Nigeria confirmed 6,288 Lassa fever cases in six years, and one in six died

Nigeria recorded 6,288 laboratory-confirmed Lassa fever cases and 975 deaths between 2020 and 2025, a case fatality rate of 15.5% among confirmed patients that showed no downward trend across the six years [s1]. That is the headline finding of a national longitudinal analysis of 307 consecutive weeks of surveillance data — the most complete recent picture of a haemorrhagic fever that Nigeria reports year after year but rarely controls [s1].

Lassa fever is caused by a virus carried by the multimammate rat, Mastomys, which sheds it in urine and droppings that contaminate food and household surfaces [s2]. Most human infections are mild: the World Health Organization estimates that about 80% of people infected have no or only mild symptoms, and roughly one in five progress to severe disease [s2]. The overall case fatality across all infections is about 1%, but among patients sick enough to be hospitalised it is 15% and above [s2]. The disease is endemic in Benin, Ghana, Guinea, Liberia, Mali, Nigeria and Sierra Leone, with Nigeria reporting the largest recorded burden [s2].

What the six-year record shows

The analysis drew on 47,029 suspected cases reported nationally over the period, of which 6,288 were confirmed [s1]. The gap between those two numbers is itself a finding. The mean laboratory-confirmation rate — the share of suspected cases that received a confirmatory test result — was 11.2%, and it fell over time, from 18.4% in 2020 to 9.9% in 2025 (p < 0.001) [s1]. A declining confirmation rate does not mean the disease is retreating; it means a shrinking fraction of the people flagged as possible cases are being tested to resolution, which the authors read as consistent with, though not proof of, systematic under-ascertainment [s1].

The 15.5% case fatality rate among confirmed patients sits far above WHO's 1% figure for all infections, and the two are not in conflict [s1][s2]. WHO's estimate spans every infection, including the large majority that are mild or never come to medical attention; the Nigerian surveillance figure counts only laboratory-confirmed cases, which skew heavily toward the severely ill patients who reach a hospital and get tested [s1][s2]. Read that way, a confirmed-case fatality of roughly one in six is closer to WHO's "15% and above" for hospitalised severe disease than to the population-wide 1% [s1][s2]. What is striking is the absence of improvement: over six years, better treatment access or earlier presentation did not move the confirmed-case fatality rate [s1].

A season that tracks the weather

Lassa fever in Nigeria is sharply seasonal, and the analysis quantified how closely that season follows the climate. Confirmed cases were inversely associated with weekly rainfall (Spearman's rho −0.589, p < 0.0001) and positively associated with mean temperature (rho 0.462, p < 0.0001) — that is, cases rise in the hot, dry months and fall as the rains arrive [s1]. Peak-season incidence ran 5.3-fold higher than during off-peak weeks [s1]. The pattern fits the ecology: the dry season concentrates rodents around human dwellings and stored food, raising the chance of contact.

The study also caught a signature of the COVID-19 pandemic. The 2020 national lockdown was associated with an immediate drop in confirmed cases but no matching drop in suspected cases [s1]. In other words, people kept being flagged as possible Lassa infections while the laboratory pipeline that confirms them faltered — a reminder that surveillance numbers measure the health system as much as the virus.

The limits of the picture

This is an ecological analysis of routine surveillance data, and the authors are explicit that it cannot establish cause [s1]. A case fatality rate built from confirmed cases is sensitive to who gets tested; if testing tilts further toward the sickest patients over time, the ratio can stay high even if the underlying disease is not becoming more lethal [s1]. Case-mix variation and detection bias may account for part of the flat fatality trend rather than a true failure of care [s1]. And a falling confirmation rate could reflect either genuinely wider casting of the suspected-case net or a real erosion of laboratory capacity — the data cannot separate the two [s1].

What the record does establish is a disease that Nigeria detects in large numbers, confirms in a shrinking minority, and loses roughly one in six confirmed patients to, year after year, on a clockwork seasonal cycle keyed to temperature and rainfall. The authors argue the response that fits that profile is a resilient, climate-informed, One Health surveillance system — one that links human, rodent and environmental data ahead of the dry-season peak rather than counting cases after it [s1]. Whether Nigeria's confirmation rate stabilises or keeps sliding is the metric to watch in the next season.

This article is informational and is not medical advice.

Sources

• [s1] Osong Public Health and Research Perspectives, "Temporal dynamics, climate associations, and surveillance performance of Lassa fever in Nigeria, 2020–2025: a national longitudinal analysis," published 29 June 2026. https://doi.org/10.24171/j.phrp.2026.0226
• [s2] World Health Organization, "Lassa fever — fact sheet." https://www.who.int/news-room/fact-sheets/detail/lassa-fever