Reported case fatality as a target and trial metric in endemic zoonoses: a framework and ecological analysis of Lassa fever surveillance in Nigeria, 2017–2026
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Introduction
Reported case fatality ratios (CFRs) are used to set national targets, such as Nigeria’s goal of a Lassa fever CFR below 10%, and to plan vaccine and therapeutic trials, yet they also move with testing, death reporting and where cases occur. Using the rise in Nigeria’s reported Lassa fever CFR from 18.9% at week 23 of 2025 to 25.0% in 2026 as a test, I evaluated what published data can show.
Methods
This ecological study extracted weekly state-level counts from 474 Nigeria Centre for Disease Control and Prevention situation reports (2017-2026). Following a pre-specified plan, years were compared at phase-matched points, year-on-year changes were decomposed into within-state and between-state components, and the ascertainment change needed to explain 2026 was calculated. Comparing the published cumulative and summed weekly series was added after the plan was fixed.
Results
By week 36 of 2026, 70 deaths had been added to the cumulative totals outside any week’s figures (4 in all of 2025), so the published 2026 CFR exceeded 2025 by 5.4 to 6.2 percentage points at every comparison point. On the weekly series, 2026 did not differ from 2025 at the phase-matched midpoint (+0.3 percentage points, 95% CI -5.0 to 7.6), and within-state (-1.1, 95% CI -4.7 to 3.0) and between-state (+0.5, -2.0 to 2.9) components spanned zero. An additional 279 unconfirmed survivors (6% of suspected cases not confirmed) would bring the published 2026 CFR down to the 2025 level.
Conclusion
The 2026 rise came from deaths added to cumulative totals, not the weekly counts, and cannot be attributed to care. Reported CFRs alone cannot track national targets or set trial baselines. Data already collected in case-based surveillance (specimens tested, sampling at or after death, outcome ascertainment, late-added deaths by week of confirmation) could be published to close this gap.
Key messages
What is already known on this topic.
Reported case fatality ratios (CFRs) are used to set targets and plan trials for endemic zoonoses such as Lassa fever, but methods for separating a change in reported CFR from changes in testing and reporting were developed for epidemics spread person to person.
What this study adds.
In Nigeria, the published 2026 rise in Lassa fever CFR came from deaths added to cumulative totals outside any week’s figures; on the weekly figures it did not differ from 2025, and the published reports allow only 2 of 11 mechanisms that move a reported CFR to be evaluated fully.
How this study might affect research, practice or policy.
A minimum reporting set deliverable from data already collected in case-based surveillance (specimens tested, the share sampled at or after death, the share with a known outcome and late-added deaths by week of confirmation) would let national CFR targets be tracked and give trialists and funders baselines that separate surveillance from severity.
