Effectiveness of oral cholera vaccine in preventing cholera among fishermen in Lake Chilwa, Malawi: A case-control study

Background In response to a cholera outbreak among mobile, difficult-to-reach fishermen on Lake Chilwa, Malawi in 2016, a novel vaccine distribution strategy exploited the proven vaccine thermostability. Fishermen, while taking the first vaccine dose under supervision, received the second dose in a sealed bag, and were told to drink it two weeks later. This study assessed short-term vaccine protection of this strategy. Methods Patients with diarrhoea admitted to health facilities around lake were interviewed and a stool sample collected for PCR testing. Vaccine effectiveness was assessed in a case-control test-negative design by comparing cases (PCR-positive for V. cholerae O1) and controls (patients with diarrhoea but PCR-negative) and with the screening method that compared the proportions of vaccinated among cholera cases versus the general fishermen population. Results Of 145 study participants, 120 were fishermen living on the lake. Vaccine effectiveness at three-months was 90.0% [95% CI: 38.8; 98.4] among fishermen and 83.3% [95% CI: 20.8; 96.5] among all participants in the case-control test-negative design, and 97.5% [95% CI: 90.9; 99.3] with the screening method. Conclusion This strategy was effective in providing short-term protection in fishermen against cholera. Further research is needed to determine the adding value of the second dose and to identify the optimal vaccination strategies for different contexts.