Figure S2.

Relationship between the Number of Travel-Associated Zika Cases and the Correlation between Local and Travel Incidence Rates, Related to Figure 2

To determine the number of travel-associated infections needed to infer the shape of a local outbreak, we compared the total travel-associated Zika cases from each exposure-reporting country/territory combination (x axis) with Pearson correlation between the local and travel incidence rates corresponding to the combination (y axis). The travel-associated Zika cases were totaled from 2016-2017. For the Pearson correlations between local-travel incidence rates, monthly incidence values from 2016-2017 were compared. When there were > 20 travel-associated cases, the local-travel Pearson r was > 0.5, indicating a strong positive correlation and that the travel cases can help determine the shape of the local outbreak. The lone exception to that finding was from travelers from Barbados diagnosed in the United Kingdom (UK) because the travel cases miss the locally reported Zika virus peak during January-February, 2016, but they correlate with the second local peak during July-October, 2016 (Figure 2A). In our dataset, there were 25 Zika virus infections diagnosed in Italy with recent travel to Cuba, 30 diagnosed in Spain, and 98 diagnosed in Florida. These totals are all within the range of strong positive correlations between local and travel incidence, justifying their use to infer the local Zika outbreak dynamics in Cuba (Figure 2B).