Gérardin P. auteur aut IRD Guernier V. auteur aut IRD Perrau J. auteur aut IRD Fianu A. auteur aut IRD Le Roux K. auteur aut IRD Grivard P. auteur aut IRD Michault A. auteur aut IRD de Lamballerie Xavier auteur aut IRD Flahault A. auteur aut IRD Favier F. auteur aut IRD text journalArticle eng text/pdf reformatted digital access Background: Chikungunya virus (CHIKV) caused a major two-wave seventeen-month-long outbreak in La Reunion Island in 2005-2006. The aim of this study was to refine clinical estimates provided by a regional surveillance-system using a two-stage serological assessment as gold standard. Methods: Two serosurveys were implemented: first, a rapid survey using stored sera of pregnant women, in order to assess the attack rate at the epidemic upsurge (s1, February 2006; n = 888); second, a population-based survey among a random sample of the community, to assess the herd immunity in the post-epidemic era (s2, October 2006; n = 2442). Sera were screened for anti-CHIKV specific antibodies (IgM and IgG in s1, IgG only in s2) using enzyme-linked immunosorbent assays. Seroprevalence rates were compared to clinical estimates of attack rates. Results: In s1, 18.2% of the pregnant women were tested positive for CHIKV specific antibodies (13.8% for both IgM and IgG, 4.3% for IgM, 0.1% for IgG only) which provided a congruent estimate with the 16.5% attack rate calculated from the surveillance-system. In s2, the seroprevalence in community was estimated to 38.2% (95% CI, 35.9 to 40.6%). Extrapolations of seroprevalence rates led to estimate, at 143,000 and at 300,000 (95% CI, 283,000 to 320,000), the number of people infected in s1 and in s2, respectively. In comparison, the surveillance-system estimated at 130,000 and 266,000 the number of people infected for the same periods. Conclusion: A rapid serosurvey in pregnant women can be helpful to assess the attack rate when large seroprevalence studies cannot be done. On the other hand, a population-based serosurvey is useful to refine the estimate when clinical diagnosis underestimates it. Our findings give valuable insights to assess the herd immunity along the course of epidemics. specialized 052 8 99 2008 1471-2334 https://www.documentation.ird.fr/hor/fdi:010048276 10.1186/1471-2334-8-99 1471-2334 [F B010048276] https://www.documentation.ird.fr/hor/fdi:010048276 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers17-09/010048276.pdf IRD - Base Horizon / Pleins textes 2009-11-25 2017-10-03 fdi:010048276 fre