“Of those four variants of concern, delta is, by far, the most transmissible,” Van Kerkhove added. “If delta is identified or starts to circulate in a country where there is beta … [delta] has quickly replaced the variant there.”
The delta variant has appeared in 185 countries, as global coronavirus cases near 230 million and deaths surpass 4.7 million since the start of the pandemic.
The mu variant, which has been identified thousands of times in the United States and had raised concerns that it may be more resistant to vaccines than its sibling variants, is also being replaced in countries where delta is present, Van Kerkhove said. “Lambda and mu don’t seem to be dominant.”
By the end of July, delta had overpowered all its rivals in the United States. The “coronavirus pandemic in America has become a delta pandemic,” The Washington Post reported in August, noting that it accounted for 93.4 percent of new infections.
In the past four weeks, more than 98 percent of the coronavirus sequences submitted from the United States to the Global Initiative on Sharing All Influenza Data, a large database of novel coronavirus genome sequences in the world, were of the delta variant.
Separately on Tuesday, the WHO reclassified three other variants — eta, iota and kappa — to “monitoring” status, indicating that they “no longer pose a major added risk to global public health” as they are being outcompeted by delta.
“They are not taking hold,” Van Kerkhove said.
Ahead of a vaccination summit that President Biden is holding Wednesday, the World Health Organization also said that the number of global coronavirus deaths and infections had declined in recent weeks, reflecting increased access to vaccines in rich and some middle-income nations even as developing economies scramble to immunize their populations.
Mike Ryan, the WHO’s emergencies chief, warned in the same video event that previous dips had been followed by sharp spikes, although he said that countries with high immunization rates had less to worry about. “It shows us that vaccines work,” he said.