# CellBioGuy comments on New SARS-CoV-2 variant

• Taking an effective R value from ~1.2 to ~1.8 would WAY more than double the growth rate. I really don’t think that this makes sense, and that number for an increased R value seems like it should be referring to the unmitigated R0 value that is then reduced by behavioral interventions.

EDIT: doubling time would go from 17 days to 4 days (!) with the above change of numbers. This doesn’t fit given what is currently observed.

An R0 going up by ~0.4-0.9 also fits well with my imputation of a ~15% increase in infectiousness, as estimates of an unmitigated R0 range from circa 3 to 5.

I think this is what happens when people don’t show their work.

• EDIT: doubling time would go from 17 days to 4 days (!) with the above change of numbers. This doesn’t fit given what is currently observed.

The doubling time for the new strain does appear to be around 6-7 days. And the doubling time for London overall is currently 6 days.

If the mitigated Rt is +0.66 and the growth rate is +71% figures are inconsistent with each other as you say, then perhaps the second is mistaken and +71% means that the Rt is 71% higher, not the case growth rate, which is vaguely consistent with the Rt is +58% higher estimate from the absolute increase. Or “71% higher daily growth rate” could be right and the +0.66 could be referring to the R0, as you say.

This does appear to have been summarized as ‘the new strain is 71% more infectious’ in many places, and many people have apparently inferred the R0 is >50% higher—hopefully we’re wrong.

Computer modelling of the viral spread suggests the new variant could be 70 per cent more transmissible. The modelling shows it may raise the R value of the virus — the average number of people to whom someone with Covid-19 passes the infection — by at least 0.4,

I think this is what happens when people don’t show their work.

So either ‘R number’ is actually referring to R0 and not Rt, or ‘growth rate’ isn’t referring to the daily growth rate but to the Rt/​R0. I agree that the first is more plausible. All I’ll say is that a lot of people are assuming the 70% figure or something close to it is a direct multiplier to the Rt, including major news organizations like the Times and Ft. But I think you’re probably right and the R0 is more like 15% larger not 58/​70% higher.

EDIT: New info from PHE seems to contradict this, https://​​t.co/​​r6GOyXFDjh?amp=1

• Yeah I was going with my quick and dirty numbers from earlier. Way I see it what’s probably happening is that doubling time for Britain as a whole has been ~2 weeks both about a month ago and recently (eyeballing graph from worldometer), and you are probably talking about a new doubling time of ~1 week for this variant under identical conditions.

Redoing math for a two week starting doubling time and the stated change in doubling time you get a R value going from ~1.25 to just under or circa 1.5, so basically similar order.

News sources do not use precise language, and precise language matters here.

• Update: this from public health England explicitly says Rt increases by 0.57, https://​​twitter.com/​​DevanSinha/​​status/​​1341132723105230848?s=20

“We find that Rt increases by 0.57 [95%CI: 0.25-1.25] when we use a fixed effect model for each area. Using a random effect model for each area gives an estimated additive effect of 0.74 [95%CI: 0.44- 1.29].

an area with an Rt of 0.8 without the new variant would have an Rt of 1.32 [95%CI:1.19-1.50] if only the VOC was present.”

But for R, if it’s 0.6 not 0.8 and the ratio is fixed then another march style lockdown in the UK would give R = 0.6 *(1.32/​0.8)= 0.99

• I’ve been trying to understand this discussion (and I agree that this is one of the central questions for the model of how things will progress from here, in particular if March-style lockdowns will be sufficient or not to halt the spread of this strain). But now I’m mainly confused—isn’t such a dramatic increase in Rt incompatible with the slower increase in the graph, as pointed out by CellBioGuy?

Edit: I’ve read yesterday’s PHE investigation report, and they do explicitly confirm it is an increase of over +0.5 to the Rt under the conditions in England in weeks 44-49 of this year. So this seems like the bad possible interpretation, where it really does spread significantly more.