Still see 70% quoted as level of vaccination required for 'herd immunity'. Important to note it's now likely to be much higher. The standard (albeit rough) calculation for herd immunity threshold is (1/E) x (1-1/R) where E is vaccine effectiveness in reducing transmission... 1/
In scenario where R is 6 (plausible for Delta in susceptible populations without any restrictions), and vaccination reduces infection/infectiousness such that onwards transmission reduced by 85%, above calc suggests would need to vaccinate (1-1/6)/0.85 = 98% of population. 2/
If transmission reduction is less than this (which is likely the case for some vaccines against Delta), or R higher, then herd immunity wouldn't be achievable through current vaccines alone. This leads to three possibilities... 3/
If herd immunity through vaccination alone not possible, need to either: A) keep some control measures in place indefinitely, B) prepare for exit wave as measures relaxed, C) update what are already very good vaccines to be even more effective. 4/
We explored these ideas more in our (pre-Delta) paper earlier this year, with @dchodgey @markjit @StfnFlsch : eurosurveillance.org/content/10.280… 5/5
For avoidance of doubt – above calculations are for *otherwise fully susceptible population* (i.e. estimating effect of vaccination alone). In populations where there have been large epidemics, this accumulated immunity will reduce level of vaccination required to get R below 1.
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