Estimating case fatality risk of severe Yellow Fever cases: systematic literature review and meta-analysis

BACKGROUND

Case fatality risk (CFR), commonly referred to as a case fatality ratio or rate, represents the probability of a disease case being fatal. It is often estimated for various diseases through analysis of surveillance data, case reports, or record examinations. Reported CFR values for Yellow Fever vary, offering wide ranges. Estimates have not been found through systematic literature review, which has been used to estimate CFR of other diseases. This study aims to estimate the case fatality risk of severe Yellow Fever cases through a systematic literature review and meta-analysis.

METHODS

A search strategy was implemented in PubMed and Ovid Medline in June 2019 and updated in March 2021, seeking reported severe case counts, defined by fever and either jaundice or hemorrhaging, and the number of those that were fatal. The searches yielded 1,133 studies, and title/abstract review followed by full text review produced 14 articles reporting 32 proportions of fatal cases, 26 of which were suitable for meta-analysis. Four studies with one proportion each were added to include clinical case data from the recent outbreak in Brazil. Data were analyzed through an intercept-only logistic meta-regression with random effects for study. Values of the I2 statistic measured heterogeneity across studies.

RESULTS

The estimated CFR was 39 % (95 % CI: 31 %, 47 %). Stratifying by continent showed that South America observed a higher CFR than Africa, though fewer studies reported estimates for South America. No difference was seen between studies reporting surveillance data and studies investigating outbreaks, and no difference was seen among different symptom definitions. High heterogeneity was observed across studies.

CONCLUSIONS

Approximately 39 % of severe Yellow Fever cases are estimated to be fatal. This study provides the first systematic literature review to estimate the CFR of Yellow Fever, which can provide insight into outbreak preparedness and estimating underreporting.

Re-Emergence of Yellow Fever in Brazil during 2016-2019: Challenges, Lessons Learned, and Perspectives

Yellow fever (YF) is a re-emerging viral zoonosis caused by the Yellow Fever virus (YFV), affecting humans and non-human primates (NHP). YF is endemic in South America and Africa, being considered a burden for public health worldwide despite the availability of an effective vaccine. Acute infectious disease can progress to severe hemorrhagic conditions and has high rates of morbidity and mortality in endemic countries. In 2016, Brazil started experiencing one of the most significant YF epidemics in its history, with lots of deaths being reported in regions that were previously considered free of the disease. Here, we reviewed the historical aspects of YF in Brazil, the epidemiology of the disease, the challenges that remain in Brazil’s public health context, the main lessons learned from the recent outbreaks, and our perspective for facing future YF epidemics.

Timeliness of yellow fever surveillance, Central African Republic

During January 2007-July 2012, a total of 3,220 suspected yellow fever cases were reported in the Central African Republic; 55 were confirmed and 11 case-patients died. Mean delay between onset of jaundice and case confirmation was 16.6 days. Delay between disease onset and blood collection could be reduced by increasing awareness of the population.

Yellow Fever in Africa: estimating the burden of disease and impact of mass vaccination from outbreak and serological data

BACKGROUND

Yellow fever is a vector-borne disease affecting humans and non-human primates in tropical areas of Africa and South America. While eradication is not feasible due to the wildlife reservoir, large scale vaccination activities in Africa during the 1940s to 1960s reduced yellow fever incidence for several decades. However, after a period of low vaccination coverage, yellow fever has resurged in the continent. Since 2006 there has been substantial funding for large preventive mass vaccination campaigns in the most affected countries in Africa to curb the rising burden of disease and control future outbreaks. Contemporary estimates of the yellow fever disease burden are lacking, and the present study aimed to update the previous estimates on the basis of more recent yellow fever occurrence data and improved estimation methods.

