bioRxiv2016
BackgroundZika virus (ZIKV) is an arbovirus in the same genus as dengue virus and yellow fever virus. ZIKV transmission was first detected in Colombia in September 2015. The virus has spread rapidly across the country in areas infested with the vector Aedes aegypti. As of March 2016, Colombia has reported over 50,000 cases of Zika virus disease (ZVD).\n\nMethodsWe analyzed surveillance data of ZVD cases reported to the local health authorities of San Andres, Colombia, and Girardot, Colombia, between September 2015 and January 2016. Standardized case definitions used in both areas were determined by the Ministry of Health and Colombian National Institute of Health at the beginning of the ZIKV epidemic. ZVD was laboratory-confirmed by a finding of Zika virus RNA in the serum of acute cases. We report epidemiological summaries of the two outbreaks. We also use daily incidence data to estimate the basic reproductive number R0 in each population.\n\nFindingsWe identified 928 and 1,936 laboratory or clinically confirmed cases in San Andres and Girardot, respectively. The overall attack rate for reported ZVD detected by healthcare local surveillance was 12{middle dot}13 cases per 1,000 residents of San Andres and 18{middle dot}43 cases per 1,000 residents of Girardot. Attack rates were significantly higher in females in both municipalities. Cases occurred in all age groups but the most affected group was 20 to 49 year olds. The estimated R0 for the Zika outbreak in San Andres was 1{middle dot}41 (95% CI 1{middle dot}15 to 1{middle dot}74), and in Girardot was 4{middle dot}61 (95% CI 4{middle dot}11 to 5{middle dot}16).\n\nInterpretationTransmission of ZIKV is ongoing and spreading throughout the Americas rapidly. The observed rapid spread is supported by the relatively high basic reproductive numbers calculated from these two outbreaks in Colombia.\n\nFundingThis work was supported by National Institutes of Health (NIH) U54 GM111274, NIH R37 AI032042 and the Colombian Department of Science and Technology (Fulbright-Colciencias scholarship to D.P.R). The funding source had no role in the preparation of this manuscript or in the decision to publish this study.\n\nResearch in ContextO_ST_ABSEvidence before this studyC_ST_ABSThe ongoing outbreak of Zika virus disease in the Americas is the largest ever recorded. Since its first detection in April 2015 in Brazil, around 500,000 cases have been estimated, and the virus is spreading rapidly in the Americas region. There are many unanswered questions about the transmissibility and pathogenicity of the virus. Limited data are available from recent outbreaks occurring in islands in the Pacific, and little epidemiological data is available on the current outbreak.\n\nWe searched PubMed on March 12, 2016, for epidemiological reports on Zika virus outbreaks using the search terms \"Zika\" AND \"Basic reproductive number\". We applied no date or language restrictions. Our search identified one previous paper assessing the basic reproductive number, R0 of Zika virus in Yap Island, Federal State of Micronesia and in French Polynesia, but no papers estimating R0 using data from the Latin American Zika outbreak. Because of the sparsity of the data, we could not do a detailed systematic review at this point in time.\n\nAdded value of this studyWe report detailed epidemiological data on outbreaks in San Andres and Girardot, Colombia. Because such reports are currently unavailable, we provide early information on age and gender effects and the functioning of local and national surveillance in the second-most affected country in this epidemic. We provide early estimates of R0. Our results can be used by mathematical modelers to understand the future impact of the disease and potential spread.\n\nImplications of all the available evidenceWe report attack rates similar to those reported in the Yap Island outbreak. We find that Zika impacts individuals of all ages, though the most affected age group is 20 to 49 years of age. The surveillance system detected more cases among women in both areas, though this finding may be attributable to reporting bias. Our estimates of R0 imply that Zika has the capacity for widespread transmission in areas with the vector.