Research article

Estimating Chikungunya prevalence in La Réunion Island outbreak by serosurveys: Two methods for two critical times of the epidemic

Patrick Gérardin¹ , Vanina Guernier² , Joëlle Perrau¹ , Adrian Fianu¹ , Karin Le Roux³ , Philippe Grivard³ , Alain Michault³ , Xavier de Lamballerie⁴ , Antoine Flahault^2,5 and François Favier¹

¹  Centre d'Investigation Clinique – Épidémiologie Clinique (CIC – EC) de La Réunion (Institut National de la Santé et de la Recherche Médicale/Centre Hospitalier Départemental – Groupe Hospitalier Sud Réunion/Union Régionale des Médecins Libéraux de la Réunion), Groupe Hospitalier Sud Réunion, BP 350, 97448 Saint Pierre cedex, La Réunion

²  Unité de Recherche 707, Institut National de la Santé et de la Recherche Médicale, Faculté de Médecine Saint – Antoine, 27 rue de Chaligny, 75012 Paris, France

³  Service de Bactériologie-Parasitologie-Virologie et Hygiène, Groupe Hospitalier Sud Réunion, BP 350, 97448 Saint Pierre cedex, La Réunion

⁴  Unité des Virus Emergents, Faculté de Médecine, 27 bd Jean Moulin, 13005 Marseille, France

⁵  École des Hautes Études en Santé Publique, avenue Léon Bernard, 35000 Rennes, France

author email corresponding author email

BMC Infectious Diseases 2008, 8:99doi:10.1186/1471-2334-8-99

Published:	28 July 2008

Abstract

Background

Chikungunya virus (CHIKV) caused a major two-wave seventeen-month-long outbreak in La Réunion 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.