World Health Organization (WHO) Director-General Tedros Adhanom Ghebreyesus on Wednesday called on countries to stay vigilant in their fight against the COVID-19 pandemic as the omicron variant spreads, warning that “complacency now will cost lives.”

Speaking at a press briefing, Ghebreyesus said omicron has now been detected in 57 countries and has the potential to have a “major impact” on how the pandemic unfolds going forward.

“In South Africa, the number of omicron cases is increasing quickly. However, omicron was detected when transmission of delta was very low, so it had little competition,” said Ghebreyesus.

“It will therefore be important to monitor carefully what happens around the world, to understand whether omicron can outcompete delta. For that reason, we call on all countries to increase surveillance, testing and sequencing.”

He stressed that data on the omicron variant is still sparse and the threat it poses is still not well understood.

“Any complacency now will cost lives,” Ghebreyesus said.

“The steps countries take today, and in the coming days and weeks will determine how omicron unfolds. If countries wait until their hospitals start to fill up, its too late. Dont wait. Act now.”

Though data is sparse, health officials have so far cautioned against panicking in response to omicron. Anecdotal reports have suggested that the strain causes milder illness, though more concrete evidence has yet to be released.

On Wednesday, Pfizer said three doses of its COVID-19 vaccine appear to be effective at protecting against omicron, based on preliminary studies.

“Although two doses of the vaccine may still offer protection against severe disease caused by omicron strain, its clear from these preliminary data that protection is improved with a third dose of our vaccine, Pfizer CEO and chairman Albert Bourla said.