Increasingly, it looks like omicron’s takeover from delta is assured — and that delta is unlikely to resurface in a meaningful way, even after omicron has burned through the population. In many states, omicron now makes up 99% or more of all coronavirus infections, according to an analysis by Trevor Bedford, a biostatistician and biologist at the Fred Hutchinson Cancer Research Center in Seattle. Bedford and his colleagues have been tracking genetic sequences collected from COVID-19 patients and have found that omicron infections began to outnumber delta infections in mid- to late December. Even in states with relatively less omicron, the variant is responsible for 80% or more of cases, Bedford wrote on Twitter on Jan. 5.

When omicron first emerged, researchers worried that both variants might co-circulate. It was clear that omicron could spread like wildfire, but it wasn’t clear whether this was due to an intrinsically better ability to spread, or whether omicron was just evading immunity in vaccinated and previously infected people, giving it targets that delta didn’t have. As it turns out, however, omicron does evade immunity. But it’s also intrinsically two to three times more transmissible than delta, and that explains omicron’s world domination.