You go out to a bar with friends this week, and you’re planning to visit your elderly relatives in a few days. You feel healthy, and you even get a COVID-19 test out of caution. The result comes back negative. Is it safe to go?
Not exactly, experts say. How well COVID-19 tests work in people who feel healthy is still a key unknown of the pandemic. The test may overlook the small but growing amounts of virus in someone who has not yet developed symptoms, who could still go on to spread COVID-19 without knowing it. So as Americans weigh the risks of attending protests, rallies, birthday parties, dinners, and all of the social gatherings that make up normal life, they will have to contend with the uncertainty that a negative test result does not rule out infection. “If they go do a risky contact, they can’t get a ‘Get out of jail free’ card. They can’t just get tested and feel fine,” A. Marm Kilpatrick, a disease ecologist at UC Santa Cruz, says.
Understanding false negatives from COVID-19 tests is especially important because people who do not yet know that they’re sick play a major role in the spread of COVID-19. A study based on data in and around China suggests that 44 percent of transmission comes from presymptomatic cases. The United States has not isolated people who say they feel sick as aggressively as China has, so it likely has a higher proportion of symptomatic transmission, Benjamin Cowling, an epidemiologist at the University of Hong Kong, who co-authored the study, says. But China’s experience makes clear that simply isolating people once they are sick is not enough. “We can’t ignore presymptomatic transmission,” Cowling says. “Even if you manage to stop some of the transmission from going on by doing isolation cases, you still will have this presymptomatic transmission, which keeps the epidemic going.”