Noting that the variant has already been detected in many countries and that closing borders often has limited effect, the World Health Organization called for frontiers to remain open.

Dr. Francis Collins, director of the National Institutes of Health in the United States, meanwhile, emphasized that there is no data yet that suggests the new variant causes more serious illness than previous COVID-19 variants.

“I do think it’s more contagious when you look at how rapidly it spread through multiple districts in South Africa. It has the earmarks therefore of being particularly likely to spread from one person to another. … What we don’t know is whether it can compete with delta,” Collins said on CNN’s “State of the Union.”

Collins echoed several experts in saying the news should make everyone redouble their efforts to use the tools the world already has, including vaccinations, booster shots and measures such as mask-wearing.

“I know, America, you’re really tired about hearing those things, but the virus is not tired of us,” Collins said.

The Dutch public health authority confirmed that 13 people who arrived from South Africa on Friday have so far tested positive for omicron. They were among 61 people who tested positive for the virus after arriving on the last two flights to Amsterdam’s Schiphol airport before a flight ban was implemented. They were immediately put into isolation, most at a nearby hotel.