Interesting Tweet from epidemiologist and mathematician Adam Kucharski at the London School of Tropical Medicine:
Quote:
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.
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.
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:
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
We explored these ideas more in our (pre-Delta) paper earlier this year, with https://eurosurveillance.org/content....26.20.2100428 5/5
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https://twitter.com/AdamJKucharski