BACKGROUND: COVID-19 patients with long incubation period were reported in 
clinical practice and tracing of close contacts, but their epidemiological or 
clinical features remained vague.
METHODS: We analyzed 11,425 COVID-19 cases reported between January-August, 2020 
in China. The accelerated failure time model, Logistic and modified Poisson 
regression models were used to investigate the determinants of prolonged 
incubation period, as well as their association with clinical severity and 
transmissibility, respectively.
RESULT: Among local cases, 268 (10.2%) had a prolonged incubation period of 
> 14 days, which was more frequently seen among elderly patients, those residing 
in South China, with disease onset after Level I response measures 
administration, or being exposed in public places. Patients with prolonged 
incubation period had lower risk of severe illness (ORadjusted = 0.386, 95% CI: 
0.203-0.677). A reduced transmissibility was observed for the primary patients 
with prolonged incubation period (50.4, 95% CI: 32.3-78.6%) than those with an 
incubation period of ≤14 days.
CONCLUSIONS: The study provides evidence supporting a prolonged incubation 
period that exceeded 2 weeks in over 10% for COVID-19. Longer monitoring periods 
than 14 days for quarantine or persons potentially exposed to SARS-CoV-2 should 
be justified in extreme cases, especially for those elderly.