The spread of the virus was also being driven by the context of people mixing more, WHO saidGeneva:
The World Health Organization said Friday that the spread of Omicron was down to a combination of factors including the make-up of the Covid-19 variant and increased social mixing.
Maria Van Kerkhove, the WHO's Covid-19 technical lead, said people therefore needed to think about reducing their exposure to the virus and take control over its transmission, following a week of record numbers of new cases.
Van Kerkhove said Omicron transmitted very efficiently between people due to a number of reasons.
Firstly, its mutations allow the virus to adhere to human cells more easily.
"Second, is that we have what is called immune escape. And this means that people can be reinfected either... if they had a previous infection or if they've been vaccinated," she said, in comments circulated by the WHO.
"The other reason is that we are seeing replication of Omicron in the upper respiratory tract -- and that's different from Delta and other variants, including the ancestral strain which replicated in the lower respiratory tract, in the lungs."
But in addition to these factors, the spread of the virus was also being driven by the context of people mixing more, spending more time indoors in the northern hemisphere winter, and not adhering to measures such as physical distancing.
Just under 9.5 million new Covid-19 cases were reported to the WHO last week -- a record, up 71 percent on the week before.
"The general public out there, what you need to worry about is just reducing your exposure to the virus," said Van Kerkhove.
"We want people to understand and feel empowered that they have some control over infection."
She also added that avoiding developing ongoing Long Covid symptoms was "reason enough" to try to prevent getting infected with the disease in the first place.