High fever returns and several body systems are affected, usually the liver and the kidneys. In this phase, people are likely to develop jaundice (yellowing of the skin and eyes), hence the name yellow fever, associated with dark urine and abdominal pain with vomiting.
Yellow fever (YF) is a mosquito borne viral illness caused by an arbovirus of the family flaviviridae, genus flavivirus, encompassing positive single stranded RNA viruses. The virus was isolated for the first time in 1927 in a male patient .Transmission is primarily by mosquitoes. After an incubation period of 3- 6 days, yellow fever infection can cause the onset of different clinical features, ranging from a self – limited or mild febrile illness with flu like symptoms in most of the cases to severe hemorrhage and liver disease.
The analysis of data on asymptomatic infection, mild disease, severe disease (fever with jaundice or hemorrhagic symptoms) and fatalities collected in 11 studies involving Africa and South America during the 1969– 2011 was used to estimate the probability of each infection outcome. In cases of yellow fever virus infections, the probability of being asymptomatic was 55%, where as the probability of developing mild and severe diseases were 33% and 12% respectively. The probability of death for people experiencing severe disease was 47%. Symptoms include fever, muscle pain, nausea, vomiting and fatigue. Such variety in the clinical spectrum makes yellow fever diagnosis difficult. In those patients presenting a severe infection, hemorrhagic fever can develop leading to the death of the infected subject.
The case fatality rate has been estimated as 20 – 50% in patients with severe symptoms accounting for approximately78, 000 deaths every year, although misdiagnosis and under reporting might be responsible for under estimation of mortality rate. Epidemiological and genetic studies sustain the hypothesis that yellow fever virus originated in Africa and would be introduced in the 16th century by the trading of slaves from endemic African countries into countries of the western region of America ,causing outbreaks there between the 17th and 18th centuries. Yellow fever virus is transmitted to people primarily through the bite of infected Aedes or Haemagogus species mosquitoes.
Mosquitoes acquire the virus by feeding on infected primates (human or non human / monkeys) and then can transmit the virus to other primates. People infected with yellow fever virus are infectious to mosquitoes shortly before the onset of fever and up to 5 days after onset. Concerning yellow fever transmission from the seventeenth century until the end of the nineteenth century, it was argued that it could occur by water and or human contacts, sustaining the idea that the germ penetrated the body through the respiratory system.
Following the epidemics that occurred in Philadelphia in 1793, Cadiz in 1800 and Barcelona in 1821 – 1822, where in the absence of direct contact between the patients could not have had a role in their spread, the hypothesis regarding the modality of the disease transmission progressively changed at the end of 18th century, sustain the idea that direct contact could not be responsible for yellow fever transmission. The virus is maintained in nature by transmission between non- human primates via blood feeding mosquitoes and transovarial transmission in competent vectors. Since non – human primates represents one of the reservoirs, the yellow fever virus cannot be eradicated and the identification of vectors in different habitats has led to the establishment of three distinct transmission cycles; like wild, semi domestic and domestic. A domestic or urban yellow fever cycle occurs when the virus is introduced into highly populated areas with elevated mosquito density by infected people that’s, after having contracted the virus in the jungle come back into urban areas.
Here the virus can be transmitted from person to person by competent urban mosquitoes leading to the onset of uncontrolled outbreaks with devastating consequences. Uganda is a yellow fever endemic country with high – risk of transmission. The country introduced yellow fever into routine immunization schedule in October 2022, during the integrated child health days (ICHDs). The yellow fever vaccine is safe and effective and a single dose provides life – long protection against yellow fever. It provides effective immunity within 10 days for 80 – 100% of people vaccinated, and within 30 days for more than 99% of people vaccine.
All children aged 9 months and above, and adults are eligible for vaccination. A booster dose is not needed. Before the introduction of the yellow fever vaccine into routine immunization, the overall population immunity in Uganda was low ( 4.2%) and this was attributed to past reactive vaccination activities supported by ICG in focal districts like Masaka and Koboko in 2019 and Yumbe , Moyo , Buliisa , Maracha and Koboko in 2020. Further the country had history of outbreak reported in 2020(Buliisa, Maracha and Moyo districts, 2019(Masaka and Koboko districts), 2016(Masaka, Rukungiri and Kalangala districts) and in 2010, when 10 districts were affected in northern Uganda.
Despite the introduction of the vaccine in the routine immunization schedule, the national coverage has remained low at 29%. This low level of population immunity poses a high risk of yellow fever outbreaks. To rapidly raise the population immunity against yellow fever, the country planned to conduct phased implementation of yellow fever preventive mass vaccination campaigns (PMVC) in 2023 and 2024 with a goal of eliminating yellow fever epidemics in Uganda by 2026 and objectives of vaccinating at least 90% of eligible population aged 1 to 60 years, effectively engage all stake holders and mobilize all communities for yellow fever vaccination, effectively communicate and engage communities on risk of yellow fever, how to reduce risk plus strengthening the surveillance for yellow fever and finally to strengthen routine immunization by improving uptake and reporting for new vaccines with emphasis to the life course approach.
Amidst all challenges, a population of 14437098 (90%) of the eligible population including the refugees was targeted in around 6 day campaign rolled from second to eighth of April 2024. A combination of fixed and mobile post vaccination strategies were used to implement the program in health facilities, schools and selected villages. The phase one of vaccination was conducted in June, 2023 in 51 districts in 6 regions of Kigezi, Tooro, Lango, Westnile, Acholi and Bunyoro. Yellow fever virus still represents a major threat in low resource countries in both South America and Africa Uganda inclusive despite the presence of an effective vaccine. Yellow fever outbreaks are not only due to insufficient vaccine coverage for insufficient vaccine supply, but also to the increase in people without history of vaccination living in endemic areas.
Globalization, continuous population growth, urbanization associated with inadequate public health infrastructures and climate changes constitute important promoting factors for the spread of this virus to tropical and subtropical areas in mosquito – infested regions capable of spreading the disease Before 20th century, yellow fever devastated communities including the developed world of America and Europe, in the middle of this century, an effective vaccine which is effective in preventing yellow fever was developed. Anyone travelling to an area where yellow fever is known to exist should find out about having the vaccine at least 10 to 14 days before departure, though some countries may insist on a valid immunization certificate before a person can be allowed in.
A single vaccine dose provides at least 10 years protection and a person may be protected for life. Minor side effects secondary to vaccination are reported which are though manageable including headache, low grade fever, muscle pain, tiredness, soreness at the site of injection. The vaccine is deemed safe for individuals aged 9 months to 60 years with those below 9 months, pregnant women unless the risk is unavoidable, breastfeeding mothers, people who are allergic to eggs and people with weakened immune systems also unless the risk of yellow fever is unavoidable including those with HIV or people receiving chemotherapy and radiotherapy are all not eligible for vaccination.
To reduce exposure to mosquitoes, experts advise, where possible to avoid outdoor activities during dawn, dusk and early evening, when mosquitoes are most prevalent. Covering the skin as much as possible by wearing long sleeved shirts and long pants plus use of mosquito repellant agents are also among the preventive strategies. Without vaccination, yellow fever disease will always remain a threat to our existence.
