  Background. During 2021, the COVID-19 pandemic was characterized by the
emergence of lineages with increased fitness. For most of these variants,
quantitative information is scarce on epidemiological quantities such as the
incubation period and generation time, which are critical for both public
health decisions and scientific research. Method. We analyzed a dataset
collected during contact tracing activities in the province of Reggio Emilia,
Italy, throughout 2021. We determined the distributions of the incubation
period using information on negative PCR tests and the date of last exposure
from 282 symptomatic cases. We estimated the distributions of the intrinsic
generation time (the time between the infection dates of an infector and its
secondary cases under a fully susceptible population) using a Bayesian
inference approach applied to 4,435 SARS-CoV-2 cases clustered in 1,430
households where at least one secondary case was recorded. Results. We
estimated a mean incubation period of 4.9 days (95% credible intervals, CrI,
4.4-5.4; 95 percentile of the mean distribution: 1-12) for Alpha and 4.5 days
(95%CrI 4.0-5.0; 95 percentile: 1-10) for Delta. The intrinsic generation time
was estimated to have a mean of 6.0 days (95% CrI 5.6-6.4; 95 percentile: 1-15)
for Alpha and of 6.6 days (95%CrI 6.0-7.3; 95 percentile: 1-18) for Delta. The
household serial interval was 2.6 days (95%CrI 2.4-2.7) for Alpha and 2.4 days
(95%CrI 2.2-2.6) for Delta, and the estimated proportion of pre-symptomatic
transmission was 54-55% for both variants. Conclusions. These results indicate
limited differences in the incubation period and intrinsic generation time of
SARS-CoV-2 variants Alpha and Delta compared to ancestral lineages.
