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Introduction
Yellow fever is a viral infection caused by a flavivirus (Aedes aegypti mosquito), which is spread in sub-Saharan Africa and tropical South America, particularly in areas with dense forests or jungles. This is largely dependent on the climate and the presence of the environment. These mosquitoes thrive in and near human habitations where they breed in even the cleanest water.
Humans and monkeys are most commonly infected with the yellow fever virus. Mosquitoes transmit the virus back and forth between monkeys, humans, or both.
How Does Yellow Fever Transmit?
When a mosquito bites a human or a monkey infected with yellow fever, the virus enters the mosquito's bloodstream and circulates before settling in the salivary glands. When the infected mosquito bites another monkey or human, the virus then enters the host's bloodstream, where it may cause illness.
Yellow Fever Epidemic in Nigeria
In Nigeria, yellow fever is an endemic and seasonal disease, occurring often in areas with poor sanitation, thus making its outbreaks not unusual. Although issues relating to yellow fever in Nigeria haven’t been frequently reported, thus making the disease look quite dynamic in nature, the existence of such disease in our communities (especially rural areas) can’t be undermined.
On the 23rd of October, 1969, the virus research laboratory of the University of Ibadan, Oyo State, was notified of a suspected case of yellow fever occurring in Jos, Plateau State; where the diagnosis was confirmed and the widespread of the disease made it to be recognized as the first epidemic case of yellow fever in Nigeria since 1953. Between September and the end of December 1969, an estimated total of 252 patients with yellow fever were hospitalized. The case-fatality ratio for hospitalized patients was approximately 40%. The diagnosis of yellow fever was confirmed by virus isolation, serology, or pathology in 55 patients. It is estimated that up to 100,000 cases of yellow fever may have occurred during the epidemic.
After this major outbreak, Nigeria has witnessed more outbreaks in various States across the nation (2011, 2016, and 2017). Since September 2017, yellow fever cases have been reported across several states in Nigeria. From 1 January to 31 August 2021, a total of 1,312 suspected cases were reported in 367 Local Government Areas (LGAs) across 36 States and the Federal Capital Territory (FCT).
A total of 45 blood samples were sent to the Institute Pasteur in Dakar (IPD) and 31 samples tested positive by Plaque reduction neutralization test (PRNT). Of these 31 PRNT-positive cases, twelve cases had a history of yellow fever vaccination. Two deaths were reported among the remaining 19 non-vaccinated PRNT-positive cases. These 19 PRNT-positive cases were reported from: Enugu (seven cases), Anambra (three cases), Benue (three cases), Delta (two cases), Oyo (two cases) Niger (one case), and Osun (one case) State.
Nigeria has documented gaps in population immunity against yellow fever. According to WHO-UNICEF 2020 estimates, the national immunization coverage for yellow fever was 54% in 2020 which is below the threshold of 80% necessary to protect against outbreaks. In the nine states reporting PRNT-positive cases, routine immunization coverage declined between 2018 and 2020 and was below 80% in 2020. These states include Anambra, Benue, Delta, Enugu, Imo, Niger, Ondo, Osun, and Oyo. Six states reported coverage below 50% (Anambra, Delta, Enugu, Imo, Osun, and Oyo states).
Between 2019 and 2020, preventive mass vaccination campaigns were conducted in six (all LGAs) of the nine states. Coverage was reported to be high (>90%) in Delta and Ondo States but lower (<80%) in Anambra, Benue, Niger, Osun, and Oyo States. Additionally, in Enugu State, nine out of 17 LGAs organized reactive mass vaccination campaigns in 2020, while in Imo State, mass vaccination activities have not been organized in recent years.
According to recent reports by the Nigeria Centre for Disease Control (NCDC) as of 30th September 2022, a total of 1,601 suspected cases have been reported from 463 LGAs in 36 states including the FCT in Nigeria to the World Health Organization (WHO) during the year 2022. Fifteen (15) deaths were recorded from these suspected cases with one hundred and seventy-four (11%) of 1,601 suspected cases receiving at least one dose of the yellow fever vaccine.
According to World Health Organization operatives in Nigeria, there is currently a moderate risk at these affected regions level due to the possible movement of the individuals of affected States to adjacent areas and neighboring countries, particularly if there is the arrival of unvaccinated visitors to the State.
To address the risk, the country has engaged in a multi-year plan to complete preventive mass vaccination campaigns targeting all eligible persons aged 9 months to 44 years old, supported through EYE and global partners. Since 2017, a total of 22 of 36 states including FCT have completed these campaigns. National and State public health authorities are currently responding to several concurrent outbreaks (COVID-19 pandemic, Lassa fever, and a widespread cholera epidemic), which are straining the available limited resources, especially human resources to conduct investigations and response activities. Additionally, the recent relaxation of COVID-19 measures could increase population mobility and the potential risk of spreading yellow fever to urban areas.
Yellow Fever Vaccine & Prevention
In preventing yellow fever from becoming an epidemic or spreading from one human to another, people must understand the various ways to prevent yellow fever, including the yellow fever vaccine.
Yellow Fever Vaccine
Yellow fever vaccine is the most important means of preventing yellow fever. This vaccine is used to prevent infection by the yellow fever virus. This vaccine works by causing your body to produce its protection (antibodies) against the virus. It is safe, and affordable and a single dose provides life-long protection against yellow fever disease. A booster dose of yellow fever vaccine is not needed. The vaccine provides effective immunity within 10 days for 80–100% of people vaccinated, and within 30 days for more than 99% of people vaccinated. It can only be given at authorized Yellow Fever Vaccination Centers.
Vaccination against yellow fever is recommended for all persons 9 months of age and older who are traveling to or living in African countries such as Nigeria, South America, or other countries where there is yellow fever infection and for people who are traveling to countries that require yellow fever immunization (certificate of vaccination). It is also needed by other people who might come into contact with the yellow fever virus.
By the International Health Regulations (IHR), countries have the right to require travelers to provide a certificate of yellow fever vaccination. If there are medical grounds for not getting vaccinated, this must be certified by the appropriate authorities. The certificate of vaccination for yellow fever is valid for 10 years beginning 10 days after the first vaccination, or on the date of the second vaccination if within 10 years of the first injection.
Pregnant women should be vaccinated only if they must travel to areas where there is an epidemic of yellow fever and they cannot be protected from mosquito bites.
Some people cannot have the yellow fever vaccine when it is recommended. People who cannot have the vaccine include:
Several vaccination strategies are used to prevent yellow fever disease and transmission: routine infant immunization; mass vaccination campaigns designed to increase coverage in countries at risk; and vaccination of travelers going to yellow fever endemic areas.
In high-risk areas where vaccination coverage is low, prompt recognition and control of outbreaks using mass immunization is critical. It is important to vaccinate most (80% or more) of the population at risk to prevent transmission in a region with a yellow fever outbreak.
Countries requiring vaccine card/certificate for people 9 months and above before entering includes especially those arriving from countries with a risk of yellow fever transmission include:
