R0 is not remotely immutable. It is a function of people’s behaviour and physical infrastructure as well as physical properties of the virus (which are themselves likely changing, especially early in a pandemic, as the virus evolves).
It is not affected by levels of exposure, because R0 is defined as the infection rate in the absence of any exposure.
‘Immutable’ is a tricky word. Let’s be more specific about what R0 does and doesn’t include:
Viral evolution: yes (ex: R0 for Omicron is higher than R0 for Delta)
Immunity from vaccination against this pathogen: no
Immunity from prior infection by this pathogen: no
Immunity from vaccination for or prior infection by other pathogens: varies depending on how close the other pathogens are and whether we consider this to be one long outbreak or several (this generally doesn’t seem very principled)
General behavior of the population: yes (ex: R0 is lower in populations that socialize mostly outdoors)
Behavior changes in response to this pathogen: usually no (ex: people moving socializing outdoors or starting masking in response to this virus does not decrease its R0, except that some papers define it differently so that it does)
I think maybe a lot of the disagreement here is whether you consider these to be R0 changing vs different R0s for different scenarios?
Overall, this means that if you’re going to use the R0 from a paper it’s worth putting a good bit of effort into seeing how this particular paper is using it.
It’ll tend to change with things population, social conventions, etc. For the herd animal populations it was originally applied to you can pretty much ignore all of that but not for humans. Especially for things like coronaviruses with a high k where R0 is driven by the fat tail of the distribution. In a small village where most bat/human coronavirus crossovers tend to happen the village size limits how large a superspreader event can be. Not so in a city. And then you have things like Ebola spread being partially driven by funereal customs.
I guess a better way of putting that is that R0 is fixed for a particular population but humans are composed of many different populations, just like there are other populations of different species a virus can also infect which might have their own R0s as well.
Huh? R0 is immutable.
R0 is not remotely immutable. It is a function of people’s behaviour and physical infrastructure as well as physical properties of the virus (which are themselves likely changing, especially early in a pandemic, as the virus evolves).
It is not affected by levels of exposure, because R0 is defined as the infection rate in the absence of any exposure.
‘Immutable’ is a tricky word. Let’s be more specific about what R0 does and doesn’t include:
Viral evolution: yes (ex: R0 for Omicron is higher than R0 for Delta)
Immunity from vaccination against this pathogen: no
Immunity from prior infection by this pathogen: no
Immunity from vaccination for or prior infection by other pathogens: varies depending on how close the other pathogens are and whether we consider this to be one long outbreak or several (this generally doesn’t seem very principled)
General behavior of the population: yes (ex: R0 is lower in populations that socialize mostly outdoors)
Behavior changes in response to this pathogen: usually no (ex: people moving socializing outdoors or starting masking in response to this virus does not decrease its R0, except that some papers define it differently so that it does)
I think maybe a lot of the disagreement here is whether you consider these to be R0 changing vs different R0s for different scenarios?
Overall, this means that if you’re going to use the R0 from a paper it’s worth putting a good bit of effort into seeing how this particular paper is using it.
It’ll tend to change with things population, social conventions, etc. For the herd animal populations it was originally applied to you can pretty much ignore all of that but not for humans. Especially for things like coronaviruses with a high k where R0 is driven by the fat tail of the distribution. In a small village where most bat/human coronavirus crossovers tend to happen the village size limits how large a superspreader event can be. Not so in a city. And then you have things like Ebola spread being partially driven by funereal customs.
I guess a better way of putting that is that R0 is fixed for a particular population but humans are composed of many different populations, just like there are other populations of different species a virus can also infect which might have their own R0s as well.
Only “in a population that has not previously encountered the disease”, which no longer applies to this planet.