Yellow fever virus in Haemagogus leucocelaenus and Aedes serratus mosquitoes, southern Brazil, 2008

Yellow fever virus (YFV) was isolated from Haemagogus leucocelaenus mosquitoes during an epizootic in 2001 in the Rio Grande do Sul State in southern Brazil. In October 2008, a yellow fever outbreak was reported there, with nonhuman primate deaths and human cases. This latter outbreak led to intensification of surveillance measures for early detection of YFV and support for vaccination programs. We report entomologic surveillance in 2 municipalities that recorded nonhuman primate deaths. Mosquitoes were collected at ground level, identified, and processed for virus isolation and molecular analyses. Eight YFV strains were isolated (7 from pools of Hg. leucocelaenus mosquitoes and another from Aedes serratus mosquitoes); 6 were sequenced, and they grouped in the YFV South American genotype I. The results confirmed the role of Hg. leucocelaenus mosquitoes as the main YFV vector in southern Brazil and suggest that Ae. serratus mosquitoes may have a potential role as a secondary vector.     

Entomological investigation of a sylvatic yellow fever area in São Paulo State, Brazil

Following reports of two autochthonous cases of sylvatic yellow fever in the State of S?o Paulo, Brazil, in 2000, entomological surveys were conducted with the objective of verifying the occurrence of vector species in forest environments close to or associated with riparian areas located in the western and northwestern regions of the State. Culicidae were captured in 39 sites distributed in four regions. Haemagogus leucocelaenus and Aedes albopictus were the most abundant species and were captured in all the regions studied. H. leucocelaenus was the most abundant species in the municipalities of Santa Albertina and Ouroeste, where the two cases of sylvatic yellow fever had been reported. Mosquitoes from the janthinomys/capricornii group were only found at eight sites in the S?o José do Rio Preto region, while Sabethes chloropterus was found at one site in Ribeir?o Preto. H. leucocelaenus showed its capacity to adapt to a secondary and degraded environment. Our results indicate a wide receptive area for yellow fever transmission in the State of S?o Paulo, with particular emphasis on the possibility of H. leucocelaenus being involved in the maintenance of this sylvatic focus of the disease.     

Scanning electron microscopy of eggs of Haemagogus leucocelaenus (Diptera: Culicidae)

OBJECTIVE: To observe morphological details of the eggs of Haemagogus (Conopostegus) leucocelaenus, seen for the first time via scanning electron microscopy (SEM), with morphometric analysis of the main structures. METHODS: Eggs of Hg. leucocelaenus were obtained from females captured in the Biological Reserve of Tinguá, State of Rio de Janeiro. Some of the eggs were kept for hatching and others underwent processing for scanning electron microscopy studies. Three eggs were submitted to morphometric analysis. The material was fixed in 2.5% glutaraldehyde and postfixed in 1% osmium tetroxide, both in 0.1M, pH 7.2 sodium cacodylate buffer, then dehydrated in ethanol and dried using the critical point method. This was then set up on metallic supports, covered with gold and observed using the Jeol 5310 scanning electron microscope. Measurements were made with the aid of the Semafore analysis software coupled to the electron microscope. RESULTS: The eggs presented elliptical outlines of approximately 574 mm in length and 169 mm in width, with an egg index (l/w ratio) of 3.39 mm. The exochorion was extremely regular and had ornamentation that was usually hexagonal but sometimes pentagonal. Tubercles were observed on the chorionic cells, symmetrically arranged in relation to the longitudinal axis. Inside the cells, there were smaller, individualized tubercles, some arranged peripherally and others grouped to a greater or lesser extent in the center. The surface of the chorionic reticulum did not present rugosity. The micropylar apparatus was formed by a prominent continuous collar of 8.32 mm in thickness, with a slightly irregular surface. The micropylar disk was very evident, and was continuous with the collar. The micropyle was seen at the center of this disk, measuring 1.6 mm and with a micropylar apparatus of 27.3 mm in diameter. CONCLUSIONS: The ornamentation of the exochorion presents differences in relation to the tubercles of chorionic cells and the external chorionic reticulum between the eggs of Hg. Leucocelaenus, in comparison with the eggs of Hg. janthinomys and Hg. equinus, and also in relation to those of Aedes aegypti, Ae. albopictus and Ae. bahamensis. In various aspects, the eggs of Hg. leucocelaenus have more resemblance to those of Hg. Equinus than those of Hg. janthinomys, with greater differences presented in relation to the eggs of Hg. spegazzinii and Hg. lucifer.     