METHODS AND FINDINGS

Generalised linear regression models were fitted to a dataset of the locations of yellow fever outbreaks within the last 25 years to estimate the probability of outbreak reports across the endemic zone. Environmental variables and indicators for the surveillance quality in the affected countries were used as covariates. By comparing probabilities of outbreak reports estimated in the regression with the force of infection estimated for a limited set of locations for which serological surveys were available, the detection probability per case and the force of infection were estimated across the endemic zone. The yellow fever burden in Africa was estimated for the year 2013 as 130,000 (95% CI 51,000-380,000) cases with fever and jaundice or haemorrhage including 78,000 (95% CI 19,000-180,000) deaths, taking into account the current level of vaccination coverage. The impact of the recent mass vaccination campaigns was assessed by evaluating the difference between the estimates obtained for the current vaccination coverage and for a hypothetical scenario excluding these vaccination campaigns. Vaccination campaigns were estimated to have reduced the number of cases and deaths by 27% (95% CI 22%-31%) across the region, achieving up to an 82% reduction in countries targeted by these campaigns. A limitation of our study is the high level of uncertainty in our estimates arising from the sparseness of data available from both surveillance and serological surveys.

CONCLUSIONS

With the estimation method presented here, spatial estimates of transmission intensity can be combined with vaccination coverage levels to evaluate the impact of past or proposed vaccination campaigns, thereby helping to allocate resources efficiently for yellow fever control. This method has been used by the Global Alliance for Vaccines and Immunization (GAVI Alliance) to estimate the potential impact of future vaccination campaigns.

Dynamic viral dissemination in mice infected with yellow fever virus strain 17D

Arboviruses such as yellow fever virus (YFV) are transmitted between arthropod vectors and vertebrate hosts. While barriers limiting arbovirus population diversity have been observed in mosquitoes, whether barriers exist in vertebrate hosts is unclear. To investigate whether arboviruses encounter bottlenecks during dissemination in the vertebrate host, we infected immunocompetent mice and immune-deficient mice lacking alpha/beta interferon (IFN-α/β) receptors (IFNAR⁻/⁻ mice) with a pool of genetically marked viruses to evaluate dissemination and host barriers. We used the live attenuated vaccine strain YFV-17D, which contains many mutations compared with virulent YFV. We found that intramuscularly injected immunocompetent mice did not develop disease and that viral dissemination was restricted. Conversely, 32% of intramuscularly injected IFNAR⁻/⁻ mice developed disease. By following the genetically marked viruses over time, we found broad dissemination in IFNAR⁻/⁻ mice followed by clearance. The patterns of viral dissemination were similar in mice that developed disease and mice that did not develop disease. Unlike our previous results with poliovirus, these results suggest that YFV-17D encounters no major barriers during dissemination within a vertebrate host in the absence of the type I IFN response.

[Trends in yellow fever mortality in Colombia, 1998-2009]

INTRODUCTION

Yellow fever is a neglected tropical disease, thus, knowing the trends in mortality from this disease in Colombia is an important source of information for decision making and identifying public health interventions.

OBJECTIVE

To analyze trends in yellow fever mortality in Colombia during the 1998-2009 period and the differences in the morbidity and mortality information sources for the country, which affect indicators such as the lethality one.

MATERIALS AND METHODS

This is a descriptive study of deaths by yellow fever according to the Departamento Administrativo Nacional de Estadística and the incidence of the disease according to the Instituto Nacional de Salud . We used secondary sources of information in the calculation of proportions of socio-demographic characteristics of the deceased and epidemiological measures of lethality, incidence and mortality from yellow fever by department of residence of the deceased.

RESULTS

Yellow fever deaths occur primarily in men of working age residing in scattered rural areas, who were members of the regimen vinculado, and who were living in the eastern, southeastern, northern and central zones in the country. We observed inconsistencies in the reports that affect the comparative analysis.

CONCLUSION

The inhabitants of the departments located in national territories and Norte de Santander have an increased risk of illness and death from yellow fever, but this information could be underestimated, according to the source of information used for its calculation.

