Methodological elements for a reflection on the data of the sars-cov-2 epidemic

Abstract

Findings of the seroprevalence survey conducted by Istat between May 25 and July 15 2020, on a sample of 64,660 people, show that only 2.5% of Italian people developed antibodies to SARS-CoV-2, a prevalence very far from the hypothesis of achieving herd immunity. Starting from the comment on these results, we summarized some of the main indicators used to evaluate the epidemic curves (R, R0, Rt) and the concept of herd immunity. R0, basic reproduction number, represents the average number of secondary cases we expect to observe from a single primary case in a population with no immunity to the disease before prevention and containment measures have been planned. Rt, effective reproduction number, is calculated over time and considers how the outbreak progresses, as a result of the containment measures and of people who might have gained immunity, because they survived from infection or were vaccinated. We presented the issue of herd immunity, or community immunity, a state of protection in a population obtained because the number of people in the population who are immune to infectious disease is above a critical threshold, resulting in a protection even for those who are not immune.