Production of yellow fever 17DD vaccine virus in primary culture of chicken embryo fibroblasts: yields, thermo and genetic stability, attenuation and immunogenicity

While a good vaccine against yellow fever (YF) virus has been available for decades, the basic technology for the production of YF vaccine in chicken embryos has remained substantially unchanged since the 1940s. Here we describe the highly efficient and economic production of the 17DD strain of YF virus in chicken embryo fibroblast (CEF) cell cultures with viral titers ranging from 6.3 to 6.7 log10PFU/mL. Thermostability of two different formulations (5 and 50-dose vials) of the CEF vaccine virus was found to be as high as the current vaccines retaining the minimal titer required for YF 17D vaccines. The production passage in CEF did not lead to the selection of genetic variants as shown by nucleotide sequence analyses of the CEF-derived vaccine lots or the sequence of viruses recovered from monkeys experimentally inoculated with the CEF virus. YF 17DD virus produced in CEF was also indistinguishable from its seed lot virus parent in terms of plaque size and immunogenicity in mice and monkeys. Comparison of the CEF virus and the seed lot virus made in chicken embryo in the internationally accepted monkey neurovirulence test (MNVT) revealed a higher clinical score for the former. The differences in central nervous system (CNS) histological scores for monkeys inoculated with the chicken embryo and experimental CEF vaccines were at the borderline level of statistical significance. These data warrant further studies on the monkey attenuation of other batches of CEF-derived vaccines.     

A yellow fever epizootic in Zika Forest, Uganda, during 1972: Part 2: Monkey serology.

During the 1972 yellow fever epizootic in Zika Forest, Uganda, sera from 21 monkeys shot in a number of forests around the Entebbe area were tested for the presence of a number of arbovirus antibodies. All sera were tested for antibodies against Chikungunya (CHIK), O'nyong-nyong (ONN), Zika, yellow fever (YF) West Nile (WN) and Wesselsbron (WESS) by the haemagglutination-inhibition (HI) test. Because of the crossreaction within the flaviviruses (group B arboviruses) mouse protection test (PT) was also carried out on the sera against YF, WESS and Zika viruses. Serological studies carried out on monkey sera from different parts of Uganda, including the Entebbe area, during 1968 gave results which reflected a surprisingly low rate of YF immune monkeys (3%) throughout the country compared with the rate of over 40% immune monkeys obtained by Haddow et al. in 1951. 40% of the monkey sera collected during 1972 were immune to YF by the PT. Since no YF virus had been isolated between 1968 and 1972 the results indicate strongly that the monkeys in the Entebbe area were involved in the epizootic of 1972. No sick or dead monkeys were found in all the forests checked around Entebbe area during the epizootic. This indicates that the animal-to-animal cycle of the equatorial African forests involved the mild endemic infection characteristic of a virus in its natural habitat and infecting its natural host.     

Vector competence of Brazilian Aedes aegypti and Ae. albopictus for a Brazilian yellow fever virus isolate

Because the potential urban yellow fever (YF) mosquito vectors Aedes aegypti and Ae. albopictus are at historical highs in Brazil, both in terms of density and geographical range, we assessed the risk of an urban YF epidemic in Brazil. We evaluated and confirmed in a laboratory setting the vector competence of Brazilian Ae. aegypti for a currently circulating strain of YF virus, and investigated the potential for Brazilian Ae. albopictus to transmit YF.     