Immune correlates of protection against yellow fever determined by passive immunization and challenge in the hamster model

Live, attenuated yellow fever (YF) 17D vaccine is highly efficacious but causes rare, serious adverse events resulting from active replication in the host and direct viral injury to vital organs. We recently reported development of a potentially safer β-propiolactone-inactivated whole virion YF vaccine (XRX-001), which was highly immunogenic in mice, hamsters, monkeys, and humans [10,11]. To characterize the protective efficacy of neutralizing antibodies stimulated by the inactivated vaccine, graded doses of serum from hamsters immunized with inactivated XRX-001 or live 17D vaccine were transferred to hamsters by the intraperitoneal (IP) route 24h prior to virulent, viscerotropic YF virus challenge. Neutralizing antibody (PRNT(50)) titers were determined in the sera of treated animals 4h before challenge and 4 and 21 days after challenge. Neutralizing antibodies were shown to mediate protection. Animals having 50% plaque reduction neutralization test (PRNT(50)) titers of ≥40 4h before challenge were completely protected from disease as evidenced by viremia, liver enzyme elevation, and protection against illness (weight change) and death. Passive titers of 10-20 were partially protective. Immunization with the XRX-001 vaccine stimulated YF neutralizing antibodies that were equally effective (based on dose response) as antibodies stimulated by live 17D vaccine. The results will be useful in defining the level of seroprotection in clinical studies of new yellow fever vaccines.

Epidemic yellow fever in Borno State of Nigeria: characterisation of hospitalised patients

BACKGROUND

In 1990, an outbreak of a febrile illness with high mortality was reported in border villages, later spreading to other areas of Borno State of Nigeria.

OBJECTIVE

To present a report of the investigation of that outbreak, with emphasis on the characterisation of hospitalised patients.

METHODS

Selected centres reporting cases of acute febrile illness during the months of August to December, 1990 were visited, to establish surveillance. Case investigation forms were used to obtain clinical and demographic data; and blood samples were obtained from patients for analyses. Only hospitalised patients with adequate clinical information from three centres were included in the analysis.

RESULTS

The outbreak, which involved five of the six health zones in the state, and spread into adjoining Gongola state and the Cameroun Republic, was caused by the yellow fever virus. Fever, central nervous system (CNS) involvement, jaundice and haemorrhage were the most common clinical manifestations of 102 hospitalised patients. Eighty -three (81%) of hospitalised patients died and most within two days of admission. CNS manifestations were more common in dying patients than in survivors.

CONCLUSION

The reasons for this rare outbreak of yellow fever in the dry Savannah belt of Borno State remain unclear. Improved surveillance and more effective prevention strategies are needed to avert the recurrence of such outbreaks.

Activity of T-1106 in a hamster model of yellow Fever virus infection

Yellow fever virus (YFV) causes 30,000 deaths worldwide, despite the availability of a vaccine. There are no approved antiviral therapies for the treatment of YFV disease in humans, and, therefore, these studies were designed to investigate the anti-YFV properties of T-1106, a substituted pyrazine, in a hamster model of YFV disease. Intraperitoneal (i.p.) treatment with 100 mg/kg of body weight/day of T-1106 starting 4 h prior to virus inoculation and continuing twice daily through 7 days post-virus inoculation (dpi) resulted in significantly improved survival, alanine aminotransferase levels in the serum, weight gain, and mean day to death. Virus titer in the liver at 4 dpi was significantly reduced in treated animals, as determined by both quantitative real-time PCR and infectious cell culture assay. No toxicity (weight loss or mortality) was observed at a dose of 100 mg/kg/day in sham-infected control animals. The observed minimal effective dose of T-1106 was 32 mg/kg/day administered either by oral or i.p. treatment. Therapeutic treatment was effective in significantly improving survival when T-1106 was administered beginning as late as 4 days after virus challenge with twice-daily treatment for 8 days at a dose of 100 mg/kg/day. With favorable safety, bioavailability, and postviral challenge treatment efficacy, T-1106 was effective in the treatment of disease in hamsters infected with YFV and should be further studied for potential use as a therapy for human YFV disease.

Clinical and epidemiological characteristics of yellow fever in Brazil: analysis of reported cases 1998-2002

