Lessons learnt from 288 COVID-19 international cases: importations over time, effect of interventions, underdetection of imported cases

Francesco Pinotti; Laura Di Domenico; Ernesto Ortega; Marco Mancastroppa; Giulia Pullano; Eugenio Valdano; Pierre-Yves Boelle; Chiara Poletto; Vittoria Colizza

We analyzed clusters of transmission generated by imported cases (index case(s) in each cluster) to estimate undetected importations. A cluster can be seeded by more than one index case when local transmissions are epidemiologically linked to more cases traveling together (e.g. infected family members traveling together). We modelled the number of such 'cluster seeds', i.e. groups of index cases, with a multinomial distribution depending on the portion of cluster seeds of size 1 or greater than 1 (for simplicity, this was taken as 2), on the probability of detection of a seed, and on occurrence of secondary transmission. The likelihood function was a function of: the number G of observed clusters with one index case; the number H of observed clusters with more than 1 index cases; the number J of detected index cases not leading to onward transmission; the number of clusters whose index cases have not been identified; and the number of undetected imported cases that did not generate any cluster. can be estimated through likelihood maximization from the records of G , H , J, .