But on Tuesday, the WHO warned the world might not get so lucky with the next variant.

“The next variant of concern will be more fit, and what we mean by that is it will be more transmissible because it will have to overtake what is currently circulating,” Maria Van Kerkhove, the World Health Organization’s technical lead on COVID-19, said in a press briefing on Tuesday. “The big question is whether or not future variants will be more or less severe.”

The WHO defines a “variant of concern” as any that either weakens the effectiveness of vaccines or increases transmissibility or virulence to a degree “of global public health significance.” Between Delta and Omicron, the WHO labeled variants like Lambda and Mu as “variants of interest.” But Lambda and Mu did not spread quickly or broadly enough to warrant “variant of concern” status.

Omicron is the only variant of concern the WHO has labeled as such after Delta. Omicron’s enhanced ability to evade vaccines as well as its heightened transmissibility makes the now dominant strain between two and four times as transmissible as Delta.

Naturally, the WHO’s next variant of concern will be in some way more virulent than Omicron is now. But some experts believe that future COVID-19 strains will eventually weaken, even as the virus grows more infectious.

“There is a natural selection that the virus will become less virulent to become more transmissible…A virus is a parasite. It requires a host to survive. The worst thing for the virus is to kill off its host,” Shiu-Wan Chan, lecturer and principal investigator in molecular virology at the University of Manchester told Fortune last week. However, not everyone agrees.

Zeynep Tufekci, a sociologist and pandemic expert, told the New York Times it is “purely luck” if a virus becomes weaker as it evolves, and that viruses “don’t care if you eventually die as long as they transmit through you.”

Van Kerkhove appears to side with Tufekci’s view. The WHO official warned Tuesday that there is “no guarantee” the COVID virus will get weaker as it evolves and that although the world might hope that’s the case, “we can’t bank on it.”

Health authorities might find out whether Omicron’s successor is more or less deadly than the current variant sooner than they hoped. This week the WHO announced that a new Omicron subvariant called BA.2, which is often dubbed “stealth Omicron,” should be investigated and studied separately from the globally dominant Omicron subvariant, BA.1.

“The BA.2 descendant lineage, which differs from BA.1 in some of the mutations, including in the spike protein, is increasing in many countries,” the WHO said. “Investigations into the characteristics of BA.2, including immune escape properties and virulence, should be prioritized independently (and comparatively) to BA.1.”

Some scientists believe BA.2 exhibits enough characteristics that are distinct from Omicron’s original strain for it to be considered its own variant of concern. The stealth Omicron strain—named for a now disproved theory that it is more difficult to detect in tests—has already become the most dominant COVID-19 strain in Denmark, after arriving there late last month.

The BA.2 strain accounts for 45% of new cases in Denmark, and has been detected in over 40 countries including the U.S. But, for now, there is no evidence that BA.2 is more deadly than Omicron’s first strain. Denmark’s Statens Serum Institute said in a statement last week that “initial analysis shows no differences in hospitalizations for BA.2 compared to BA.1.”

Hopefully, the declining rate of hospitalization will continue if and when COVID evolves again.