The head of the World Health Organization says the Omicron coronavirus variant is probably spreading in most countries, even where it has not been detected yet.

WHO Director-General Tedros Adhanom Ghebreyesus said at a news conference on Tuesday that 77 countries have now reported cases of the mutant strain. He said, "Omicron is spreading at a rate we have not seen with any previous variant."

Tedros voiced concern that people are dismissing Omicron as mild. He warned that "Even if Omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems."

Tedros called on countries to take thorough antivirus measures, such as vaccinations and masks.

The WHO chief referred to a vast gap in rates of vaccination between countries. He noted that, "The emergence of Omicron has prompted some countries to roll out booster programs for their entire adult populations," while "41 countries have still not been able to vaccinate 10 percent of their populations."

Tedros reiterated calls for the equitable distribution of vaccines worldwide, saying that "If we allow inequity to continue, we allow the pandemic to continue."