Abstract
Background: Since the first case of Lassa fever was reported in Plateau State, Nigeria in 1969, several outbreaks have occurred, and many more cases are being reported every year. This study aimed to describe the trends in Lassa fever outbreak in Plateau State, Nigeria from 2015 to the first half of 2019.
 Methodology: A time series study was carried out employing a retrospective analysis of records of Lassa fever cases (from January 2015 – June 2019) obtained from Plateau State Epidemiological Unit. Cases were identified using the WHO case definition. Age-adjusted cumulative incidence, case positivity and case fatality rates were calculated for each year to study the trends. Data were retrieved and analyzed using Microsoft Excel.
 Results: Cases were mainly of the younger age group and were more concentrated in the Northern senatorial zone. Despite the increasing trend in incident cases from 5 in 2015 to 126 in 2019, and age-adjusted cumulative incidence from 1 case per 10,000 population in 2015 to 348 per 10,000 population in 2019, case fatality rate showed a downward trend from 29.6% in 2016 to 12.9% in 2019. Case positivity rates varied from 27.3% to 34.1% to 19.4% and back to 27.3% from 2016 to 2019, and the cumulative Case Positivity Rate was 25.7%.
 Conclusion: The incidence of Lassa fever in Plateau State showed an increasing trend while that of mortality was on a downward trend. More efforts should be put in place by the State government and other stakeholders to reduce the burden of this disease.
Highlights
Lassa fever is an acute viral haemorrhagic fever caused by Lassa virus, a single stranded RNA virus from the family arenaviridae [1]
Despite the increasing trend in incident cases from 5 in 2015 to 126 in 2019, and age-adjusted cumulative incidence from 1 case per 10,000 population in 2015 to 348 per 10,000 population in 2019, case fatality rate showed a downward trend from 29.6% in 2016 to 12.9% in 2019
The incidence of Lassa fever in Plateau State showed an increasing trend while that of mortality was on a downward trend
Summary
Lassa fever is an acute viral haemorrhagic fever caused by Lassa virus, a single stranded RNA virus from the family arenaviridae [1]. It is a highly infective zoonotic infection that is transmitted to man primarily by the reservoir multimamate rat, Mastomys natalensis species, through its faecal or urine contamination of food and drinks. Since Lassa fever was first discovered in 1969 in Lassa, Borno State, Nigeria It has remained endemic in some parts of West Africa (including Sierra Leone, Liberia, Guinea, and Nigeria) and flares up in epidemics [1]. Since the first case of Lassa fever was reported in Plateau State, Nigeria in 1969, several outbreaks have occurred, and many more cases are being reported every year. This study aimed to describe the trends in Lassa fever outbreak in Plateau State, Nigeria from 2015 to the first half of 2019
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