title: High rate of subclinical chikungunya virus infection and association of neutralizing antibody with protection in a prospective cohort in the Philippines.
abstract: BACKGROUND
  Chikungunya virus (CHIKV) is a globally re-emerging arbovirus for which previous studies have indicated the majority of infections result in symptomatic febrile illness. We sought to characterize the proportion of subclinical and symptomatic CHIKV infections in a prospective cohort study in a country with known CHIKV circulation.
  
  
  RESULTS
  A prospective longitudinal cohort of subjects ≥6 months old underwent community-based active surveillance for acute febrile illness in Cebu City, Philippines from 2012-13. Subjects with fever history were clinically evaluated at acute, 2, 5, and 8 day visits, and at a 3-week convalescent visit. Blood was collected at the acute and 3-week convalescent visits. Symptomatic CHIKV infections were identified by positive CHIKV PCR in acute blood samples and/or CHIKV IgM/IgG ELISA seroconversion in paired acute/convalescent samples. Enrollment and 12-month blood samples underwent plaque reduction neutralization test (PRNT) using CHIKV attenuated strain 181/clone25. Subclinical CHIKV infections were identified by ≥8-fold rise from a baseline enrollment PRNT titer <10 without symptomatic infection detected during the intervening surveillance period. Selected CHIKV PCR-positive samples underwent viral isolation and envelope protein-1 gene sequencing. Of 853 subjects who completed all study procedures at 12 months, 19 symptomatic infections (2.19 per 100 person-years) and 87 subclinical infections (10.03 per 100 person-years) occurred. The ratio of subclinical-to-symptomatic infections was 4.6:1 varying with age from 2:1 in 6 month-5 year olds to 12:1 in those >50 years old. Baseline CHIKV PRNT titer ≥10 was associated with 100% (95%CI: 46.1, 100.0) protection from symptomatic CHIKV infection. Phylogenetic analysis demonstrated Asian genotype closely related to strains from Asia and the Caribbean.
  
  
  CONCLUSIONS
  Subclinical infections accounted for a majority of total CHIKV infections. A positive baseline CHIKV PRNT titer was associated with protection from symptomatic CHIKV infection. These findings have implications for assessing disease burden, understanding virus transmission, and supporting vaccine development.
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: In-Kyu
authors.initials: IK
authors.lastname: Yoon
authors.affiliation: Philippines-AFRIMS Virology Research Unit, Cebu City, Philippines.
authors.forename: Maria Theresa
authors.initials: MT
authors.lastname: Alera
authors.affiliation: Philippines-AFRIMS Virology Research Unit, Cebu City, Philippines.
authors.forename: Catherine B
authors.initials: CB
authors.lastname: Lago
authors.affiliation: Cebu City Health Department, Cebu City, Philippines.
authors.forename: Ilya A
authors.initials: IA
authors.lastname: Tac-An
authors.affiliation: Cebu City Health Department, Cebu City, Philippines.
authors.forename: Daisy
authors.initials: D
authors.lastname: Villa
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Stefan
authors.initials: S
authors.lastname: Fernandez
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Butsaya
authors.initials: B
authors.lastname: Thaisomboonsuk
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Chonticha
authors.initials: C
authors.lastname: Klungthong
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Jens W
authors.initials: JW
authors.lastname: Levy
authors.affiliation: Philippines-AFRIMS Virology Research Unit, Cebu City, Philippines.
authors.forename: John Mark
authors.initials: JM
authors.lastname: Velasco
authors.affiliation: National Epidemiology Center, Department of Health, Manila, Philippines.
authors.forename: Vito G
authors.initials: VG
authors.lastname: Roque
authors.affiliation: Department of Epidemiology, Johns Hopkins School of Public Health, Baltimore, Maryland, United States of America.
authors.forename: Henrik
authors.initials: H
authors.lastname: Salje
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Louis R
authors.initials: LR
authors.lastname: Macareo
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand; Department of Medicine, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Laura L
authors.initials: LL
authors.lastname: Hermann
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Ananda
authors.initials: A
authors.lastname: Nisalak
authors.affiliation: Division of Infectious Diseases and Immunology, Department of Medicine, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America.
authors.forename: Anon
authors.initials: A
authors.lastname: Srikiatkhachorn
mesh_terms: D000293:Adolescent; D000328:Adult; D057134:Antibodies, Neutralizing / Q000097:blood / Q000276:immunology*; D000914:Antibodies, Viral / Q000097:blood / Q000276:immunology*; D001208:Asia; D058345:Asymptomatic Infections / Q000453:epidemiology; D001483:Base Sequence; D017691:Caribbean Region; D065632:Chikungunya Fever / Q000453:epidemiology* / Q000276:immunology* / Q000635:transmission / Q000821:virology; D002646:Chikungunya virus / Q000276:immunology* / Q000302:isolation & purification; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D005838:Genotype; D006801:Humans; D007074:Immunoglobulin G / Q000097:blood; D007075:Immunoglobulin M / Q000097:blood; D007223:Infant; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009500:Neutralization Tests; D010679:Philippines / Q000453:epidemiology; D010802:Phylogeny; D011446:Prospective Studies; D017423:Sequence Analysis, RNA; D055815:Young Adult
chemical_list: D057134:Antibodies, Neutralizing; D000914:Antibodies, Viral; D007074:Immunoglobulin G; D007075:Immunoglobulin M