The BA.4 and BA.5 sub-variants of Omicron made up 12 percent and 43 percent, respectively, of COVID-19 variants globally as of June 26, the World Health Organization said on Wednesday.

The U.N. health agency said in its latest weekly report that the number of new COVID-19 cases rose by 18 percent in the last week, with more than 4.1 million cases reported globally. Tedros Adhanom Ghebreyesus, the WHO director-general, said cases were driven mostly by the Omicron variants BA.4 and BA.5. 

Among sequences of Omicron, the dominant variant circulating globally, BA.4 and BA.5 spread quickly and accounted for more than half of the sequences, replacing that of the BA.2, which represented 25 percent.

Tedros called for all countries to immunize their most vulnerable populations.

"The world should target 100 percent vaccination of older people and health workers, as well as strengthening the wall of immunity against COVID towards and beyond 70 percent," he said.

WHO has set the target for 70 percent global vaccination coverage by mid-2022 last year. As of June 2022, only 58 of WHO's 194 Member States had reached this target, and in low-income countries, just 37 percent of healthcare workers had received a complete course of primary vaccination, WHO said.

(With input from AP)