Reported numbers probably too low

When Nevada casinos were permitted to reopen on June 4, the natural concern was how the influx of people gathered indoors would affect the spread of the novel coronavirus. According to new data from the Nevada Department of Health and Human Services, hundreds of people tested positive for COVID-19 either while visiting the state or shortly after they returned home.

As of August 15, “at least” 530 Nevada visitors tested positive for the virus – an increase of nearly 200 from July 25, when 347 infected people had been reported. The data reaches back to June 1, just before the casinos reopened.

All but 11 of the confirmed positive cases were from tests done while the visitors were still in Nevada.

poor reporting systems have likely led to a severe under-counting of positive cases in visitors

Professor Samuel Scarpino, head of Northeastern University’s Emergent Epidemics Lab, told the Las Vegas Review-Journal that poor reporting systems have likely led to a severe under-counting of positive cases in visitors. People’s home states only voluntarily notify Nevada if a test comes back positive; Arizona, Ohio, and California are the only states to have sent such reports.

“My hunch would be that those numbers are low because of reporting issues, not because they’re actually low,” Scarpino said.

Study shows wide visitor dispersal

A recent study by ProPublica backs up Scarpino’s opinion that the figures may be much lower than reported. Las Vegas, ProPublica says, may very well be a COVID hub for the rest of the United States.

ProPublica asked X-Mode and Tectonix to analyze anonymous cell phone data to track the travels of people going in and out of Las Vegas. The data represents about 5% of cell phone users and was gathered primarily via weather and fitness apps that report users’ locations.

In a Friday-Monday period in mid-July, 26,000 smartphones were identified on the Las Vegas Strip. Thousands of those same phones pinged from every one of the 48 contiguous states except for Maine in that same four-day period.

The U.S. saw a surge in COVID-19 cases during this same time; health officials point to data like this to explain some of that. As the country continues to relax its mitigation efforts and people increase their mobility, the virus spreads.

Contract tracing difficult with COVID-19

The data and subsequent mapping of smartphone movement also illustrates both the importance and difficulty of COVID-19 contact tracing.

Kimberly Hertin, disease surveillance supervisor in the Office of Epidemiology and Disease Surveillance for the Southern Nevada Health District, told ProPublica that the two-week incubation period of the virus makes it “pretty much impossible” to determine the source of a case in a tourist. By the time a person tests positive at home, there is almost no telling if they contracted COVID-19 in Las Vegas or before they went on the trip.

And contact tracing is centered around the person who has the virus, not the source of their infection. The goal is to keep the infected person from spreading it.

“Once you’ve reached the point of community spread with this virus, it’s hard to jump to that conclusion of any clusters or outbreaks,” Hertin said.

contact tracing systems tend to be developed more for lower-spread diseases

On top of that, contact tracing systems tend to be developed more for lower-spread diseases like E. coli or salmonella where a source can be determined much more easily. Something that spreads as quickly as COVID-19 – there have been more than 6 million confirmed cases in the United States and almost 68,000 in Nevada – is too difficult for contact tracing networks to handle.