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COVID-19: moving towards an endemic equilibrium

In the early days of the COVID-19 pandemic it was possible to imagine a world where the SARS-CoV-2 virus would eventually disappear. A virus free equilibrium with no restrictions on mobility, mixing and non-pharmaceutical interventions (NPIs) was possible. With a basic reproduction number (R0) of around 2.5 for the original wild strain of SARS-CoV-2, the virus would eventually have disappeared if over 60% of the total population acquired permanent 100% immunity against further infection, whether by vaccination or infection and recovery.

But, as new variants have arrived, with higher basic reproduction numbers, and as vaccine protection against infection has turned out to be well below 100%, the prospect of viral elimination has disappeared. Estimates of basic reproduction numbers are very uncertain, but a reasonable estimate of R0for the Alpha variant is around 3.8 and for the Delta variant is around 5.2. Meanwhile, vaccine efficacy against infection looks to be around 70%. Put together, the immunity thresholds for viral elimination exceed 100% of the total population for both the Alpha and Delta variants. The means that the effective reproduction number (Re), which in equilibrium is driven down relative to R0 only by immunity, exceeds one even after everyone has acquired immunity. The virus free equilibrium, which requires Re to be less than one, is not possible. Thus, even if the entire population is vaccinated, infections will continue as the pool of susceptible individuals is replenished by fading immunity, the emergence of new strains and population growth.


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