Yellow fever (YF), an arboviral infection of major public health importance in Brazil, is associated with high mortality and high epidemic potential. We analysed confirmed YF cases from the National Surveillance System from 1998-2002 and assessed risk factors for death among hospitalised patients. Variables assessed included age, gender, clinical signs and laboratory findings. A logistic regression model was used to identify independent predictors of death among hospitalised patients. From 1998-2002, among 2117 suspected YF cases reported to Brazil's Ministry of Health, 251 (11.9%) had confirmed YF, of whom 217 (86.5%) were hospitalised and the case fatality rate was 44.2%. Factors associated with higher mortality in univariate analysis included male gender (relative risk (RR) 1.96, 95% CI 1.17-2.28), age >40 years (RR 2.61, 95% CI 1.25-5.45), jaundice (RR 2.66, 95% CI 2.12-3.35), serum aspartate aminotransferase (AST) >1200 IU/l (RR 1.84, 95% CI 1.23-2.74), alanine aminotransferase >1500 IU/l (RR 2.09, 95% CI 1.38-3.17), total bilirubin >7.0mg/dl (RR 2.33, 95% CI 1.44-3.78), direct bilirubin >5.0mg/dl (RR 2.29, 95% CI 1.33-3.94) and blood urea nitrogen >100mg/dl (RR 5.77, 95% CI 1.43-23.22). In multivariate analysis, elevated AST and jaundice remained independently associated with higher mortality. These findings suggest that selected clinical and laboratory indicators may help clinicians recognise potentially fatal cases of YF.

Yellow fever vaccination: How much is enough?

In recent years, a growing number of serious adverse events (including deaths) associated with the yellow fever (YF) vaccine has been reported. If YF vaccination were incorporated in routine programs, administered to children, the risk of deaths from this vaccine would be minimized provided that mortality of children vaccinated below 1 year were negligible. However, in affected areas the vaccine is administered to all age groups. This poses a dilemma to public health authorities - what proportion of a population subject to low risk of YF outbreaks should be vaccinated in order to minimize the total number of serious adverse events (including deaths) due both to natural infection and vaccination? In other words, how much vaccination is safe? Our results suggest that, depending on the age-specific rates of developing vaccine-induced serious adverse events and the risk of yellow fever outbreaks, the optimum proportion to vaccinate may be lower than the proportion that would prevent an epidemics or even be zero. We also show that the vaccine should not be applied to individuals older than 60 years of age because the risk of serious adverse events (including deaths) is higher for that age class. Our work is instrumental to the discussion on the optimum strategy to vaccinate affected populations against yellow fever. Therefore, the aim of this work is to estimate the optimum proportion to vaccinate against YF taking into account the risks of serious adverse events associated with both the vaccine and natural infection.

[Yellow fever--a tropical danger]

Yellow fever is an acute life-threatening viral infection. In the case of the urban type, the virus is transmitted from one human to another by the Aedes mosquito vector. In particular travelers to Africa from non-endemic areas run a high risk of clinical disease in case of infection. For prophylaxis, a reliable live virus vaccine is available.

Age-related risk of adverse events following yellow fever vaccination in Australia

Reports of six deaths internationally, including one from Australia, plus other cases of severe systemic adverse events following yellow fever (YF) vaccination have raised concern about the safety of YF vaccine, particularly among older vaccinees. We investigated the age-related reporting rates of adverse events following YF vaccination reported to the Australian Adverse Drug Reactions Advisory Committee for the period 1993 to 2002. The reporting rate of systemic adverse events leading to hospitalisation or death was significantly higher among vaccinees aged > or = 65 years [reporting rate ratio (RRR) 8.95, 95% confidence interval (CI) 1.49-53.5] or > or = 45 years (RRR 5.30, 95% CI 1.33-21.2) compared with younger YF vaccinees. The higher reporting rates among older vaccinees are similar to those identified in the United States of America. The data highlight the importance of assessing the destination-specific risk, especially for older travellers to yellow fever endemic areas, and careful monitoring of those who are vaccinated.

A Belgian traveler who acquired yellow fever in the Gambia

A 47-year-old Belgian woman acquired yellow fever during a 1-week vacation in The Gambia; she had never been vaccinated against yellow fever. She died of massive gastrointestinal bleeding 7 days after the onset of the first symptoms. This dramatic case demonstrates that it is important for persons to be vaccinated against yellow fever before they travel to countries where yellow fever is endemic, even if the country, like The Gambia, does not require travelers to be vaccinated.

