Clip from an article on latest BA.2 variant.
'So what's going on? Firstly, both the BA.1 and BA.2 variants do not have an intrinsically much higher growth rate than any previous variant. Instead, they have spread globally for two main reasons. First, both can infect people who were protected from earlier viral strains by vaccines and/or previous infection because their spike proteins (the region which neutralising antibodies, which protects people from infection by the virus, bind to) are drastically different to any previous Covid strain.
This means that in terms of protection against infection, even populations with high rates of vaccination or prior infection were essentially encountering it as an almost novel virus.
The second reason behind the explosive growth of Omicron is that its incubation time (the time interval between one infection and the next) is much shorter than for any previous Covid lineage. BA.2 has an even shorter incubation time than BA.1.
Shorter incubation can lead to explosive growth in daily case numbers over a short period of time even if each infected person passes on the virus to a similar number of other hosts.
Despite the ability of BA.1 and BA.2 to readily infect and re-infect people, vaccines and prior infections still protect from severe disease and death, thanks to T-cells that cannot be bypassed by the virus as they target hundreds of different sites across the viral genome.
Moreover, contrary to antibodies which are largely identical in different people, each person mounts a unique T-cell response that targets different sites in the viral genome. As such, a virus simply cannot evolve to escape T-cell recognition at the level of the host population.'