Undetected infectives in the Covid-19 pandemic

OBJECTIVES: Epidemiological investigations and mathematical models have revealed that the rapid diffusion of Covid-19 can mostly be attributed to undetected infective individuals who continue to circulate and spread the disease: finding their number would be of great importance in the control of the epidemic. METHODS: The dynamics of an infection can be described by the SIR model, which divides the population into susceptible [Formula: see text] , infective [Formula: see text] , and removed [Formula: see text] subjects. In particular, we exploited the Kermack-McKendrick epidemic model, which can be applied when the population is much larger than the fraction of infected subjects. RESULTS: We proved that the fraction of undetected infectives, compared to the total number of infected subjects, is given by [Formula: see text] , where [Formula: see text] is the basic reproduction number. The mean value [Formula: see text] for the Covid-19 epidemic in three Italian regions yielded a percentage of undetected infectives of 52.4% (52.2%–52.6%) compared to the total number of infectives. CONCLUSIONS: Our results, straightforwardly obtained from the SIR model, highlight the role of undetected carriers in the transmission and spread of the SARS-CoV-2 infection. Such evidence strongly recommends careful monitoring of the infective population and ongoing adjustment of preventive measures for disease control until a vaccine becomes available for most of the population.