| Who ought to be tested for Covid-19? That question has loomed over America's response to the coronavirus from the earliest days, when arriving travelers from infection hot spots weren't screened unless they had symptoms. It's looming still. One answer is: pretty much everyone, as often as possible. Vulnerable nursing home residents, anyone on a college campus, bus drivers and factory workers. Ubiquitous testing, the argument goes, can detect the virus's silent spread and allow carriers to isolate before they pass it on.  Discarded rapid Covid-19 test equipment in a bio hazard waste bin at San Francisco International Airport. Photographer: David Paul Morris/Bloomberg But we don't have enough supply to test everyone all the time. So health officials, doctors, employers and schools must decide how to allocate the tests we do have. Those decisions must balance the public health need for surveillance – picking up unknown, invisible infections – with diagnostic tests needed to tell whether a sick person's illness is Covid or not. Right now, the decisions often seem more random than based on clear evidence for when a test is warranted. Some nursing home residents are tested weekly, a vigilant surveillance regime. But because of bottlenecks at labs, they sometimes don't have the results back before it's time to take a new test. Some college campuses are testing students weekly, while others select a sample group or test those who show symptoms. The U.S. Centers for Disease Control and Prevention escalated the confusion last week when it changed a guideline on its website to suggest that people who have close contact with someone who tested positive for Covid-19 may not need to get tested themselves. The move was widely panned as political and denounced by doctors and infectious-disease experts. A new, low-cost test from Abbott Laboratories is set to boost the supply this fall, which could ease the trade-off between surveillance and diagnostic testing. But until the virus in the U.S. is tamed to much lower levels, or testing capacity exponentially increases, the question of who, when and how often to test won't be easy to answer. –John Tozzi | 
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