Since 2017, Nigeria has experienced large outbreaks of Lassa fever (LF) [1,2]. The Nigeria Centre for Disease Control (NCDC) activated its Emergency Operations Centre for the outbreak response. However, it was difficult to determine the emergency phase for LF outbreak response because of its endemicity. Because there are ongoing sporadic LF cases throughout the year , a single case of the disease cannot be the trigger to determine the emergency phase of LF such as the case of Ebola . The World Health Organization advocated the use of alerts for malaria when weekly cases exceed the 75th percentile of cases from the same week in previous years . However, applying similar thresholds for LF seems too low to implement intensive response in resource-limiting setting. Additionally, the thresholds do not take consideration of the capacity of response activity itself.
Here, we describe the development of composite indicators to determine the emergency phase for LF outbreak response in Nigeria. The composite indicators consist of seven criteria to reflect the outbreak situation from various aspects.
1. Number of confirmed cases
Emergency phase designation criteria thresholds were developed using statistical criteria . The thresholds were developed as the mean plus two standard deviations of the weekly number of confirmed cases for the period of 2 weeks before and after a particular week in the past 3 years (eg, data from week 3-7 in 2016-2018 for the development of threshold for week 5 in 2019). This accounts for variation in LF incidence throughout the year and the previous years.
2. Number of states with cases exceeding bed capacity
Even if the number of confirmed cases at the national level was below the threshold defined in criterion 1, the outbreak could surpass local health care capacity. Therefore, we monitored the number of cases against bed capacity in LF treatment center at the state level.
3. Number of suspected cases
The number of confirmed cases would be less meaningful if there was no adequate surveillance activity to detect LF cases. Therefore, the number of suspected cases including both laboratory-confirmed and laboratory-negative cases was monitored to ascertain sufficient surveillance activity. To consider seasonal fluctuation of the disease, the lower thresholds were developed as the mean minus one standard deviation of the number of suspected cases in the past 3 weeks in the same year.
4. Case fatality rate
Timely and appropriate medical care, such as ribavirin administration and renal dialysis, was reported to improve prognosis of patients with LF [1,5]. Unexpectedly high case fatality rate indicates malfunction of the health care system such as delayed referral to treatment center and poor compliance with case management guidelines . We monitored weekly case fatality rate and compared it with that in the previous year.
5. Infection among health care workers
Nosocomial human-to-human transmission of the disease can be prevented by appropriate standard precaution strategy. The criterion is met when there are ≥2 confirmed cases among health care workers.
6. Safe burial
Unsafe burial is considered to be a mode of secondary transmission of viral hemorrhagic fever. National guidelines stated that a safe and dignified burial should be conducted for every LF corpse by designated safe burial team . There should be no unsafe burial of LF patient.
7. Turnaround time of laboratory test
Timely diagnosis of LF is important for better clinical management of patients and for prevention of further spread of the disease. Although in Nigeria, a national network of laboratories covers the entire country for LF diagnosis, the surge of submission of samples during outbreak affected sample transportation and diagnostic test, resulting in the delay in turnaround time for diagnostic results. We monitored the proportion of samples in which the time from sample collection to diagnostic test report was ≥48 hours and compared this proportion with that observed in the previous year.
Using the seven composite indicators, “Emergency” of the outbreak was defined as either “criterion 1 and one more criterion” or “three or more of criterion 2–7.” Table 1 shows the situation of LF outbreak in 2019. Emergency phase started from week 3, and the emergency phase was declared over when there was no “emergency” in 4 consecutive weeks (week 22). The NCDC have used this approach to declare the emergency phase and its containment to change intensity of outbreak response. This kind of composite indicators considering various aspects of an outbreak could be useful to determine emergency phase not only for LF but also for other endemic diseases. Determination of emergency phase for outbreak response would enable us to effectively and efficiently allocate resources particularly in resource-limiting settings.
|Wk 1||Wk 2||Wk 3||Wk 4||Wk 5||Wk 6||Wk 7||Wk 8||Wk 9||Wk 10||Wk 11||Wk 12||Wk 13||Wk 14||Wk 15||Wk 16||Wk 17||Wk 18||Wk 19||Wk 20||Wk 21||Wk 22||Wk 23||Wk 24|
|Criterion 1||Indicator||Number of confirmed cases*||25||35||74||77||68||37||25||23||39||52||23||15||16||11||3||6||8||11||4||6||3||3||6||4|
|Threshold||Calculated from data in the past 3 years||38||42||44||48||61||64||64||62||59||44||30||17||16||16||13||10||9||7||7||6||6||5||8||9|
|Criterion 2||Indicator||Number of states with cases exceeding bed capacity||0||0||3||4||3||1||1||1||1||2||1||0||0||0||0||0||0||0||0||0||0||0||0||0|
|Criterion 3†||Indicator||Number of suspected cases*||64||130||170||193||217||200||163||119||182||215||152||137||126||88||84||90||59||79||77||100||100||65||67||65|
|Lower threshold||Calculated from data in the past 3 weeks||65||59||48||67||132||169||190||165||120||122||123||151||126||125||91||76||84||61||60||60||72||79||68||57|
|Criterion 4||Indicator||Case fatality rate (%)||28.0||22.9||18.9||14.3||20.6||27.0||24.0||26.1||20.5||21.2||17.4||33.3||12.5||9.1||0||0||12.5||36.4||0||16.7||0||33.3||16.7||25.0|
|Threshold||Data from the previous year||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0||25.0|
|Criterion 5||Indicator||Number of confirmed cases among healthcare workers||0||0||1||1||4||3||1||1||0||0||1||0||1||0||0||0||1||0||0||0||0||0||0||0|
|Criterion 6||Indicator||Number of unsafe burial||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0||0|
|Criterion 7||Indicator||Proportion of samples that took ≥48 hours from sample collection to diagnostic test report (%)||20.3||26.0||20.0||17.6||17.9||19.0||19.5||29.5||15.6||10.3||15.2||22.0||17.2||12.0||28.8||14.9||17.5||17.9||12.7||10.9||13.3||15.6||12.3||9.4|
|Threshold||Data from the previous year||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0||22.0|
|Emergency defined by the composite indicators‡||YES||YES||YES||YES||YES||YES||YES||YES|
|Emergency phase‡||Start||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||Ong.||(End) §||Ong.||Ong.||Ong.||Ong.||End|
Wk – week, Ong. – ongoing
*The definition of suspected and confirmed LF cases used for the indicators was described elsewhere .
†Criterion 3 is met when indicator is ≤ the thresholds. For others, each criterion is met when indicator is ≥ the thresholds.
‡Interpretation of composite indicators to define emergency phase is explained in detail in the main texts.
§According to the definition, week 17 should have been the end of the emergency phase. However, the containment of emergency phase was not declared then because resurge of the number of cases was recognized during week 18 before completion of assessment for week 17.