Serological responses to killed oral cholera vaccine (OCV) when given 4 years after initial receipt of OCV in Cameroon: A randomized controlled trial.

Oral cholera vaccination (OCV) is one of strategies to help control cholera with two doses recommended as initial vaccination and booster doses recommended 3-5 years later to prevent future outbreaks. There is no actual recommendation on the number of doses required during booster vaccination. An OCV campaign conducted in Mogode-Cameroon provided an opportunity to determine if revaccination four years after would induce a booster response. 350 people living in Mogode-Cameroon, who received 0, 1 or 2 doses of OCV four years earlier, were stratified into age groups, < 5, 5-14, and >14 and randomized into two arms. Arm A received a single dose and Arm B received two doses 14 days apart. We aimed to determine if people who had received OCV four years earlier responded with a booster immune response compared to those not vaccinated. Serum samples were collected at baseline and at multiple time points after vaccination to measure vibriocidal antibody titers. For Arm A, vibriocidal GMT significantly increased nine days after the first dose with a response rate of 61-70%; however, early vibriocidal response (day 4-6) was rare (22%) and were similar in those who had received vaccine four years earlier and those who did not. Ogawa, but not Inaba, vibriocidal responses were higher on day 9 for those who had been vaccinated earlier, but vibriocidal titers during subsequent timepoints were similar in all study arms. In Arm B, there were no early response (day 4-6) after the second dose. Overall, OCV induced a measurable immune response by day 9, but no evidence of a rapid booster response was detected following revaccination. Since there was no evidence that the earlier vaccination stimulated a booster response when vaccination is repeated, this suggests that persons being revaccinated should receive the two doses as was recommended initially. Trial registration: Pan African Clinical Trials Registry under the reference PACTR202102660004195.