Abstract:
Introduction
In cholera endemic areas, the periodicity of cholera outbreaks remains unpredictable, making it difficult to organize preventive efforts. Lack of data on duration of protection conferred
by oral cholera vaccines further makes it difficult to determine when to deploy preemptive
vaccination. We report on the immunogenicity and waning of immunity to Shanchol™ in
Lukanga Swamps.
Methods
We enrolled a cohort of 223 participants aged between 18 and 65 years old from whom
serum samples were collected at baseline, day 28 before administration of the second dose,
and consecutively at 6, 12, 24, 30, 36, and 48 months. Vibriocidal antibody titres were measured and expressed as geometric mean titres. Box plots and 95% CI were computed at
each visit for both Inaba and Ogawa. Seroconversion was defined as a four fold or greater
increase in antibody titres compared to baseline titres.
Results
Overall, seroconversion against V. cholerae Inaba and Ogawa after 1st dose was 35/134
(26%) and 34/134 (25%) respectively. We observed a statistical difference in seroconversion between the two subgroups of baseline titres (low <80 and high �80) for both Inaba
(p = 0.02) and Ogawa (p<0.0001). From a baseline of 13.58, anti-Ogawa GMT increased to
21.95 after the first dose, but rapidly waned to 14.52, 13.13, and 12.78 at months 6, 12 and
24 respectively, and then increased to 13.21, 18.67 and 23.65 at months 30, 36 and 48
respectively. A similar trend was observed for anti-Inaba GMT across the same time points.
Conclusion
We found that Shanchol™ was immunogenic in our study population and that vibriocidal
antibodies may not be a good marker for long-term immunity. The observed rise in titres
after 36 months suggests natural exposure, and this may be a critical time window opening
for natural transmission in an endemic areas. We recommend re-vaccination at this time
point in high risk areas.