Shortage of vaccines during a yellow fever outbreak in Guinea

A yellow fever epidemic erupted in Guinea in September, 2000. From Sept 4, 2000, to Jan 7, 2001, 688 instances of the disease and 225 deaths were reported. The diagnosis was laboratory confirmed by IgM detection in more than 40 patients. A mass vaccination campaign was limited by insufficient international stocks. After the epidemic in Guinea, the International Coordinating Group on Vaccine Provision for Epidemic Meningitis Control decided that 2 million doses of 17D yellow fever vaccine, being stored as part of a UNICEF stockpile, should be used only in response to outbreaks.

[Yellow fever epidemiology in Brazil]

We have carried out a meticulous time-space-analysis of the incidence of yellow fever in humans in Brazil from 1954 to 1972 and especially from 1973 to 1999. This study has added to our knowledge of the epidemiology of yellow fever and enabled us to redefine epidemiological zones and determine their geographical limits. The endemic area is located within the Amazon basin; here cases are scattered and generally limited in number. However, there are also "foci of endemic emergence" within this area, where cases are less rare, although occurrence remains irregular. The epidemic area is for the most part situated outside the Amazon basin, to the north east and particularly to the south. It has been divided into two parts according to whether the occurrence of yellow fever is cyclic or sporadic. The epidemics, which are all sylvatic, follow either a circular path (in the forest area) or a linear path (in forest-galleries of the savannah area). The study of the development of the 3 main epidemics (1972-74; 1979-82; 1986-92) in the cyclic emergence area showed that, on each occasion, the yellow fever virus appeared at a particularly active outbreak site located in the "serra dos Carajás", and from there, it followed the courses of the Tocantins and Araguaia rivers upstream, moving southwards during the "pre-epidemic phase" which may be visible due to the occurrence of a few cases, or may remain invisible. Subsequently the virus reached the emergence area, where it appeared in the form of epidemics. In this zone, it also followed privileged south-western pathways, moving from one hydraulic basin to another along the upstream courses of the rivers. Almost exactly the same pathways have been identified for each of the 3 epidemics studied. The distances travelled by the virus over a period of one year--when it goes rapidly--can reach several hundred kilometers. On the other hand, it may be stationary for a period of one or two consecutive years, occasionally three, remaining present in the area but infecting humans only rarely if at all. The virus occasionally leaves the cyclic emergence area and appears in the sporadic emergence area to the east, in the states of Bahia, Minas Gerais and São Paulo, and, as a consequence, moving onto other hydraulic basins. The small river basins in Maranhão and NorthWest states, as well as in the northern part of the state of Roraima also form part of the sporadic emergence area. The epidemics that occur here are directly linked to the endemic area and are only preceded by sometimes indiscernible epizootics and can consequently not be foreseen. Again the virus appears to use privileged pathways to reach the sporadic emergence areas where human and monkey populations are generally only partially immunised against yellow fever and where contact with mosquitoes is intense despite the fact it is limited in space and time, being restricted to the often narrow strip of trees along the water courses. Other routes used by the virus may be the Madeira, Xingu and Tapajós rivers, the scene of outbreaks observed in the state of Rondônia and in the north of Mato Grosso, where ongoing environmental changes are likely to result in an increasing number of outbreaks in the coming years. Since the discovery of the sylvatic cycle of yellow fever in 1933, not only the extent of the epidemiological areas has changed, but also their limits. Ecological modifications that are currently taking place in the Amazon basin, which is an endemic reservoir of the virus, will inevitably facilitate an increase the contact between humans and vectors. While more and more urban areas harbour populations of Aedes aegypti, the domestic and urban vector of yellow fever, it is particularly important to try to protect human populations living in emergence zones and epidemic areas and thus to prevent the arrival of the virus in towns via humans with viremia--in other words the much feared urbanisation of yellow fever in Brazil.

An epidemic of sylvatic yellow fever in the southeast region of Maranhao State, Brazil, 1993-1994: epidemiologic and entomologic findings

Yellow fever virus transmission was very active in Maranhao State in Brazil in 1993 and 1994. An investigation was carried out to evaluate the magnitude of the epidemic. In 1993, a total of 932 people was examined for yellow fever from Maranhao: 70 were positive serologically, histopathologically, and/or by virus isolation, and another four cases were diagnosed clinically and epidemiologically. In Mirador (17,565 inhabitants), the incidence was 3.5 per 1,000 people (case fatality rate [number of deaths/number of cases diagnosed] = 16.4%), while in a rural yellow fever risk area (14,659 inhabitants), the incidence was 4.2 and the case-fatality rate was 16.1% (10 of 62). A total of 45.2% (28 of 62) asymptomatic infections were registered. In 1994, 49 serum samples were obtained and 16 cases were confirmed (two by virus isolation, two by seroconversion, and 12 by serology). No fatal cases were reported. In 1993, 936 potential yellow fever vectors were captured in Mirador and a single strain was isolated from a pool of Haemagogus janthinomys (infection rate = 0.16%). In 1994, 16 strains were isolated from 1,318 Hg. janthinomys (infection rate = 1.34%) and one Sabethes chloropterus (infection rate = 1.67%). Our results suggest that this was the most extensive outbreak of yellow fever in the last 20 years in Brazil. It is also clear that the lack of vaccination was the principal reason for the epidemic, which occurred between April and June, during the rainy season, a period in which the mosquito population in the forest increases.

Genetic studies of flavivirus resistance in inbred strains derived from wild mice: evidence for a new resistance allele at the flavivirus resistance locus (Flv)

Studies of genetic resistance to flavivirus infection in laboratory mice have led to the development of a single model in which resistance is conferred by an autosomal dominant gene designated Flvr. Because of evidence suggesting that wild mice carry virus resistance genes which are not present in laboratory mice, we compared flavivirus resistance in the inbred strains CASA/Rk, CAST/Ei, and MOLD/Rk, which are derived directly from wild mice, and the congenic strains C3H/RV (Flvr/Flvr) and C3H/HeJ (Flvs/Flvs). Resistance to the Murray Valley encephalitis virus strain OR2 and the 17D vaccine strain of yellow fever virus was assessed by determining the lethality of intracerebral infection and by measuring virus replication in the brain. The resistance of the CASA/Rk and CAST/Ei strains resembled the resistance of C3H/RV mice, whereas the resistance of the MOLD/Rk strain was intermediate between those of C3H/RV and C3H/HeJ mice. Genetic analyses showed that resistance in both the CASA/Rk and MOLD/Rk strains is conferred by single autosomal dominant alleles at the Flv locus. Our data indicate that flavivirus resistance in the CASA/Rk strain is due to a gene which is similar or identical to Flvr, whereas resistance in the MOLD/Rk strain is due to a previously undescribed gene which we designate Flvmr to indicate minor resistance to flavivirus infection. Since genetic resistance to flaviviruses is rare in laboratory mice, the CASA/Rk and MOLD/Rk strains will be valuable for further investigation of this phenomenon.

Yellow fever epidemics and mortality in the United States, 1693-1905

Yellow fever epidemics struck the United States repeatedly in the 18th and 19th centuries. The disease was not indigenous; epidemics were imported by ship from the Caribbean. Prior to 1822, yellow fever attacked cities as far north as Boston, but after 1822 it was restricted to the south. Port cities were the primary targets, but the disease occasionally spread up the Mississippi River system in the 1800s. New Orleans, Mobile, Savannah, and Charleston were major targets; Memphis suffered terribly in 1878. Yellow fever epidemics caused terror, economic disruption, and some 100,000-150,000 deaths. Recent white immigrants to southern port cities were the most vulnerable; local whites and blacks enjoyed considerable resistance.

Mosquitoes, leeches and medicine in Charleston, South Carolina (1670-1861)

Charleston, South Carolina, USA has a rich history extending over 300 years. The plantations of the Carolina Low Country produced rice, indigo and cotton. Mosquitoes, which abounded in the wetlands, transmitted diseases such as Yellow Fever and epidemics occurred almost every year. African slaves, however, had a natural immunity to the disease and, together with the climate and the nature of the work, this shaped the economic and social system of the area. Physicians offered all sorts of cures to deal with the medical problems of the area and apothecaries also sold many nostrums including leeches. Newspaper advertisements reveal that these bloodsuckers were extensively used for many decades.

