The Epidemiology and Transmissibility of Zika Virus in Girardot and San Andres Island, Colombia
Authors: Rojas Diana Patricia , Dean Natalie E. , Yang Yang , Kenah Eben , Quintero Juliana , Tomasi Simon , Ramirez Erika L. , Kelly Yendy , Castro Carolina , Carrasquilla Gabriel , Halloran M. Elizabeth , Longini Ira M. 
Published: April 2016

Background

Zika 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).

Methods

We 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 R0in each population.

Findings

We 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 12·13 cases per 1,000 residents of San Andres and 18·43 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 R0for the Zika outbreak in San Andres was 1·41 (95% CI 1·15 to 1·74), and in Girardot was 4·61 (95% CI 4·11 to 5·16).

Interpretation

Transmission 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.

Funding

This 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.

Research in Context

Evidence before this study

The 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.

We 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,R0of Zika virus in Yap Island, Federal State of Micronesia and in French Polynesia, but no papers estimatingR0using 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.

Added value of this study

We 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 ofR0. Our results can be used by mathematical modelers to understand the future impact of the disease and potential spread.

Implications of all the available evidence

We 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 ofR0imply that Zika has the capacity for widespread transmission in areas with the vector.