For example: during the current covid 19 outbreak the reported numbers are number of infections after various measures like social distancing or quarantine have been used - so it doesn't make much sense to compare it to the r0 if a disease like flu, where similar measures aren't used.
Is there an objective measurement? Something like what are the chances of getting infected in some controlled conditions
R0 is the basic reproduction number describing how many cases are expected to be infected by a single case. This value is not a biological constant.
With an R0 (reproduction number) estimated at 2.5 (based on China), many experts predict that between 20 and 60% of the world population could get infected (which means at least 0.5M deaths).
However, the R0 varies during the course of an epidemic as a result of a series of determinants and interventions
such as containment and mitigation factors, and eventually herd immunity and vaccination.
https://www.isglobal.org/en/coronavirus-lecciones-y-recomendaciones
R0 is the objective measure you're looking for.
The "basic reproduction number", R0, is the rate of rate of spread independent of any external factors such as quarantine or social distancing. You appear to be confusing it with R, the "effective reproduction number", which measures the rate of spread after countermeasures and population immunity have been applied.
I don't think it makes any sense to discuss such a part now. Although there have been sars and mers, covid-19 is the first virus it encounters, and it takes a long time for the data to be properly cleaned up. That is why it is most accurate to interpret it based on what is currently being presented. The power that covid-19 spreads is the most powerful when compared to the infectious diseases we have encountered so far. In general, it can be spread by talking about 5 minutes in or around 1m, but there are reports that have been spread even less than the circumstances.
