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

Among imported cases with full information, the delay from travel to hospitalization was longer in cases that generated secondary transmissions (mean of 10 ± 0.97 days compared to 5.5 ± 0.67 days, p=0.003). Overall, the duration from travel to first event (whether symptom onset, or hospitalization for asymptomatic) was also longer, although the difference was not statistically significant (5.0 ± 0.9 days vs. 3.7 ± 0.5 days p= 0.08). Durations of hospitalization were instead comparable among the two groups of cases (1.5 ± 0.7 days vs. 2.6 ± 0.4 days for cases that generated or did not generate secondary transmissions, respectively). Including imported cases with missing information through imputation, we found the same trend though smaller in magnitude and not statistically significant (delay from travel to hospitalization 9.8 ± 1.2 vs. 8.3 ± 0.5 days p= 0.3; delay from travel to onset 5.8 ±1.1 vs. 4.2 ±0.5 p= 0.16, for cases that generated or did not generate secondary transmissions, respectively). This suggests that importations with missing information may be closer in characteristics to index cases leading to onward transmission.