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


CONCLUSIONS
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.
Gérardin
Patrick
P
Centre d'Investigation Clinique-Epidémiologie Clinique de La Réunion, Institut National de la Santé et de la Recherche Médicale, Groupe Hospitalier Sud Réunion, Saint Pierre. patrick.gerardin@chr-reunion.fr
Guernier
Vanina
V
Perrau
Joëlle
J
Fianu
Adrian
A
Le Roux
Karin
K
Grivard
Philippe
P
Michault
Alain
A
de Lamballerie
Xavier
X
Flahault
Antoine
A
Favier
François
F
D018354:Alphavirus Infections / Q000453:epidemiology* / Q000276:immunology; D000914:Antibodies, Viral / Q000097:blood* / Q000276:immunology; D002646:Chikungunya virus / Q000276:immunology / Q000302:isolation & purification*; D003430:Cross-Sectional Studies; D004196:Disease Outbreaks; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D006801:Humans; D007074:Immunoglobulin G / Q000097:blood; D007075:Immunoglobulin M / Q000097:blood; D011247:Pregnancy; D011251:Pregnancy Complications, Infectious / Q000453:epidemiology*; D015995:Prevalence; D017508:Reunion / Q000453:epidemiology; D018571:Sentinel Surveillance; D016036:Seroepidemiologic Studies
D000914:Antibodies, Viral; D007074:Immunoglobulin G; D007075:Immunoglobulin M