Camacho A, et al. - Researchers ascertained the key epidemiological features of the cholera epidemic, including the drivers of cholera transmission during the outbreak. Across Yemen, the small first cholera epidemic wave seeded cholera during the dry season. In April 2017, when the rains returned, widespread cholera transmission was triggered that led to the large second wave. During the ongoing 2018 rainy season, if transmission remains active, cholera could resurge. Hence, current control efforts must be enhanced by the health authorities and partners to mitigate the risk of a new cholera epidemic wave in Yemen.

Methods

• A national cholera surveillance system was set up by the Yemen Health Authorities to collect information on suspected cholera cases presenting at health facilities.
• Symptom onset date, age, severity of dehydration, and rapid diagnostic test result were included in the individual variables.
• Experts confirmed the suspected cholera cases by culture, and a subset of samples had additional phenotypic and genotypic analysis.
• They first conducted descriptive analyses at national and governorate levels.
• The epidemic was divided the into 3 time periods: the first wave (Sept 28, 2016, to April 23, 2017), the increasing phase of the second wave (April 24, 2017, to July 2, 2017), and the decreasing phase of the second wave (July 3, 2017, to March 12, 2018).
• The changes in cholera transmission over time was reconstructed by estimating the instantaneous reproduction number, R_t.
• Finally, authors estimated the relationsip between rainfall and the daily cholera incidence during the increasing phase of the second epidemic wave by fitting a spatiotemporal regression model.

Results

• As per data, from Sept 28, 2016, to March 12, 2018, 1,103,683 suspected cholera cases (attack rate 3·69%) and 2,385 deaths (case fatality risk 0·22%) were reported countrywide.
• Findings suggested that 2 distinct waves were consisted in the epidemic with a surge in transmission in May, 2017, corresponding to a median R_t of more than 2 in 13 of 23 governorates.
• That the same Vibrio cholerae O1 Ogawa strain circulated in both waves was suggested in the microbiological analyses.
• Results demonstrated a positive, non-linear, relationship between weekly rainfall and suspected cholera incidence in the following 10 days; the relative risk of cholera after a weekly rainfall of 25 mm was 1·42 (95% CI 1·31–1·55) vs a week without rain.