Examination of the envelope glycoprotein of yellow fever vaccine viruses with monoclonal antibodies

Two panels of envelope glycoprotein reactive monoclonal antibodies (mAbs) were prepared against yellow fever (YF) 17D vaccine viruses. Five mAbs were prepared against the World Health Organization 17D-204 avian leukosis virus-free secondary seed virus and eight mAbs against 17DD vaccine manufactured in Brazil. The majority of these mAbs were type-specific and displayed differing reactions in neutralization tests. One, B14, would only neutralize YF vaccine virus grown in invertebrate cells. Others would differentiate 17D-204 and 17DD vaccines, from different manufacturers, in neutralization tests when the viruses were grown in vertebrate cells. The data indicate that heterogeneity exists between the epitopes that elicit neutralizing antibody on YF vaccine from different manufacturers.     

Yellow fever and Zika virus epizootics and enzootics in Uganda

Data of monkey serology are presented which, together with past evidence, support the view that yellow fever (YF) virus circulates in its primary sylvan host populations, i.e., forest monkeys, in an enzootic state in Bwamba County in western Uganda but as series of epizootics in the forest-savanna mosaic zone of central Uganda. Evidence of an epizootic of Zika virus at the Zika Forest near Entebbe is described which occurred in two episodes, the first (in 1969) apparently following the build-up of non-immune monkey populations since a previous epizootic of 1962-63 and the second (in 1970) when Aedes africanus biting densities rose. This was followed only 18 months later by an intensive epizootic of YF virus, contradictory to the hypothesis that Zika virus alone would suppress subsequent epizootics of YF virus in nature, at least when redtail monkeys are involved. Conclusions are finally reviewed in the light of more recent evidence of transovarial flavivirus transmission in mosquitoes, pointing out that phlebotomine sandflies also require fresh attention.     

Mosquito (Diptera,Culicidae) ecology in the Iguaçu National Park,Brazil: 1 Habitat distribution

A study of the mosquito fauna in the Igua?u National Park focused on population behavior in four biotopes with different types of plant cover inside the Park. Systematic bimonthly diurnal and nocturnal human bait and Shannon trap captures were conducted in both forest and domiciliary environments over the course of 24 months. A total of 20,273 adult mosquito specimens belonging to 44 species were collected: Ochlerotatus serratus (10.3%), Haemagogus leucocelaenus (9.7%), Mansonia titillans (9.6%), and Chagasia fajardoi (8.8%) were the most frequently captured mosquitoes. Anopheles cruzii, Runchomyia theobaldi, Wyeomyia aporonoma, and Wy. confusa were captured almost exclusively in well-preserved areas with dense forest cover. Culex nigripalpus, Oc. pennai, Oc. serratus, Sabethes purpureus, and Sa. albiprivus were captured in three essentially sylvatic biotopes. Species captured in the forest areas around a dam were: An. albitarsis s.l., An. galvaoi, An. evansae, An. fluminensis, Coquillettidia venezuelensis, Cq. juxtamansonia, Wy. quasilongirostris, and Onirion personatum, Ch. fajardoi, Cq. fasciolata, Cq nitens, and Ma. titillans were the most frequently captured species in a residential area.     

Absence of YF-neutralizing antibodies in vulnerable populations of Brazil: A warning for epidemiological surveillance and the potential risks for future outbreaks

Yellow Fever (YF) is an acute febrile illness caused by yellow fever virus (YFV), a mosquito-borne flavivirus transmitted to humans and non-human primates. In Brazil, YF is a public health threat and may cause recurrent epidemics, even with the availability of a vaccine. We evaluated the sero-status for YFV in 581 individuals living in a risk area for YF in Brazil. The area presents history of cases and is located in the southeast region of country where outbreaks of YF have been reported since 2016. Through, a PRNT assay, we found 25.8% of individuals lacking YF-neutralizing antibodies. Furthermore, neutralizing antibodies were not detected in 10 individuals with proven vaccination. Our findings reinforce the importance of surveillance systems and the need of an urgent intensification of immunization programs in regions with YFV circulation. Monitoring susceptible individuals that could act as potential disseminators for YFV in risk areas should also be considered.     

