Interpreting changes in reported case fatality in endemic zoonoses: an analytical framework and ecological analysis of Lassa fever surveillance in Nigeria, 2017–26
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Background
Nigeria’s reported Lassa fever case fatality ratio (CFR) rose from 18.9% at week 23 of 2025 to 25.0% in 2026, prompting concerns over failing clinical care. Because surveillance artefacts can also alter CFRs, candidate mechanisms that published data can resolve were identified and evaluated.
Methods
This ecological study extracted weekly state-level counts from 474 Nigeria Centre for Disease Control and Prevention situation reports (2017-2026). Per a pre-specified plan, years were compared at phase-matched points, year-on-year CFR changes were decomposed into within-state and between-state components, and tipping-point ascertainment shifts calculated. Weekly incident and cumulative published totals were compared post hoc.
Findings
Of 11 candidate mechanisms, 2 could be evaluated fully and 5 in part. On weekly incident counts, 2026 did not differ from 2025 at the phase-matched midpoint (+0.3 percentage points, 95% CI -5.0 to 7.6); within-state (-1.1, 95% CI -4.7 to 3.0) and between-state (+0.5, -2.0 to 2.9) components spanned zero. Conversely, the published cumulative CFR was 5.4 to 6.2 percentage points higher because 70 deaths were added retrospectively by week 36 outside weekly reports. Reconciling cumulative figures would require 279 missed survivors (6% of suspected cases not confirmed).
Interpretation
The 2026 CFR rise came from deaths added to cumulative totals outside the weekly counts, and published data cannot attribute it to care. Endemic zoonotic CFRs cannot serve as clinical outcome measures unless testing volumes, deaths added late, and post-mortem sampling are published alongside case counts.
Funding
US NSF-NIH-NIFA EEID; UK BBSRC; Birgitta Sintring Fellowship (Uppsala University).
Research in context
Evidence before this study
PubMed and Europe PMC, including preprints, were searched on 13 October 2026, with no date or language restriction, for studies separating changes in a reported case fatality ratio (CFR) from changes in ascertainment or reporting (strategy in the appendix). Such methods exist mainly for diseases spread in chains of human transmission, such as COVID-19. Detection that depends on the outcome has been described for Nipah virus and plague, and analyses of Nigeria’s Lassa fever surveillance take the reported CFR as the outcome. No framework was found for interpreting year-on-year or seasonal changes in the reported CFR of a zoonosis with recurrent spillover, or for evaluating whether public surveillance data can show that such a change reflects outcome.
Added value of this study
Of 11 mechanisms identified that can move a reported CFR, published NCDC situation reports allow 2 to be evaluated fully and 5 in part. Years were compared at matched points in their transmission seasons, each year-on-year change since 2018 was split into within-state and between-state change, and the change in ascertainment needed to explain the 2026 figure was estimated. On the weekly figures, 2026 did not differ from 2025, and neither the within-state nor the between-state change differed from zero. The published rise came from deaths added to the printed cumulative totals outside any week’s figures, in amounts that vary widely between years.
Implications of all the available evidence
A reported CFR moves with who is tested, how and when deaths are counted and where cases occur, as well as with care, so a change in it cannot on its own be read as a change in outcome, in Lassa fever or in other endemic zoonoses compared year on year. The data that would separate these explanations (specimens tested, the share of cases sampled at or after death, the share with a known outcome, and deaths added late by week of confirmation) are collected but not published. Publishing them would let progress towards national CFR targets be tracked and would support planning of Lassa fever vaccine trials.
