Abstract:
This study investigates the transmission dynamics of COVID-19 and Lassa
fever, with particular attention to the risks and implications of co-infection. By
dividing the human population into epidemiological compartments, the models
capture the real course of disease spread in a structured and realistic way. The
mathematical correctness of the models was validated by proving positivity, boundedness,
and reliability of solutions for public health interpretation.
For COVID-19, the basic SYR framework was analysed to obtain the reproduction
number RY , and the model was further extended to an SEAIHR structure
to include exposed, asymptomatic, infectious, and hospitalised individuals. For
Lassa fever, a deterministic compartmental model was developed and subjected
to stability analysis. Numerical simulations were carried out for both diseases to
assess intervention strategies and transmission behaviour.
The COVID–19 analysis revealed that the disease can be eliminated when
RY < 1, but once RY > 1, infection becomes persistent. The extended SEAIHR
model also produced the overall reproduction number R0, showing how reductions
in contact rates, timely detection, effective hospital care, and faster recovery can
substantially suppress transmission.
For Lassa fever, the disease-free equilibrium remained stable only when the
reproduction number was kept below one. Simulations highlighted the significant
influence of asymptomatic carriers and showed that no single intervention—
whether treatment, health education, or rodent control—can fully control the disease
on its own. Instead, the most meaningful reduction in cases occurred when
human-focused measures were combined with strong rodent control, leading to the
elimination of infectious rodents by the 35th day.
Together, these results emphasise the urgency of studying co-infections in regions
such as West Africa, where both diseases circulate at the same time. Incorporating
optimal control theory provides a systematic and cost-effective framework
for coordinating interventions across multiple pathways of transmission.
This study therefore deepens our understanding of the dynamics of both COVID–
19 and Lassa fever and offers practical insights for improving interventions, epivdemic preparedness, and public health responses. The centre-manifold analysis
further shows that both models experience a forward transcritical bifurcation at
R0 = 1: once transmission exceeds this threshold, a stable endemic state emerges.
This reinforces a critical message—maintaining transmission below the threshold
is essential for preventing long-term persistence of either disease.
