Journal of Mosquito Research, 2024, Vol.14, No.5, 264-275 http://emtoscipublisher.com/index.php/jmr 269 The epidemic was characterized by a delayed response in some areas, with emergency vaccination campaigns initiated only after the disease had spread significantly. The average response time ranged from 15 to 132 days, indicating room for improvement in surveillance and rapid response measures (Umar et al., 2020). This delay was particularly problematic in rural and remote regions, where access to healthcare services is limited, and local vaccination coverage was low. As a result, the outbreak underscored the need for stronger health infrastructure and more efficient coordination in public health response efforts. 4.2 Vaccination campaigns: recent successes and challenges In response to the outbreak, Nigeria implemented a series of mass vaccination campaigns, which were critical in controlling the spread of the virus. The campaigns, supported by the International Coordinating Group (ICG) on Vaccine Provision, targeted populations under 45 years of age, with over 45 million individuals vaccinated between 2017 and 2019. These efforts contributed to a decline in cases by boosting population immunity, particularly in high-risk areas. However, challenges persisted, including logistical barriers, vaccine shortages, and vaccine hesitancy among certain communities. One major challenge was the implementation of fractional-dose vaccination during periods of vaccine shortage. This approach allowed a larger population to receive some level of protection but raised concerns about the duration of immunity provided by smaller doses (Ahuka-Mundeke et al., 2018). Furthermore, the COVID-19 pandemic disrupted routine immunization services, exacerbating the risk of YF spread due to delayed catch-up campaigns and reduced vaccination rates during the pandemic lockdowns (Gaythorpe et al., 2021). Despite these setbacks, the success of reactive vaccination efforts in slowing the outbreak demonstrated the importance of rapid deployment and mass immunization campaigns in outbreak settings. 4.3 Impact on immunization policy The recurring YF outbreaks in Nigeria prompted significant changes in immunization policies at the national and regional levels. The outbreak response emphasized the need for integrating yellow fever vaccination into routine immunization schedules, particularly for children under the Expanded Programme on Immunization (EPI). This change aimed to increase the baseline immunity in younger populations and prevent future outbreaks (Jean et al., 2020). Moreover, the outbreak accelerated the implementation of the World Health Organization’s Eliminate Yellow Fever Epidemics (EYE) strategy in Nigeria, which focuses on increasing vaccination coverage, improving surveillance systems, and ensuring rapid response capabilities. The EYE strategy aims to prevent future outbreaks through preemptive vaccination campaigns targeting at-risk populations before the occurrence of cases (Adogo and Ogoh, 2019). Additionally, the integration of yellow fever vaccination into broader health programs allowed for a more comprehensive approach to managing the disease, combining vaccination with vector control and health education initiatives to reduce transmission. 4.4 Long-term immunity and the role of booster doses The yellow fever vaccination campaign in Nigeria highlighted debates about the duration of immunity provided by a single dose of the 17D vaccine, especially in populations vaccinated during infancy. While the World Health Organization (WHO) currently endorses a single dose for lifelong immunity, emerging evidence suggests that immunity can wane, particularly in children vaccinated at nine months of age (Domingo et al., 2019). Studies in West African populations, including Ghana and Mali, have shown that seropositivity rates decline significantly within 6 years post-vaccination, raising concerns about the adequacy of a single-dose strategy in endemic regions. This decline in immunity has led to discussions on the potential need for booster doses, particularly for individuals at higher risk, such as those living in endemic areas or healthcare workers with frequent exposure to the virus (Vasconcelos and Barrett, 2019). Additionally, research in Nigeria indicates that a considerable proportion of adults might require a booster dose to maintain protective immunity, especially considering that natural exposure to the virus can be unpredictable and variable across different regions (Campi-Azevedo et al., 2019). As a result,
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