  Background: Travel is a potent force in the emergence of disease. We
discussed how the traveler case reports could aid in a timely detection of a
disease outbreak. Methods: Using the traveler data, we estimated a few
indicators of the epidemic that affected decision making and policy, including
the exponential growth rate, the doubling time, and the probability of severe
cases exceeding the hospital capacity, in the initial phase of the COVID-19
epidemic in multiple countries. We imputed the arrival dates when they were
missing. We compared the estimates from the traveler data to the ones from
domestic data. We quantitatively evaluated the influence of each case report
and knowing the arrival date on the estimation. Findings: We estimated the
travel origin's daily exponential growth rate and examined the date from which
the growth rate was consistently above 0.1 (equivalent to doubling time < 7
days). We found those dates were very close to the dates that critical
decisions were made such as city lock-downs and national emergency
announcement. Using only the traveler data, if the assumed epidemic start date
was relatively accurate and the traveler sample was representative of the
general population, the growth rate estimated from the traveler data was
consistent with the domestic data. We also discussed situations that the
traveler data could lead to biased estimates. From the data influence study, we
found more recent travel cases had a larger influence on each day's estimate,
and the influence of each case report got smaller as more cases became
available. We provided the minimum number of exported cases needed to determine
whether the local epidemic growth rate was above a certain level, and developed
a user-friendly Shiny App to accommodate various scenarios.
