“The Conversation” Editor’s note: The omicron variant of rapid surge in cases globally. We asked a team of virologists and immunologists from the University of Colorado Boulder to weigh in on some of the pressing questions that people are asking about the new variant.
How is omicron different from previous variants?
There are two key differences between omicron and previous variants of the SARS-CoV-2 virus that emerged in late 2019. Early data suggests that omicron cases are milder than infections caused by the delta variant. On the flip side, omicron is far more transmissible – meaning it spreads easier – than previous variants. It can be confusing to think about the overall effects of a milder virus that is also far more infectious.
When the delta variant became dominant and displaced alpha in the summer of 2021, it managed to do so because it was between 40% and 60% more transmissible. Now, the omicron variant is even more transmissible than delta.
It’s difficult to put numbers around how intrinsically more transmissible one variant is than another, because human behaviors and vaccination percentages are constantly in flux. Those factors, together with transmissibility, affect how a virus fares in a population.
In comparison with the original strain of SARS-CoV-2, omicron contains 72 mutations throughout its genome. Some of these mutations account for the complex new features that characterize this variant. Half of those changes are in the spike protein, the critical surface protein that enables the virus to latch on and infect cells. It is also the key virus feature that is recognized by the human immune system.
Why is omicron spreading so quickly?
Initial studies suggest that omicron is more effective at reproducing in the upper airways, including the nose, throat, and mouth, than earlier variants, making it more similar to a common cold virus. If data from these preliminary studies holds up, then it may help explain omicron’s high transmissibility: Viruses replicating in the upper airways may spread more easily, although the reasons for this are not completely understood.
In addition, omicron is often able to evade existing immunity long enough to start an infection, cause symptoms and transmit onward to the next person. This explains why reinfections and vaccine breakthrough infections seem to be more common with omicron.