Symptomatic Versus Inapparent Outcome in Repeat Dengue Virus Infections Is Influenced by the Time Interval between Infections and Study Year Magelda Montoya 1,2. , Lionel Gresh 2. , Juan Carlos Mercado 1 , Katherine L. Williams 3 , Maria Jose ´ Vargas 1 , Gamaliel Gutierrez 2 , Guillermina Kuan 4 , Aubree Gordon 3,5 , Angel Balmaseda 1 , Eva Harris 3 * 1 Laboratorio Nacional de Virologı ´a, Centro Nacional de Diagno ´ stico y Referencia, Ministry of Health, Managua, Nicaragua, 2 Sustainable Sciences Institute, Managua, Nicaragua, 3 Division of Infectious Diseases and Vaccinology, School of Public Health, University of California, Berkeley, California, United States of America, 4 Centro de Salud So ´ crates Flores Vivas, Ministry of Health, Managua, Nicaragua, 5 Division of Epidemiology, School of Public Health, University of California, Berkeley, California, United States of America Abstract Four dengue virus serotypes (DENV1-4) circulate globally, causing more human illness than any other arthropod-borne virus. Dengue can present as a range of clinical manifestations from undifferentiated fever to Dengue Fever to severe, life- threatening syndromes. However, most DENV infections are inapparent. Yet, little is known about determinants of inapparent versus symptomatic DENV infection outcome. Here, we analyzed over 2,000 DENV infections from 2004 to 2011 in a prospective pediatric cohort study in Managua, Nicaragua. Symptomatic cases were captured at the study health center, and paired healthy annual samples were examined on a yearly basis using serological methods to identify inapparent DENV infections. Overall, inapparent and symptomatic DENV infections were equally distributed by sex. The mean age of infection was 1.2 years higher for symptomatic DENV infections as compared to inapparent infections. Although inapparent versus symptomatic outcome did not differ by infection number (first, second or third/post-second DENV infections), substantial variation in the proportion of symptomatic DENV infections among all DENV infections was observed across study years. In participants with repeat DENV infections, the time interval between a first inapparent DENV infection and a second inapparent infection was significantly shorter than the interval between a first inapparent and a second symptomatic infection. This difference was not observed in subsequent infections. This result was confirmed using two different serological techniques that measure total anti-DENV antibodies and serotype-specific neutralizing antibodies, respectively. Taken together, these findings show that, in this study, age, study year and time interval between consecutive DENV infections influence inapparent versus symptomatic infection outcome, while sex and infection number had no significant effect. Moreover, these results suggest that the window of cross-protection induced by a first infection with DENV against a second symptomatic infection is approximately 2 years. These findings are important for modeling dengue epidemics and development of vaccines. Citation: Montoya M, Gresh L, Mercado JC, Williams KL, Vargas MJ, et al. (2013) Symptomatic Versus Inapparent Outcome in Repeat Dengue Virus Infections Is Influenced by the Time Interval between Infections and Study Year. PLoS Negl Trop Dis 7(8): e2357. doi:10.1371/journal.pntd.0002357 Editor: Alan L. Rothman, University of Rhode Island, United States of America Received January 25, 2013; Accepted June 27, 2013; Published August 8, 2013 Copyright: ß 2013 Montoya et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by the National Institutes of Health (1R01AI099631 to AB and HHSN2722001000026C to EH, AB), the Dengue Vaccine Institute (DV-11-07 to EH) and the Pediatric Dengue Vaccine Initiative (VE-1 to EH). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: eharris@berkeley.edu . These authors contributed equally to the manuscript. Introduction Dengue is a major health problem globally, with more than 40% of the world’s population at risk and over a hundred countries affected by epidemics [1]. In the past 50 years, the incidence of dengue has increased considerably, affecting tens of millions of people annually. Dengue is caused by an enveloped, positive-sense RNA virus in the genus Flavivirus of the Flaviviridae family, which is transmitted by mosquitoes of the Aedes genus. There are four serotypes of dengue virus (DENV): DENV-1, DENV-2, DENV-3 and DENV-4. Infection with DENV can be subclinical (inapparent infection) or cause a variety of clinical manifestations ranging from undifferentiated illness and Dengue Fever (DF) to severe life-threatening syndromes Dengue Hemor- rhagic Fever (DHF) and Dengue Shock Syndrome (DSS) [2]. Very little is known about the determinants of inapparent versus symptomatic DENV infection outcome. By definition, inapparent infections are not detected in routine surveillance and can only be captured in the context of prospective cohort or index cluster studies. In a cohort study in Thailand, study year, total DENV infection incidence in the current and previous year, circulating DENV serotype and the number of circulating serotypes were identified as factors influencing inapparent versus symptomatic infection outcome [3,4]. Analysis of infection outcome is further complicated by immune responses to multiple infections with different DENV serotypes, which can be either protective or PLOS Neglected Tropical Diseases | www.plosntds.org 1 August 2013 | Volume 7 | Issue 8 | e2357