First analyses support the idea that #B117 #SARSCoV2 #VOC has an increased transmissibility but unchanged clinical severity compared to already circulating variants. Is really an increase in transmission more problematic than an increase in lethality? Well, it depends. Thread 1/N
To keep things simple, let us, once again, take the canonical SIR model (Kermack & McKendrick, 1927) and look at how the cumulative mortality of COVID-19 would vary through time assuming no initial immunity (nor vaccination) but public health measures such that R < R0 = 3. 2/N
We explore 3 hypothetical scenarios: 1 (in grey) is the baseline -- as if the mutations carried by the variant were epidemiologically neutral; 2 (in pink) simulates a 50% increase in transmission and 3 (in blue) represents a 50% increase in lethality. 3/N
Through a strongly slowed down, yet not under control, epidemic with a background R equal to 1.1, the emergence of a more transmissible variant is in any point more worrying than a deadlier variant: both the immediate (hospital strain) and final death toll are far greater. 4/N 
However, under a more rapidly spreading epidemic with background R = 1.6, the final death toll is greater in the case of the deadlier variant, while the more contagious variant still induce a steeper initial increase in population mortality. 5/N
To generalise, this graph shows that an increase in transmission has a greater impact on the two long-term metric (the final attack rate and the herd immunity threshold, both weighted by the IFR) compared to an increase in lethality only for small values of R. 6/N
7/N thanks @threadreaderapp unroll
8/N threadreaderapp unroll
This (mean-field hence simplified) approach suggests that the long-term death toll of a deadlier variant would actually be greater than an equivalently more transmissible variant in areas where the epidemic is already rapidly spreading @AdamJKucharski @threadreaderapp unroll 8/8
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