Trevor Bedford addresses this question on his twitter. Rt relies on both transmissibility and immune escape. In other words, just because Omicron is spreading fast does not mean it is more transmissible than Delta, but could be a combination of transmissibility and getting around immunity. Also says estimates of current fitness advantage of Omicron are likely to get lower in the coming days as bias from preferential sequencing eases as more surveillance samples are sequenced. He ultimately does not think it is likely that Omicron is as transmissible as Delta and is more likely to fall near Gamma (because Omicron lacks some of the mutations that make Delta so transmissible). Not necessarily good news because he thinks Omicron's immune escape is driving the spread. Shown here in a graph:
I like this graph he shared which points to where we need the vaccines to go:
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