“I think the Delta variant for me is the one that's most concerning because of the increased transmissibility,” said Dr. Maria Van Kerkhove, the agency’s Technical Lead for COVID-19, speaking during an online question and answer session.

“It's doubly transmissible compared to the ancestral strain, which means that it can spread to more people.”

Virus evolving

Dr. Van Kerkhove said that Delta continues to evolve and scientists are studying to see how the virus might be changing, with new variants continuing to emerge.

Last week, WHO announced it was closely monitoring the Mu variant, also known as B.1621, which was first identified in Colombia in January 2021.  It is among five “variants of interest” the agency is tracking at the global level.

Mu has a number of mutations that suggest it could be more resistant to vaccines, WHO said at the time, noting that further research will be needed.

Dr. Van Kerkhove reported that the proportion of Mu cases in South America is increasing, but numbers are decreasing in other countries where the Delta variant is circulating.

Dr. Michael Ryan, Head of WHO’s Health Emergencies Programme, explained that viruses essentially compete against each other.  Currently Delta “tends to outcompete other variants”, he said.

While more COVID-19 variants are to be expected, “not every variant means the sky is going to fall in,” he added.  “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.”

‘Worrying’ plateau

Globally, the overall COVID-19 caseload is “quite a worrying situation”, according to Dr. Van Kerkhove. 

While cases have plateaued, some 4.5 million are reported each week, with deaths hovering around 68,000 weekly, and both numbers are underestimates.

Dr. Van Kerkhove said WHO is seeing “a lot of circulation among unvaccinated people” but there are also positive developments, including a reduction in hospitalizations and deaths among those who have been inoculated against the disease.

“But globally. it's quite worrying,” she said.  “We shouldn't be having this number of cases around the world, especially because we have the tools that really can prevent that from happening.”