'Herd immunity' has been reached during previous epidemics of influenza, measles and seasonal coronaviruses. But it's subsequently been lost (and then regained). What are some of the reasons for this? 1/

Here we're using technical definition of 'herd immunity', i.e. sufficient immunity within a population to push R below 1 in absence of other control measures. But reaching this point doesn't mean R will stay below 1 forever. Here four things to be aware of... 2/

A: Population turnover. Over time, new births mean an increase in % of population susceptible. This will eventually lead to R>1 and new (but smaller) outbreaks - the more transmissible the infection, the sooner this recurrence will happen. More: https://twitter.com/AdamJKucharski/status/1282975984338141185?s=20 3/

B: Waning. For seasonal coronaviruses, immunity can also wane (e.g. https://www.nature.com/articles/s41591-020-1083-1 & https://www.nature.com/articles/s41467-020-18450-4). Again, this will result in increase in susceptibility over time, and hence potential for R>1 in future. 4/

However, it's worth noting that subsequent infections for seasonal coronaviruses can come with reduced severity, so although transmission still occurs, burden can be lower during subsequent epidemics: https://twitter.com/AdamJKucharski/status/1294985940524507136?s=20 5/

C: Evolution. In the case of flu, antigenic evolution can mean immunity built against recent strains doesn't fully protect against new ones ( https://www.nature.com/articles/nrmicro.2017.118). Note: there's no evidence this is influencing SARS-CoV-2 dynamics so far ( https://theconversation.com/coronavirus-mutations-what-weve-learned-so-far-145864). 6/

D: Population migration. The immunity profile of a population may change over time as susceptible individuals arrive/leave. E.g. in 2014/15 there was mass cholera vaccination in IDP camp in South Sudan; by 2016, over 80% were susceptible ( https://journals.plos.org/plosntds/article?rev=1&id=10.1371/journal.pntd.0006257) 7/

The COVID-19 pandemic will eventually end because of accumulated immunity, either following infection or - ideally - from future vaccine. But even if this means R<1 initially, the above shows there are number of long-term possibilities to consider. 8/8