More than half of U.S. counties don’t have a single coronavirus testing site, according to a recent report by Castlight, a health software company.Why it matters: That leaves a wide swath of the country — particularly rural areas — vulnerable to undetected coronavirus outbreaks, especially as lockdown measures ease. Asking people to travel long distances to get a coronavirus test is both unrealistic and potentially dangerous. By the numbers: 54% of all counties don’t have a testing site. Among counties with 50,000 or more people, 38% don’t have any testing sites. Among rural counties with fewer than 10,000 residents, 68% don’t have any.And even among counties that do have testing sites, 58% don’t have the capacity to meet minimum recommended testing levels, which Castlight defined as 1% of their population every week.In Texas, for example, big cities have enough testing capacity to meet Castlight's thresholds, but there's a cluster of 26 counties — and 315,000 residents — in the middle of the state with no testing access.The big picture: Commitment from retailers like CVS, Walgreens and Walmart to open new testing sites could help close some of these gaps.CVS recently announced that it plans to have opened up to 1,000 self-swab testing sites around the country by the end of May.These sites will primarily be located at pharmacies with drive-through capabilities, although in a handful of cases, the company is establishing parking lot test sites.Some cities, including New Orleans, Nashville and Seattle, have set up mobile testing in order to reach communities without adequate testing access. The bottom line: There’s no geographic barrier that prevents the coronavirus from spreading to counties without the ability to test for it, especially in states that are reopening while their caseloads are still high.