(NEXSTAR) – Just when it feels like there’s some positive news in the fight against COVID-19, a new variant rears its ugly head, sending case numbers and hospitalizations soaring. But does that mean the previous variant is no longer a concern?

Well, maybe — but not necessarily, according to experts.

The ongoing COVID-19 health crisis, first declared a global pandemic by the World Health Organization back in March 2020, began following the discovery of SARS‑CoV‑2, new type of coronavirus. As the months wore on, new variants began to emerge, the most concerning of which included alpha, beta and delta, among a few others. And most recently, the omicron variant has become dominant in the U.S., outpacing delta within only a few months of the first detected cases.

“Omicron just pushed delta to the side, just like delta pushed everything else to the side,” Dr. Davidson Hamer, a professor of Global Health and Medicine at the Boston University’s Schools of Public Health and Medicine, tells Nexstar.

Hamer, however, acknowledges that earlier variants of concern (VOC), like alpha, beta or gamma, may still be circulating in places where “sequencing isn’t routine.” But in the U.S., where genome sequencing is more common, variants other than delta or omicron are barely detected. In fact, the CDC’s latest update on variant proportions in the U.S. indicates that earlier variants account for 0% of current cases.

Should we stop sweating the previous variants, then?

“We still worry about the previous ones in the short or mid-term,” warns Dr. Jorge Luis Salinas, a hospital epidemiologist of the Stanford University School of Medicine and a co-medical director at Stanford Healthcare. “It’s possible that they could come back around or in a derivative form.”

As Salinas explains, the variants that create public health concerns have “evolutive advantages” over previous ones, perhaps resulting in increased transmissibility, more severe illnesses or the ability to evade vaccine protections.

“It’s more likely that novel variants are going to rise, that have even more mutations, that make the virus more fit to spread,” Hamer says.

“Usually, it’s sort of the old strain that evolves, or evolves further, but It never really goes back to where it came from.”