The pandemic is drawing to a close, World Health Organization officials said Wednesday. But they refused to rule out a plot twist that could reverse some, if not all, progress made on the disease over the past three years.

“The acute phase of the pandemic is ending,” Dr. Sylvie Briand, director of Global Infectious Hazard Preparedness and Emergency Preparedness at the WHO, told viewers of a COVID webinar hosted by the organization. She was referencing the first and often worst phase of a pandemic, during which a new virus presents a global emergency. The WHO late last month extended the COVID-19 global health emergency status by another three months, but noted that the pandemic “may be approaching an inflexion point.”

But “until no one is at risk, everyone is at risk” of a disease that continues to evolve and affect different countries to different extents, Maria Van Kerkhove, technical lead for COVID-19 at the WHO, added.

While Omicron subvariant BA.5, which surged this past summer, and its descendents continue to dominate globally, “the virus has not stabilized into a predictable pattern of evolution,” Dr. Lorenzo Subissi, technical officer of emerging zoonotic diseases at the WHO, told those tuning in.

COVID will, in all likelihood, continue to spawn variants that are increasingly more transmissible and immune-evasive—and they may or may not be more severe, he added.

Dr. Arurag Agrawal, director of the Council of Scientific and Industrial Research Institute of Genomics and Integrative Biology in India, raised the possibility of a more severe strain of COVID in the future, Omicron or otherwise.

There exists a popular belief that Omicron is a weakened version of the original COVID. But its generally more mild presentation is likely due, at least in part, to widespread T-cell immunity in many areas of the world. While T cells can’t stop disease, they blunt its impact, often making it appear more mild than it might in a population with virtually no immunity—like China before the lifting of “zero COVID” measures.

“Omicron can cause severe disease,” Agrawal said. “It is not nature’s vaccine.”

Recombinants, or combinations, of two COVID variants, like XBB.1.5—a mix of two spinoffs of so-called “stealth Omicron” BA.2—are a bit of a wildcard, according to experts.

XBB.1.5 has not shown signs of increased disease severity. But experts can’t exclude the possibility that COVID will combine with another coronavirus altogether—either in humans or in animals, from which it could spill back into humans, Lorenzo said.

What’s more, with variant surveillance on the decline and surveillance of variants in animals anemic as always, it’s possible such a development has occurred or will occur, and that humanity will be caught by surprise.

“We need to make sure this is not happening,” he said, referencing the possibility of a new coronavirus via recombination.

Agrawal called such a scenario “worst case.”

Dr. Jay Varma, chief medical adviser at the New York-based think tank Kroll Institute, told Fortune that the potential of recombination of COVID and another coronavirus doesn’t concern him any more than the potential of a more dangerous COVID variant.

“The most dangerous variant would be one that develops mutations that make it evade pre-existing immunity against severe illness and death,” he said. “In that situation, vaccines and prior infection would no longer greatly reduce the risk of hospitalization and death, as they do now, and there would, effectively, be a new pandemic.

WHO panel members on Wednesday expressed hope that the current pattern of relatively mild disease for many will hold. But, as experts point out, hopes don’t always align with reality.

Now in its fourth year of the pandemic, the world is in a difficult position, Van Kerkhove said, adding, “No one really wants to talk about COVID anymore.”

A misconception exists that the world should wrap up the COVID-19 emergency and prepare for the next, she said. But “everything we’re doing—strengthening systems, building new systems … that’s preparedness for the next one,” she said.

“Responding to COVID is pandemic preparedness for the future.”

Eventually, the once novel coronavirus will become one of the many global diseases the WHO works to keep in check, according to Van Kerkhove.

But “we’re still not there yet,” she said, adding that she remains “optimistic that we will end the emergency of COVID this year.”