Add optional baseline assumption that future Rt will regress to one

[nikosbosse]

Currently EpiNow2 makes predictions by assuming Rt will be constant in time. One could argue that based on what we observed in the past it would make more sense to assume that the pandemic is a control system where Rt will slowly revert back to one in the future. Maybe an option for that would be nice.

[seabbs]

So this seems like a valid user choice and one that is currently supported using the simulation tooling. As part of the epinow pipeline this has been integrated using forecast_infections so the first stop is to check if that would allow you to do what. you want and if not explore what else you would need.

[sbfnk]

One way to do this would be to treat EpiNow2 as a tool that doesn't provide a forecasting model (which it doesn't really) -- what it does is offer a few ways to extend the data forward.

An external package to this could implement a range of forecast models (involving e.g. susceptible depletion, vaccination, regression to 1 etc.) that feeds into the existing framework for external forcing of R (in forecast_infections, I presume).

[seabbs]

👍 This seems like the time to wake EpiSoon back up 😆

[seabbs]

Looking at the code for forecast_infections (of which I have no memory). This seems doable as it stands by passing in a very custom forecast_model but the simulation tools that in there are from EpiSoon vs the better (😆) ones available in simulate_infections. At some point forecast_infections needs a rewrite to use simualate_infections especially if this is an approach someone is taking (I have not seen anyone doing this).