The World Health Organization’s (WHO) chief on Wednesday urged wealthier nations not to prioritize coronavirus booster shots amid a “tsunami of cases” driven by the new omicron variant.

WHO Director-General Tedros Adhanom Ghebreyesus during a news conference pressed wealthy nations to spread their share of vaccines with other countries, arguing the coronavirus will continue to evolve and the pandemic will worsen “if we don’t improve our collective response.”

“Delta and omicron are twin threats that are driving up cases to record numbers,” he said. “I’m highly concerned that omicron being more transmissible [and] circulating at the same time as delta is leading to a tsunami of cases.”

Tedros has repeatedly knocked wealthier nations for prioritizing boosters or third shots of the vaccine, saying last week that “no country can boost its way out of the pandemic.”

Developing countries continue to struggle to vaccinate their populations. Low- and middle-income nations have an average 20 percent vaccination rate, compared to 80 percent in wealthier countries, according to a September update from Covax, a collaborative initiative to share vaccines with the world.