by Michael Riordan, who writes about science, technology and public policy from his home in Eastsound.

Thanks to a vigorous response to the coronavirus outbreak by the county health department led by Public Health Officer Frank James, M.D., San Juan County has thus far been spared the worst effects of the pandemic sweeping through the state and nation. But the anticipated influx of tourists coming to our islands this summer may undermine that success and probably amplify our risks.

On May 19 the County passed the thousand mark for COVID-19 testing but had registered only 15 positive cases — and absolutely no deaths. That’s a “positivity rating” of just 1.5 percent, on par with South Korea. For comparison, Washington state currently has a positivity rating of 6.7 percent and the United States a woeful 12.8 percent.

Why does this matter? And why are we doing so well?

Low positivity ratings indicate high levels of testing — in our case, about one of every 17 county residents. And after a slow start in March, when testing supplies were scarce, county public-health officials have been testing lots of “asymptomatic essential workers” in addition to those with flu-like symptoms. According to Dr. James, such individuals could be unduly exposed — for example, health-care workers, first responders, grocery clerks, pharmacy employees and ferry workers.

“COVID-19 testing is certainly making a big difference, together with early case identification and isolation of infected individuals,” said Dr. James. This aggressive testing program helps identify infected individuals quickly — before they have much chance to expose others — keeping case counts down. That knowledge then helps guide subsequent contact-tracing activities, which can, therefore, be kept manageable.

And the high number of negative test results suggests that less than 1 percent of county residents have been infected. For comparison, Skagit County has recently been seeing Covid-19 infection rates of 3 to 4 percent in its testing program. But our success rate has an unfortunate downside: over 99 percent of us remain vulnerable.

The success of the county effort has been due in part to the limited number of new people coming on island since late March when Governor Inslee’s “Stay Home — Stay Healthy” order went into effect. Apart from a “spike” of four positive cases on one day in early April, only one or two cases per day have come in, and just one in the last 32 days.

At that level, the all-important follow-up activities of contact tracing, isolation and disease surveillance have been manageable with the limited public-health staff. These activities will become much more complex and difficult as tourists arrive, some of them likely carrying the virus. If diagnosed, such infected individuals will not have residences here where they can be quarantined. And an asymptomatic tourist could come on island, infect islanders, and be long gone before anyone here experiences symptoms.

Since tourism has become essential to the island’s economy, county leaders and public-health officials face a difficult choice between life and livelihood. Perhaps we as citizens need to reassess the value of tourism here — and whether it has swollen to levels that have now become unsupportable.