Undetected infectives in the Covid-19 pandemic

• Published in: International journal of infectious diseases 2021-03, Vol.104, p.262-268
• Main Authors: Melis, Maurizio, Littera, Roberto
• Format: Article
• Language: English
• Subjects: Basic Reproduction Number; Covid-19; COVID-19 - epidemiology; COVID-19 - prevention & control; COVID-19 - transmission; COVID-19 - virology; Disease Susceptibility; Epidemiology; Humans; Italy - epidemiology; Models, Theoretical; Pandemics; SARS-CoV-2; SARS-CoV-2 - isolation & purification; SIR model; Undetected cases
• ISSN: 1201-9712; 1878-3511
• DOI: 10.1016/j.ijid.2021.01.010

•Undetected infections are about 52% of the total number of Covid-19 infections.•Coronavirus is mainly spread by asymptomatic and presymptomatic individuals.•Our model is able to describe the main characteristics of the epidemic dynamics. 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. The dynamics of an infection can be described by the SIR model, which divides the population into susceptible (S), infective I, and removed R 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. We proved that the fraction of undetected infectives, compared to the total number of infected subjects, is given by 1−1R0, where R0 is the basic reproduction number. The mean value R0=2.10 2.09-2.11 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. 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.