What we know from the experience of half a dozen countries—the first wave of countries that were really hit by the British variant—is that when the variant passed 50 percent of the variants being sampled, it really took off.

Jonathan “Jono” Quick, a public health doctor with the Rockefeller Foundation and author of “The End of Epidemics,” commenting to The Washington Post for its recent report, “As Variants Surge, Efforts in a Patchwork of Surveillance Have Ramped Up to Detect Them.”