Letter from Brasilia. Yellow fever

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Safety and efficacy of yellow fever vaccine in children less thanone-year-old

In a clinical trial of stabilized yellow fever vaccine from Institute Pasteur in 77 children aged seven to eight months, fever was the most significant immediate and delayed side effect. Fever occurred in 12 (15.6%) children with in 48 hours of vaccination while it occurred in 10 (12.9%) children within ten days of vaccination. Other recorded side effects were pain at innoculation site in four (5.2%) children and vomiting in one (1.3%) child. Temperature recorded in 20 of the 22 febrile episodes ranged from 37.8 degrees C to 38.6 degrees C. One of the two patients who had temperatures of 39 degrees C and above had malaria parasites in her blood film. All episodes of fever except one responded to antipyretic. There was no episode of febrile convulsion and no feature suggestive of encephalitis. Of the 20 children who had neutralization test carried out against yellow fever virus six weeks after vaccination, the test was positive in post vaccination sera of 12 (60%) children whose pre-vaccination sera were negative. Two others showed evidence of partial protection. Although the seroconversion rate of 60% is less than reported in adults and older children, the result of this study shows that yellow fever vaccine is safe and fairly effective in infants. It is our suggestion that if a larger trial confirms our findings, the vaccine may be incorporated into the expanded programme on immunization (EPI) to be given at the age of seven months after completion of diptheria, tetanus, pertussis and poliomyelitis vaccinations and before measles vaccination is due.

Severe yellow fever with 23-day survival

In a severe case of jungle yellow fever, acquired in the Brazilian mid-west region, clinical and laboratory evidence of hepatic and renal failure, cardiovascular disturbance, coma and bleeding disorder developed. The patient was treated in an intensive care unit and hemodialysis was performed. In spite of severe liver dysfunction, most biochemical parameters returned to normal values but the patient finally died of respiratory failure on the 23rd day, due to secondary bacterial pneumonia. A post-mortem liver biopsy showed regeneration of normal liver architecture. Without the bacterial respiratory complication, the patient probably would have survived.

Epidemic yellow fever in eastern Nigeria, 1986

An epidemic of yellow fever occurred in the eastern part of Nigeria during the second half of 1986. Oju, in Benue State, was the most heavily affected region, but yellow fever also occurred in surrounding areas, particularly Ogoja, in Cross River State. In Oju, the mean attack and mortality rates were 4.9% and 2.8%, respectively. Sex and age specific rates were highest in males and in the 20-29 yr age group. The overall case fatality rate was approximately 50%. Diagnosis was confirmed by IgM capture enzyme-linked immunosorbent assay (ELISA) and complement fixation (CF) tests. Entomological investigations implicated Aedes africanus as the epidemic vector. Oju alone probably had about 9800 cases of yellow fever with jaundice, and some 5600 deaths. Outbreaks of this nature could be prevented by inclusion of yellow fever in the Expanded Programme on Immunisation, in areas subject to recurrent epidemics.

Genetic control of survival in epidemics

Descendants of Dutch colonists, who emigrated to Surinam in the last century and survived epidemics of typhoid and yellow fever with a total mortality of about 60%, were tested for twenty-six polymorphisms. The gene frequencies were compared with those of a large Dutch control sample. An analysis of drift indicated that the variations in gene frequencies observed for C3, Gm, HLA-B, and GLO were unlikely to be due to drift. Therefore these data might indicate selection through genetic control of survival in these epidemics.

Virulence and pathogenesis of yellow fever virus serially passaged in cell culture

Viscerotropic virulence of the Asibi strain of yellow fever virus (YFV) for monkeys has been known to be lost after serial passage in HeLa cell monolayers. This phenomenon was investigated in several other mammalian and insect tissue cell lines. Assay in monkeys of original seed virus and of virus after 7 and 11 passages in a porcine kidney cell line (PK) indicated essentially equal infectivity and mortality. Moreover, monkeys receiving the passaged virus exhibited more rapid onset of disease and death than animals infected with original seed virus. Histological changes in animals inoculated with passaged virus were identical to those in animals receiving the seed virus. Virus from later passages in PK cells was also lethal for approximately 50% of the monkeys; however, evidence for progressive attenuation was seen in these preparations. Similar results were obtained with a mosquito (Aedes aegypti) cell line. In contrast to results obtained in PK and mosquito cells, YFV became essentially avirulent (nonlethal and less infective) for monkeys after only seven passages in HeLa cell cultures.