Thread by @AdamJKucharski

In discussions of future COVID dynamics and potential for ‘herd immunity’, need to remember there are four main routes to immunity that can reduce Delta transmission - even though coverage often focuses on just one or two of them. A quick thread... 1/

Because Delta transmissibility is so high, and current vaccine less effective in reducing Delta transmission, vaccination alone unlikely to get R below 1. But that doesn’t mean immunity can’t drive R below 1 in future... 2/

https://twitter.com/AdamJKucharski/status/1415587167863283713?s=20

There are four main routes to immunity to Delta:
A. Cross-protective immunity from vaccine designed against earlier variant
B. Cross-protective immunity from infection with earlier variant
C. Immunity from infection with Delta
D. Immunity from Delta-specific vaccine
3/

Much coverage of immunity against Delta focuses on route A. But what about the other 3 routes? Many countries already have substantial immunity from pre-Delta exposure (B) and are currently building immunity from Delta infections (C). 4/

Unfortunately Delta-specific vaccine (D) not yet available, but other 3 in combination will likely be enough to get R<1 eventually - we know from earlier variants that post-infection immunity against the infecting variant is generally robust (e.g. thelancet.com/journals/lance…). 5/

Over-time, responses may wane or further variants may emerge, resulting in R increasing above 1 and outbreaks occuring again (but with far lower severe disease):

https://twitter.com/AdamJKucharski/status/1316283049445855232?s=20

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This could produce similar dynamic to seasonal coronaviruses & influenza, where immunity builds and R declines against circulating variants, followed by seasonal outbreaks as viruses evolve & initial responses wane (e.g. journals.plos.org/plospathogens/… & journals.plos.org/plosbiology/ar…) 7/7