Geneva, Switzerland - A leading figure at the World Health Organization (WHO) has said it is not "advisable" to reduce Covid protocols, including self-isolation periods, as increasing case numbers spiral across the globe.

Dr. Michael Ryan, executive director of the WHO's Health Emergencies Program, warned that governments need to be "careful" about reducing restrictions in light of the Omicron variant.

The warning comes after the Centers for Disease Control (CDC) released new guidelines on Monday that slashed isolation and quarantine times in half, from 10 days to five, for those who had tested positive and no longer had symptoms.

The CDC said most transmission of the virus happens in the two days before and three days after symptoms develop.

Speaking at a WHO press conference on Wednesday, Ryan said: "Even with the previous variants, most people will incubate and show symptoms or be positive within that first six days or so, and the chances then of being positive or transmitting the disease after that are lower - but it is then for governments to make that judgment call of when to allow people out of a quarantine situation with extra tests.

"The most important thing at this moment is we need to be careful about changing tactics and strategies immediately on the basis of what we're seeing in early Omicron data.

"'Oh, it's less severe' - maybe it's not. 'It's more transmissible' - maybe it is. But we have to wait and see," the director said of the changing information.

"We have to wait and see," he reiterated.