This research has been initiated by the following important question, motivated by the current Covid-19 coronavirus pandemic:

Can we estimate the efficacy of  the government medical and social preventive measures   by a single number?

An epidemic index (EPI) is proposed as an  answer such a question. 

The EPI index is a type of the current reproduction number R_0, the current transmission rates, or the infection rates.  

The EPI represents the current rate of the epidemic spread.

When the infection rates are greater than 1,  the epidemic may grow exponentially. 

Hence, the target is to get the value of the EPI index below 1.

   Notation: EPI - Epidemic Index; ERI -Epidemic Risk Index.

The countries  in the lists have progressively been chosen to fill in the top 50 states in the new daily cases. 

March 2022: the lists have been updated, i.e.  some countries have been removed from the lists due to the low number of daily Covid19 cases and/or irregular data reporting in these countries.