The World Health Organization on Tuesday said the Covid omicron variant can result in life-threatening illness for the unvaccinated, the elderly and people with underlying conditions.

Dr. Mike Ryan, director of the WHO’s health emergencies program, said unvaccinated people face a higher risk that an omicron infection will make them severely sick and and possibly even kill them.

“Omicron still represents a massive threat to their life and a massive threat to their health,” Ryan said of the unvaccinated during a Q&A livestreamed on WHO’s social media channels Tuesday.

Ryan said vaccinated people, on the other hand, generally experience mild illness if they get a breakthrough infection.

“People should really look at this in terms of really seriously considering getting out there and getting vaccinated,” Ryan said.

Maria Van Kerkhove, the WHO’s Covid-19 technical lead, said the elderly and people with underlying conditions face an elevated risk of death from omicron compared to other groups.

“We do know that mortality increases with omicron with increasing age,” Van Kerkhove said. “We also have data from some countries that show that people with at least one underlying condition are at an increased risk of hospitalization and death, even if you have omicron as compared to delta.”

Van Kerkhove said a lower proportion of people are dying from Covid during the omicron wave, and the overall risk of severe disease and hospitalization is lower compared with delta. However, she cautioned that less severity does not mean omicron only causes mild illnesses.

“It is not just a mild disease,” Van Kerkhove said. “This is really important because people are still being hospitalized for omicron.”

Van Kerkhove warned people should not become fatalistic and resign themselves infection, cautioning that the long-term health implications of catching omicron remain unknown. She said people should get vaccinated, wear a well-fitting mask, avoid crowds and work from home if possible.

Ryan said health-outcomes from viral infections often depend on a person’s baseline level health, including whether the immune system is strong or not. People with diabetes, for example, are not as well equipped to fight off the virus.

“We can definitely say that an omicron variant causes, on average, a less severe disease in any human being — but that’s on average,” Ryan said. “There are hundreds of thousands of people around the world in hospital as we speak with the omicron variant, and for them that’s a very severe disease.”

Van Kerkhove said omicron has been detected in every country where there’s good genetic sequencing and is likely present in every nation. She said omicron is overtaking delta worldwide and becoming dominant.

The WHO reported 15 million new infections and 43,000 deaths worldwide for the week ending Jan. 3.