When the Omicron variant (now categorised as BA.1) swept across the world at the end of last year it was seen by optimists as the final chapter in the Covid story – it was so contagious it would infect essentially anyone, but would be far less likely to cause serious illness. Now a new wave of Omicron – the BA.2 variant – is becoming dominant in many parts of the world. In the UK, cases are again on the rise.
Genomic surveys show that BA.2 made up 76 per cent of new cases in England as of 5 March. The below is from the Sanger Institute:
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