Protection by natural cholera against later episodes of cholera over 10 years of follow-up in Matlab, Bangladesh: a retrospective cohort study

Abstract

SummaryBackgroundPatients with cholera have been shown to be protected against subsequent cholera for 3 years after their initial episode. We aimed to assess protection at 10 years of follow-up.MethodsIn this retrospective cohort study, cohorts of patients treated for cholera (index patients) and contemporaneously selected age-matched individuals without cholera (controls), randomly selected from the population of Matlab, Bangladesh, were assembled between 1990 and 2009 and followed for up to 10 years. Selection of participants who had no history of cholera in the 5 years before selection proceeded in secular sequence, and selection was done without replacement. Protection against subsequent treated cholera was assessed in proportional hazards models and waning of protection was assessed non-parametrically with use of smoothing of protection curves.FindingsWe included 3925 index patients and 23 550 matched controls. Patients with El Tor cholera (26 subsequent episodes among 3619 index patients) had a 48·6% (95% CI 23·1 to 65·7; p=0·0012) lower risk of El Tor cholera than controls, with no evidence of waning during up to 10 years of follow-up (p=0·87). Index patients aged 5 years and older with El Tor cholera (nine subsequent episodes among 2279 index patients) were at a 61·7% (23·6 to 80·8; p=0·0065) lower risk of El Tor cholera, whereas index patients younger than 5 years with El Tor cholera (17 subsequent episodes among 1340 index patients) had a 36·2% (–5·0 to 61·3; p=0·077) lower risk (p=0·26 for the difference by age).InterpretationProtection against El Tor cholera associated with previous El Tor cholera was moderate in magnitude and sustained over 10 years of follow-up. These findings suggest the potential for sustained, long-term protection by oral cholera vaccines in populations with endemic cholera and help inform models of cholera in endemic settings.FundingBill & Melinda Gates Foundation.