Risk assessment of yellow fever urbanization in Rio de Janeiro, Brazil

Yellow fever (YF), an arthropod-borne viral disease, occurs in regions of tropical America and Africa. Sylvatic YF is endemic in the north and west of Brazil. Urban YF, on the other hand, has not been reported in the country since 1942. However, the widespread presence of the YF urban vector in Brazil has lead to concern about the potential re-emergence of YF in urban centres. Here, we assess the risk of YF emergence in the city of Rio de Janeiro, Brazil, by estimating the probability of infected individuals arriving from YF-endemic areas, and the probability of infective individuals triggering an epidemic. We found that the risk of urban YF emergence may reach values as high as 29% during the epizootic periods but the precision of the estimate is low.     

Clinical proof of principle for ChimeriVax: recombinant live, attenuated vaccines against flavivirus infections.

ChimeriVax is a live, attenuated recombinant virus constructed from yellow fever (YF) 17D in which the envelope protein genes of YF 17D are replaced with the corresponding genes of another flavivirus. A ChimeriVax vaccine was developed against Japanese encephalitis (JE). A randomized, double-blind, outpatient study was conducted to compare the safety and immunogenicity of ChimeriVax-JE and YF 17D. Six YF immune and six non-immune adults were randomized to receive a single SC inoculation of ChimeriVax-JE (5log(10)PFU), ChimeriVax-JE (4log(10)PFU) or YF-VAX((R)) (5log(10)PFU). Mild, transient injection site reactions and flu-like symptoms were noted in all treatment groups, with no significant difference between the groups. Nearly all subjects inoculated with ChimeriVax-JE at both dose levels developed a transient, low-level viremia which was similar in magnitude and duration to that following YF-VAX). Neutralizing antibody seroconversion rates to ChimeriVax-JE was 100% in the high and low dose groups in both na?ve and YF immune subjects; seroconversion to wild-type JE strains was similar or lower than to the homologous (vaccine) virus. Mean neutralizing antibody responses were higher in the ChimeriVax-JE high dose groups (na?ve subjects LNI 1.55, PRNT(50) 254; YF immune subjects LNI 2.23, PRNT(50) 327) than in the low dose groups (na?ve subjects 1.38, PRNT(50) 128; YF immune subjects LNI 1.62, PRNT(50) 270). JE antibody levels were higher in YF immune than in na?ve subjects, dispelling concerns about anti-vector immunity. The safety and immunogenicity profile of ChimeriVax-JE vaccine appears to be similar to that of YF 17D. The new vaccine holds promise for prevention of JE in travelers and residents of endemic countries. The ChimeriVax technology platform is being exploited for development of new vaccines against dengue and West Nile.     

Is vaccinating monkeys against yellow fever the ultimate solution for the Brazilian recurrent epizootics?

Vaccinating monkeys against yellow fever (YF) has been a common practice in the beginning of the 17D vaccine development. Although it may seem strange at first sight, vaccinating monkeys as a public health strategy is, we think, feasible and theoretically could eliminate the infection among non-human primates, interrupting the virus circulation (or significantly reducing it) and therefore reducing the risk of spilling over to the human population. We propose a series of studies that could demonstrate (or not) the efficacy and feasibility of vaccinating non-human primates YF reservoirs living in green areas of urban centres to cut off or curb the virus circulation that recurrently spill over to the human population. Therefore, vaccinating monkeys in relatively small green areas of the urban centres is perhaps the ultimate solution for the Brazilian recurrent YF epizootics.Key words: Mathematical modelling, medical informatics (veterinary and medical), dengue fever

