Measles seroprevalence in Chiradzulu district, Malawi: Implications for evaluating vaccine coverage.

INTRODUCTION Self-reported measles vaccination coverage is frequently used to inform vaccination strategies in resource-poor settings. However, little is known to what extent this is a reliable indicator of underlying seroprotection, information that could provide guidance ensuring the success of measles control and elimination strategies. METHODS As part of a study exploring HIV infection and measles susceptibility, we conveniently sampled consenting HIV-uninfected patients presenting at the HIV voluntary counselling and testing centre, and HIV-infected patients presenting for regular care, in Chiradzulu district hospital, Malawi, between January and September 2012. RESULTS A total of 2106 participants were recruited between January and September 2012, three quarters of whom were HIV positive. Vaccination cards were available for just 7 participants (0.36%). 91.9% of participants were measles seropositive. Older age (OR=1.11 per year increase in age; 95%CI: 1.09-1.14) and being female (OR=1.90; 95%CI: 1.26-2.87) were both associated with significantly increased odds for seroprotection. Prior vaccination history was associated with lower odds (Odds Ratio (OR)=0.44; 95% confidence interval (CI): 0.22-0.85) for confirmed seropositivity. Previous measles infection was not significantly associated with seroprotection (OR=1.31; 95%CI: 0.49-3.51). Protection by history and serological status were concordant for 64.3% of participants <35 years old. However, analysis by age group reveals important differences in concordance between the ages, with a greater degree of discordance among younger ages. Vaccination and/or infection history as a predictor of seropositivity was 75.8% sensitive, but just 10.3% specific. CONCLUSION Reported vaccination and previous infection were poor predictors of seropositivity, suggesting these may be unreliable indicators of seroprotection status. Such serosurveys may be indicated in similar settings in which overestimation of the proportion of seroprotected individuals could have important ramifications if used to guide vaccination strategies.