Table 1 Definition of key study variables

Epidemiological week

The first epidemiological week [herein: week] was defined as the week ending on the first Saturday of January 2018; subsequent weeks however began on Sunday and ended on Saturday. The current study covered week 1 to 47 out of the 52 weeks.

Epidemiological wave

In line with 2018 epidemiological weeks and a previous study [4], an epidemiological wave (herein: wave) was defined as the time between the commencement of a peak (first week with marked increase in the numbers of reported cases) and the end of a peak (epidemiological week marked decrease in the number of reported cases before the next rise in reported cases). The variable was classified as a categorical: first wave (between week 1 and week 9), second wave (between week 10 and week 28), third wave (between week 29 and week 34), and fourth wave (between week 35 and week 47).

Age

Age was defined in years and presented as a categorical variable.

Season

Dry season was defined as the period between week 1 and 12 and week 45 to 47, while rainy season was defined as the period between week 13 and 44.

Time to health seeking

This was defined as the difference between the reported date of health seeking and reported date of illness onset. It was classified as a categorical variable: same day, 1–2 days, and more than 2 days.

Location health care was sought

This was defined as primary healthcare centre, secondary hospital, tertiary hospital, cholera treatment centre (in any of the aforementioned health facilities), private clinic, and home. Information for its classification was validated by the DSNOs or State Epidemiologists of each affected state.

Outbreak setting

The affected LGAs were classified as rural, peri-urban or urban, using criteria of the population division of the United Nations which classifies an urban area as a settlement with 20,000 or more inhabitants, of which 75% or more are engaged in work other than agriculture, and a rural area as a settlement with fewer than 20,000 inhabitants whose primary occupation is agriculture. However, in the absence of a standard classification scheme, we classified a peri-urban area as a transition zone that is neither urban nor rural in the traditional sense. The provisional classifications were then validated using the respective state DSNOs’ or Epidemiologists’ final classifications.

Hospitalization

Hospitalization was defined as the admission of a cholera case to a formal health facility for at least one night.

Cholera death

Cholera death was defined as death of an individual classified as having cholera case in line with the case definition in the NCDC guidelines.

Attack Rate (AR)

AR was defined as the ratio of cholera cases in a defined area (e.g. state) to the estimated population of that area. AR for each reporting state was calculated using the estimated population of 2018, which was based on a 3.3% projected growth rate from the 2006 national census results; the values were multiplied by 100,000 for easier interpretation of small values.

Case Fatality Rate (CFR)

CFR was defined as the ratio of individuals classified as cholera cases who die to all those classified as cholera cases (alive and dead). CFR was expressed in percentage (%).