In 2016, a yellow fever (YF) outbreak occurred in Minas Gerais, Brazil. It was characterised as a sylvan or jungle epizootic [1]. The disease spread itself to other South Eastern States of Brazil causing close to 1300 human cases and 216 confirmed deaths. All of these deaths were in human individuals who had recently visited green areas where there have been reported the deaths of non-human primates. Urban monkeys are, therefore, responsible for triggering human outbreaks of YF in Brazilian cities in 2016.Until the end of 2017, the State of São Paulo reported 501 deaths of monkeys from YF of which 177 deaths occurred in the Capital alone [2]. These deaths of monkey triggered a huge vaccination campaign of humans living in the neighbourhood of parks and green areas of Sao Paulo city, with an expected number of close to 3 million people to be vaccinated [3]. This rapid response of the health authorities is undisputedly the correct strategy to avoid the resurgence of urban YF. There are, however, complementary strategies that could cut off the virus circulation among non-human primates. Among them, we would like to propose the vaccination of monkeys, the main reservoirs of YF in Brazil [4, 5].Although it may seem strange at first sight, vaccinating monkeys is, we think, feasible and theoretically could eliminate the infection among non-human primates, interrupting the virus circulation (or significantly reducing it) and therefore reducing the risk of spilling over to the human population [6].Vaccinating monkeys against YF has been a common practice in the beginning of the 17D vaccine development. In 1928, Theiler and Sellards ([7] see also [8]) demonstrated that serum from immune humans protected monkeys from YF infection. After this first attempt to immunise monkeys, mice substituted the latter as a cheaper and more convenient animal model for the test of further vaccines. Many years later, in 1973, Mason et al., [9] directly challenged monkeys given graded doses of the 17D YF vaccine with the live virus. Forty-three of the 45 monkeys vaccinated with the dilution of 1:10^2.3 or greater weanling mouse mean lethal doses of 17D vaccine were resistant to challenge 20 weeks later with virulent strains of YF virus. In 1986, Schlesinger et al., [10] demonstrated that monkeys immunised with the YF virus non-structural protein NS1 resulted protected against the infection with the wild virus. Four out of five immunised monkeys survived the challenge, whereas all monkeys, which received ovalbumin injections, died. These are just examples that support the strategy of vaccinating monkeys as a safe and efficacious way to protect the animals against the infection.Before proposing vaccinating monkeys against YF as a routine public health strategy, however, many challenges should be overcome.The first challenge is to determine how safe and efficacious and at which dose the 17D YF vaccine is for the two currently most important monkey reservoir, namely the genus Callitrix and Alouatta [11]. Past literature, however, provide important information on YF vaccination in different species of monkeys [12].Second, the sheer area of forest in and around the city of Sao Paulo (just to examine an extreme case) possibly has a large (and unknown) number of monkeys living there that should be vaccinated [13]. The city of São Paulo has an estimated 642 km² of forested areas [14]. Assuming, just as an exercise, an animal density of 50 animal per km² [15], it should be expected 32 100 non-human primates living in the capital. If we assume a basic reproduction number of YF of 2.0 [16], the herd immunity necessary to cut off the virus circulation in the city should be about 16 000 monkeys to be vaccinated. The actual number of animals living in São Paulo, however, is likely to be much lower, and an educated guess of the target number to be vaccinated, restricted to the most affected areas, would be around 2 000 monkeys, a much more feasible figure. Future studies, however, should determine the exact number.Finally, the logistic involved in vaccinating such a number of animals is, perhaps, the most important limiting factor. The animal must be darted with anaesthetics, collected in a safety net to avoid falling into the ground, vaccinated and cared for until recovering from the anaesthesia and marked to avoid overvaccination [17]. This involves trained staff and some hours of work per monkey vaccinated. These difficulties, however, can be circumvented by additional studies, as described below.In addition, it has been argued that vaccinating monkeys would eliminate an important sentinel represented by the death of Alouatta monkeys as a surveillance evidence of YF virus circulation in certain areas. This, however, involves the ethical thorny issues on exposing or leaving unprotected one species in benefit of our own species. We will not be involved in this discussion here. Noteworthy, however, is the fact that some species like the ones from the genus Callitrix, in many circumstances, do not die from the disease, although they may be infectious to the wild mosquitoes.Systematic vaccination of wild non-human primates would certainly require appropriate regulatory agency approval for use of the vaccine in veterinary context. Therefore, studies will be required to license YF vaccines for veterinary use (see suggestion list below).In spite of all the above difficulties, we would like to recommend the following steps in future studies:

