A new epidemiological index for SARS-CoV-2 transmission

A study by the Politecnico is published in Nature Communications

Since the start of the COVID-19 pandemic, the concept of reproduction number (R) has become widely used and has been used as a parameter to assess the progress of the epidemic.

A less well-known fact about reproduction numbers is that they refer to the long-term temporal evolution of an epidemic, which means that their value determines whether a given pathogen will eventually become established as endemic in the community (R > 1) or disappear from it (R < 1), without generally providing any indication of the dynamics of disease transmission in the short term.

This observation was the starting point for the study “The epidemicity index of recurrent SARS-CoV-2 infections”, just published in Nature Communications and carried out by Politecnico di Milano researchers Lorenzo Mari, Renato Casagrandi, Stefano Miccoli and Marino Gatto, in collaboration with colleagues from Ca' Foscari University in Venice, the University of Padua and the École Polytechnique Fédérale de Lausanne.

The article presents a new metric, the outbreak index (e), which complements the standard definition of outbreak numbers by identifying conditions (e > 0) for which transient outbreak spikes may occur even if R < 1.

Applying the new model to the first months of the pandemic in Italy, the researchers showed that the reproductive number remained below one for at least four months (April-July 2020), while at the same time the outbreak index remained positive, allowing for the recurrence of outbreaks well before the autumn when the epidemic resumed.

More information

Mari, L., Casagrandi, R., Bertuzzo, E. et al.
The epidemicity index of recurrent SARS-CoV-2 infections.
Nat Commun 12, 2752 (2021).

The study is online