Why Omicron is so transmissible: By Dr Maria Van Kerkhove, WHO COVID-19 Technical lead

Dr Maria Van Kerkhove, COVID-19 Technical lead, WHO Health Emergencies Program "So, there are several reasons why we're seeing such a sharp increase in case numbers. First, is the emergence of Omicron and the circulation of Omicron, which transmits very efficiently between people. There's a number of reasons for this. First, is the mutations that it has. The virus is able to adhere to human cells more easily. It has mutations that allow it to do that. Second, is that we have what is called immune escape. And this means that people can be reinfected either from if they had a previous infection or if they've been vaccinated. Now you have to remember, vaccines are incredibly effective at preventing severe disease and death, but they don't prevent all infections and they don't prevent all onward transmission. So it is still absolutely critical to get vaccinated because it will save your life, but it doesn't prevent all infections or transmission. 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. And so this combination of factors allowed the virus to spread more easily. But what is also important to remember is that this is in the context of people mixing more, people coming into contact with others more, people not adhering to public health and social measures like distancing, wearing of a well-fitting mask, avoiding crowds, spending time more outdoors than indoors and in the northern hemisphere, where it's getting colder and we're in the winter months now, people spend more time indoors and particularly in crowds. All of those factors allow viruses to spread, whether it's Omicron or something else."