Thread by @BarclayBenedict

Some thoughts on what Nu's growth might mean in terms of transmission advantage over Delta.

Imagine R(t) for Delta was ~1 in South Africa, with Delta at low, stable levels. If community level protection from vaccination/prior infection falls from e.g. 70% (Delta) to 55% (Nu)...

then R(t) for Nu will be ~1.5, assuming R(0) is the same for both. This will allow Nu to grow fairly rapidly, and soon dominate overall cases.

If we say SA population is:

35% vaccinated
55% infected
10% neither

then 70% community protection could be a weighted average of:

75% protection (vaxxed)
80% protection (prior inf.)
0% protection (neither)

as (75*35 + 80*55 + 0*10)/100 = ~70

Meanwhile, 55% protection for Nu might be:

30% + 80% + 0% (vaxxed, prior, neither) - i.e. vaccine escape

or:

75% + 50% + 0% – i.e. less protection from prior inf.

These are, of course, purely illustrative numbers to show that there could be various underlying forces driving the *possible* transmission advantage of Nu.

Another possibility is that R(0) is higher, and that protection from vaccination/prior infection is less compromised.

Of course, the transmission advantage might be even greater, and I don't think we have enough data to put an exact number on it.

If it has a very big advantage (e.g. twice as transmissible or more), it can't be just vaccine escape - for that, too few people are vaccinated in SA.