1. Laboratory work I: the equivalent a phase I/II clinical trial to determine the safety and efficacy doses of 17D-YF vaccine per species and per size/weight of the animals;
2. Laboratory work II: starting the development of a possible oral vaccine that could be used in baits, in the same fashion as the oral rabies vaccine used in Europe to control rabies in foxes and in the USA in raccoons (we are well aware that an oral vaccine would be very difficult to work due to the lack of stability of YF virus proteins at low pH). There is, to the best of our knowledge, no YF vaccine that could be successful in an oral formulation;
3. Laboratory work III: the development of an alternative deployment of the 17D-vaccine that could be used in darts, avoiding the capturing of the animals;
4. Field work I: determining the actual size of the monkey population by one of the commonly used techniques like the capture–recapture technique [17];
5. Field work II: the equivalent of a phase III clinical trial in a pilot area to test possible vaccination strategies/logistics;
6. Theoretical work I: determining the basic reproduction number and the herd immunity threshold in order to design an optimal vaccination strategy to cut off the virus circulation in the wild;
7. Field work III: the equivalent of a phase IV clinical trial that would involve large number of animals with an optimised strategy, as calculated in step 6;
8. Theoretical work II: modelling the impact of vaccinating non-human primates against YF on the risk of re-urbanization of the infection;
9. Theoretical work III: modelling the likelihood that YF virus transmission becomes established in a human–mosquito–human (the risk of the urban cycle, see [18]) before the local non-human primates are significantly affected.

Provided the above studies support the idea of vaccinating monkeys as an effective public health strategy, the issue of costs should also be addressed. The per capita cost of vaccinating human is estimated to be around US$1.5 [3]. Therefore, the total cost of the estimated 3 million people to be vaccinated in the near future in São Paulo is US$4.5 million. In the case of the target vaccination of about 2000 monkeys in the most affected areas, considering the costs of capturing and vaccinating each monkey, there would be a ceiling cost of less than US$200.00 per monkey vaccinated.The risk of urban YF resurgence, as shown in reference [18], is dependent on the introduction of one or more infected individual into an Aedes infested area. This may occur regardless of the involvement of local non-human primates since the infected index case could acquire the infection from visiting areas in which the urban cycle is established. However, this is not the case for Brazil where the urban cycle has been interrupted for at least 70 years now. Therefore, if and when the urban cycle of YF will recur in Brazil will certainly be triggered by individuals who acquire the infection in wild areas where epizootic cycles are already well established.We are well aware of the apparently difficult hurdles that should be overcome before the vaccination of monkeys against YF becoming a public health strategy. However, not to start the necessary studies to determine how and if vaccination monkeys against YF is a feasible strategy is, in our point of view, unethical if not inconsequential from the public health perspective.To conclude, it is obvious that it is not feasible to vaccinate monkeys in the Amazon forest, but in smaller green areas of the urban centres, it is perhaps the ultimate solution for the Brazilian recurrent YF epizootics.     

Japanese encephalitis virus/yellow fever virus chimera is safe and confers full protection against yellow fever virus in intracerebrally challenged mice

Yellow fever (YF) is an acute viral haemorrhagic disease caused by the yellow fever virus (YFV), which remains a potential threat to public health. The live-attenuated YF vaccine (17D strain) is a safe and highly effective measure against YF. However, increasing adverse events have been associated with YF vaccinations in recent years; thus, safer, alternative vaccines are needed. In this study, using the Japanese encephalitis live vaccine strain SA14-14–2 as a backbone, a novel chimeric virus was constructed by replacing the pre-membrane (prM) and envelope (E) genes with their YFV 17D counterparts.The chimeric virus exhibited a reduced growth rate and a much smaller plaque morphology than did either parental virus. Furthermore, the chimera was much less neurovirulent than was YF17D and protected mice that were challenged with a lethal dose of the YF virus. These results suggest that this chimera has potential as a novel attenuated YF vaccine.     

