The chief of the World Health Organization is warning that the Omicron variant of the coronavirus is spreading significantly faster than the Delta variant.

WHO Director-General Tedros Adhanom Ghebreyesus spoke to reporters at the organization's headquarters in Geneva on Monday. It was the WHO's first in-person news conference in nearly a year and a half.

Tedros said, "There is now consistent evidence that Omicron is spreading significantly faster than the Delta variant."

He added, "It is more likely that people who have been vaccinated or have recovered from COVID-19 could be infected or re-infected."

Pharmaceutical companies Pfizer and Moderna say booster shots of their coronavirus vaccines using mRNA technology could make them effective against the Omicron variant, too.

WHO Chief Scientist Soumya Swaminathan said it would be premature in her view to jump to the conclusion that only mRNA vaccines are effective against Omicron.

Tedros referred to a study conducted by a WHO team in the Chinese city of Wuhan that has failed to pinpoint the origin of the coronavirus.

He noted that the study was hampered by a lack of transparency and data-sharing on the Chinese side.