of 401 Floridians by the American Automobile Association, for instance, found that 42% of respondents said social distancing made them less likely to evacuate during a major hurricane.

“Decision-making around evacuation is influenced by different factors,” Dahl says. “Someone’s financial status could be very different this year and this could alter evacuation patterns compared to what we observed in the past.”

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