In 2016 a method for analysing under reported data was developed by Fernández-Fontelo, Cabaña, Puig and Moriña (2016): Under-reported data analysis with INAR?hidden Markov chains, Statistics in Medicine.
The method allows an accurate estimation of the number of un-registered cases, with several applications in public health, such as the number of cases of Human Papiloma Virus, botulism and even the number of victims of domestic violence.
We are now using the same methodology, which is briefly summarized in an upcoming report to update the situation concerning COVID-19 frequently, and particularly quantify the potential under-reporting in the official registered cases by region in Spain.
Results herein can help to have a more realistic picture of the pandemic at a real time as well as to more accurately estimate essential measures such as the basic reproduction number or the fatality rate that are used by practitioners and policy makers to take actions. Figure below shows the situation of underreporting in Madrid Community and Catalonia as of March 25th.