The World Health Organization warned on Wednesday that a “tsunami” of Omicron and Delta cases will drive health systems around the world towards collapse. 

“I’m highly concerned that omicron, being more transmissible (and) circulating at the same time as delta, is leading to a tsunami of cases,” WHO head Tedros Adhanom Ghebreyesus said at an online news conference. That, he said, will put “immense pressure on exhausted health workers and health systems on the brink of collapse.”

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Top officials with the UN health agency cautioned that it’s still too early to be reassured by initial data suggesting that the omicron variant, which was first reported last month in southern Africa, leads to milder disease. It is now the dominant variant in the United States and parts of Europe.

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According to WHO’s figures, the number of COVID cases recorded worldwide increased by 11% last week compared with the previous week, with nearly 4.99 million newly reported from December 20-26. New cases in Europe were up 3% while those in the Americas rose 39% and there was a 7% increase in Africa. The global gain followed a gradual increase since October.

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WHO’s emergencies chief, Dr. Michael Ryan, said it will be important in the coming weeks to “suppress transmission of both variants to the minimum that we can.”

Ryan said that omicron infections began largely among young people, “but what we haven’t seen is the omicron wave fully established in the broader population. And I’m a little nervous to make positive predictions until we see how well the vaccine protection is going to work in those older and more vulnerable populations.”