WHO says delta remains “most worrying”

The rapidly spreading delta variant remains the “most worrying” strain of coronavirus despite the appearance of mu, World Health Organization officials said Tuesday.

The mu variant, which was added to the WHO list of “interest” variants last week, has mutations that suggest it could bypass the immune protection provided by natural infection or vaccination, Maria Van Kerkhove, responsible agency technician for Covid-19, said during a virtual press event. However, the new variant – first detected in Colombia but now confirmed in at least 39 countries – has not taken off like a delta, he said.

“The delta variant for me is the one that worries me most about increased transmissibility,” Kerkhove said, adding that it has at least twice the transmissibility of the original ancestral virus that emerged in late 2019.

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

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

Any new virus that emerges should be able to compete with the “best in class,” and right now that’s delta, said Dr. Mike Ryan, head of the WHO health emergency program. The delta variant tends to “outperform” other variants, like mu, he said.

It doesn’t matter if a new variant has genetic changes that allow it to evade vaccine protection if it can be transmitted efficiently, Ryan said.

“We’ll wait to see more” virus like the delta, he said. “Not all variants mean the sky will fall. Each variant must take into account its characteristics in terms of its potential to cause more serious diseases, its potential for transmission and its potential to escape vaccines.”

The agency controls four “worrying” variants, including the delta; alpha, first detected in the UK; beta, first detected in South Africa, and gamma, first detected in Brazil. A variant of concern is generally defined as a mutated strain that is more contagious, more lethal, or more resistant to current vaccines and treatments.

It is also closely monitoring four other variants of interest, including lambda, first identified in Peru, which have caused outbreaks in several countries and have genetic changes that could make them more dangerous than other strains.

The White House Chief Medical Officer, Dr. Anthony Fauci, addressed concerns about the mu variant last week, saying it was not an immediate threat to the United States

“We look at it, we take it seriously, but now we don’t consider it an immediate threat,” Fauci told a news conference Thursday.

The WHO has said that more studies are needed to understand the clinical features of the new variant.

Right now it’s an “variant of interest,” Ryan said Tuesday. “If it’s worrying, we really need to look at diagnoses and how we develop our vaccines.”