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A calculated risk

Georgia Tech tracks chance of COVID-19 exposure at events

If someone were to host a gathering of 25 people in Isle of Wight County — say, a birthday party — there’s currently a 46% chance that at least one of the guests will be carrying the COVID-19 virus.

In Norfolk, which has become one of the new epicenters of the virus in Hampton Roads, that same gathering of 25 people carries a 72% risk. If that gathering grows to 100 people, exposure becomes a 99% risk — almost a certainty.

That’s according to data Georgia Tech researchers have compiled to track the risk of exposure to the novel coronavirus on a county-by-county and state-by-state basis. The resulting map and dashboard, available to the public at covid19risk.biosci.gatech.edu, allows users to gauge their risk of exposure in each locality based on the size of an event, with options ranging from 10 people to 10,000.

According to Georgia Tech Assistant Professor Clio Andris, a member of the dashboard’s design team, the county-by-county risk assessment assumes every confirmed case in that locality has managed to infect between five to 10 people, each of whom may be undiagnosed and asymptomatic but still able to pass the virus onto others.

It does not, however, treat outbreaks differently than community spread, Andris confirmed. This means that if 60 residents at a nursing home tested positive for the virus in April and have been quarantined ever since, the system would weigh the risk of exposure to these people no differently than 60 positive individuals who don’t live or work together.

According to data from the U.S. Centers for Medicare and Medicaid, 63 of Isle of Wight’s 353 total cases and eight of its nine COVID-19-related deaths are tied to an outbreak at Consulate Health Care, a long-term care facility in Windsor. The facility has not reported a new confirmed case since it began releasing its site-specific case totals to the federal government in late May, and the Virginia Department of Health lists the outbreak as being closed.

Surry County, by comparison, has had only 38 confirmed cases to date, yet the risk of exposure at a gathering of 25 people was listed at 47% as of Aug. 3 — 1% higher than that of Isle of Wight for the same size event. Surry has a countywide population of just over 6,400, according to U.S. Census Bureau estimates, while Isle of Wight has more than 37,000.

“We calculate the risk as a ratio of the cases to the total population,”Andris said. “So when there is a low population … it boosts the rate.”

According to Dr. Todd Wagner, director of the Virginia Department of Health’s Western Tidewater Health District, the vast majority of COVID-19 cases district-wide continue to be from smaller social gatherings such as backyard barbecues and birthday parties. The health district encompasses all of Isle of Wight County, Southampton County and the cities of Franklin and Suffolk. Surry County is in the Crater Health District.

Andris further confirmed that the risk of exposure does not necessarily equate to the risk of infection, meaning even if someone unknowingly carrying the COVID-19 virus showed up at an event, practicing social distancing and wearing face masks could reduce the risk that the carrier would pass the virus on to other attendees.

The reverse, unfortunately, is also true.

“There can be the ‘super-spreader’ phenomenon where one case can infect a large number of people,” Wagner said. “So we don’t want to presume that just because there is a small number of cases predicted at an event, that doesn’t mean that there can’t be a large number infected.”