• The fast-spreading delta variant remains the "most concerning" coronavirus strain despite the emergence of the mu variant, WHO officials said Tuesday.
• Any new virus that emerges has to be able to compete with the "best of class," and right now that is delta, the WHO's Dr. Mike Ryan said.

The fast-spreading delta variant remains the "most concerning" coronavirus strain despite the emergence of the mu variant, World Health Organization officials said Tuesday.

The mu variant, which was added to the WHO's list of variants "of interest" last week, has mutations that suggest it could evade the immune protection provided by natural infection or vaccination, Maria Van Kerkhove, the agency's technical lead for Covid-19, said during a virtual press event. Still, the new variant — first detected in Colombia but now confirmed in at least 39 countries — hasn't taken off like delta, she said.

"The delta variant for me is the one that's most concerning because of the increased transmissibility," Kerkhove said, adding it has at least double the transmissibility of the original ancestral virus that emerged in late 2019.

Delta has rapidly spread to at least 170 countries, including the U.S., since it was first detected in India in October, quickly becoming the dominant variant in many of those regions, according to the international health agency.

Mu, also known by scientists as B.1.621, is increasing its prevalence in some South American countries but is also decreasing in other regions of the world, especially where the delta variant is already circulating, she said.

Any new virus that emerges has to be able to compete with the "best of class," and right now that is delta, said Dr. Mike Ryan, head of the WHO's health emergencies program. The delta variant tends to "outcompete" other variants, like mu, he said.

It doesn't matter if a new variant has genetic changes that allow it to evade the protection of vaccines if it can transmit efficiently, Ryan said.

"We will expect to see more" variants like delta, he said. "Not every variant means the sky is going to fall in. Each variant needs to be looked at for its characteristics in terms of its potential to cause more severe disease, its potential to transmit, its potential to escape vaccines."

The agency is monitoring four variants "of concern": alpha, which was first detected in the U.K.; beta, first detected in South Africa; gamma, first detected in Brazil, and delta. A variant of concern is generally defined as a mutated strain that's either more contagious, more deadly or more resistant to current vaccines and treatments.

The WHO is also keeping a close watch on four other variants of interest — including lambda, first identified in Peru — that have caused outbreaks in multiple countries and have genetic changes that could make them more dangerous than other strains.

White House chief medical adviser Dr. Anthony Fauci addressed concerns about the mu variant last week, saying it wasn't an immediate threat to the U.S.

"We're paying attention to it, we take everything like that seriously, but we don't consider it an immediate threat right now," Fauci said at a press briefing Thursday.

The WHO has said more studies are required to understand the clinical characteristics of the new variant.

Right now it's a "variant of interest," Ryan said Tuesday. "If it's of concern, then we really need to look at diagnostics and how we develop our vaccines."