Characterization of a viscerotropic yellow fever vaccine variant from a patient in Brazil

Although the live attenuated yellow fever (YF) 17D vaccine is considered to be one of the safest vaccines in the world today, several cases of disease associated with administration of the vaccine have been reported, including YF vaccine-associated viscerotropic disease (YF-VAVD), which was first described in 1996. All YF-VAVD isolates sequenced to date have shown very little genomic change when compared to their parental vaccine strains. In this study, we report the characterization of an isolate, BeH291597 (Brazil75), from a 1975 fatal case of YF-VAVD in Brazil. Comparison of Brazil75 with the genomic sequence of the parental 17DD vaccine strain revealed two amino acid substitutions (at positions M-49 and NS4B-240) that were unique to Brazil75. Although still a rare occurrence, this isolate suggests that YF-VAVD has been present much longer than previously recognized.     

17D yellow fever vaccines: new insights. A report of a workshop held during the World Congress on medicine and health in the tropics, Marseille, France, Monday 12 September 2005

Yellow fever (YF) is a major health problem in endemic regions of Africa and South America. It also poses a serious health risk to travellers to areas with endemic disease. Currently, there is no effective drug treatment for YF; however, 17D YF vaccines have demonstrated high rates of effectiveness and good safety profiles. This workshop was organized to review key data and issues about YF disease and currently available 17D YF vaccines. Starting with an overview of the current disease epidemiology in Africa and South America and a review of the safety data of 17D YF vaccines, data were then presented demonstrating the genetic stability of multiple production lots of a 17D YF vaccine, the immunological responses of healthy subjects post-vaccination and the long-term immunogenicity of 17D YF vaccines. Finally, the findings of the molecular characterization of 17D YF virus sub-strains recovered from rare, fatal cases of post-vaccination serious adverse events were presented. There was unanimous agreement that current 17D YF vaccines have a highly favourable benefit-risk profile when used in persons at risk of exposure to the YF virus, and that appropriate use of 17D YF vaccines will minimize the occurrence of serious adverse events post-vaccination.     

Emergence of a new arbovirus disease in Brazil. III. Isolation of Rocio virus from Psorophora Ferox (Humboldt, 1819)

A newly described flavivirus was responsible for a large encephalitis epidemic in S?o Paulo State, Brazil. The etiologic agent, Rocio virus, was isolated from human patients and sentinel mice. The natural history of the virus is unknown although presumed to be arthropod-borne. Rocio virus was isolated from a single pool containing 19 Psorophora ferox of 47 pools (283 specimens) of this species tested. The positive pool contained 16 deplete, 2 gravid, and 2 engorged mosquitoes. No isolations were made from 2183 pools of other species. The positive pool was collected during the year of the epidemic at the same approximate time and place where vertebrate isolations were made.     

Reactogenicity of yellow fever vaccines in a randomized, placebo-controlled trial

OBJECTIVE: To compare the reactogenicity of three yellow fever (YF) vaccines from WHO-17D and Brazilian 17DD substrains (different seed-lots) and placebo. METHODS: The study involved 1,087 adults eligible for YF vaccine in Rio de Janeiro, Brazil. Vaccines produced by Bio-Manguinhos, Fiocruz (Rio de Janeiro, Brazil) were administered ("day 0") following standardized procedures adapted to allow blinding and blocked randomization of participants to coded vaccine types. Adverse events after immunization were ascertained in an interview and in diary forms filled in by each participant. Liver enzymes were measured on days 0, 4-20 and 30 of the study. Viremia levels were measured on days 4 to 20 of follow-up. The immune response was verified through serologic tests. RESULTS: Participants were mostly young males. The seroconversion rate was above 98% among those seronegative before immunization. Compared to placebo, the excess risk of any local adverse events ranged from 0.9% to 2.5%, whereas for any systemic adverse events it ranged from 3.5% to 7.4% across vaccine groups. The excess risk of events leading to search for medical care or to interruption of work activities ranged from 2% to 4.5%. Viremia was detected in 3%-6% of vaccinees up to 10 days after vaccination. Variations in liver enzyme levels after vaccination were similar in placebo and vaccine recipients. CONCLUSIONS: The frequency of adverse events post-immunization against YF, accounting for the background occurrence of nonspecific signs and symptoms, was shown for the first time to be similar for vaccines from 17D and 17DD substrains. The data also provided evidence against viscerotropism of vaccine virus.