Table 7.

Formula for Incidence Estimations

Variable	Definition
ni	Number of cases (S. Typhi, iNTS, S. Paratyphi) during surveillance period in age group i
si	Number of severe typhoid fever cases during surveillance period in age group i
1ni	Population present in catchment area at the start of surveillance in age group i
2ni	Estimated population present in catchment area at the end of surveillance in age group i
ri	Recruitment proportion of eligible patients in SETA healthcare facilities in age group i
hi	Proportion of population visiting SETA health care facilities (HCUS) in age group i
Adni	Adjusted cases in age group i
Adsi	Adjusted cases of severe typhoid in age group i
PYOi	Population in person years observation in catchment area in age group i
APYOi (Adjusted PYO)	Adjusted population at risk contributing to PYO in age group i (accounting for new, lost to follow-up, deceased individuals during surveillance period)
Incidence of non-severe TF/PF/iNTS (TSAP-style a incidence estimation using multipliers of healthcare utilisation and recruitment proportion)	Incidence per 100,000 = (new lab confirmed cases)/(population at risk) x 100,000 HDSS site: Numerator: Adni Adni = ni x 1/ri Denominator: APYOi APYOi = PYOi x hi Adjusted incidence ratei in 100,000 PYO = (Adni/APYOi) x 100,000 Non-HDSS site: Numerator: Adni Adni = ni x 1/ri Denominator: APYOi 2ni = 1ni x (annual growth rate) x [(months of surveillance)/12] PYOi = [(1ni+2ni)/2] x [(months of surveillance)/12] APYOi = PYOi x hi Adjusted incidence ratei in 100,000 PYO = (Adni/APYOi) x 100,000
Incidence of severe TF	Number of severe typhoid fever cases during surveillance period in age group i: si Numerator: Adsi Adsi = si x 1/ri Denominator: APYOi APYOi = PYOi x hi Incidence rate si in 100,000 = (severe TF cases)/(population at risk) x 100,000 Adjusted incidence rate si in 100,000 PYO = (Adsi/APYOi) x 100,000
Frequency proportion of severe TF	Number of severe typhoid fever cases out of total laboratory confirmed typhoid fever cases in percentages

Abbreviations: APYO, adjusted person-years of observation; HCUS, healthcare utilization survey; iNTS, invasive nontyphoidal Salmonella; NTS, nontyphoidal Salmonella; PF, paratyphoid

fever; PYO, person-years of observation; SETA, Severe Typhoid Fever in Africa program; TF, typhoid fever; TSAP, Typhoid Fever Surveillance in Africa Program.

^avon Kalckreuth, V, Konings F, Aaby P, et al. The Typhoid Fever Surveillance in Africa Program (TSAP): Clinical, Diagnostic, and Epidemiological Methodologies. Clin. Infect. Dis 2016; 62:S9-S16.