Scientists with the World Health Organization have been alarmed by the spread of the coronavirus Omicron variant. But they confirmed on Tuesday that this strain is causing milder symptoms.

WHO officials said the variant affects the upper respiratory tract. But they said vaccines have protected people against hospitalization, severe disease, and death.

Public health experts report that Omicron is more transmissible than other variants. In many places, it has become the dominant strain within weeks.

The spread poses a threat to medical systems in areas where a high proportion of the population remains unvaccinated. But experts say even those who are vaccinated or have been infected before can get sick.