Performance assessment and validation of a plaque reduction neutralization test (PRNT) in support to yellow fever diagnostic and vaccine clinical trials.

Yellow fever (YF) virus is a mosquito-borne virus belonging to the Flaviviridae family that circulates in tropical and subtropical areas of Africa and South America. Despite the availability of an effective vaccine, YF remains a threat to travelers, residents of endemic areas, and unvaccinated populations. YF vaccination and natural infection both induce the production of neutralizing antibodies. Serological diagnostic methods detecting YF virus-specific antibodies demonstrate high levels of cross-reactivities with other flaviviruses. To date, the plaque reduction neutralization test (PRNT) is the most specific serological test for the differentiation of flavivirus infections and is considered the reference method for detecting YF neutralizing antibodies and assessing the protective immune response following vaccination. In this study, we developed and validated a YF PRNT. We optimized different parameters including cell concentration and virus-serum neutralization time period and then assessed the intra- and inter-assay precisions, dilutability, specificity, and lower limit of quantification (LLOQ) using international standard YF serum, sera from vaccinees and human specimens collected through YF surveillance. The YF PRNT has shown good robustness and 100% of intra-assay precision, 95.6% of inter-assay precision, 100% of specificity, 100% of LLOQ, and 95.3% of dilutability. The test is, therefore, suitable for use in the YF diagnostic as well as evaluation of the YF vaccine neutralizing antibody response and risk assessment studies.

The epidemiology and disease burden of children hospitalized for viral infections within the family Flaviviridae in China: A national cross-sectional study.

BACKGROUND: Viruses of the family Flaviviridae, including Japanese encephalitis virus (JEV), dengue virus (DENV), yellow fever virus (YFV) and hepatitis C virus (HCV), are widely distributed worldwide. JEV, DENV and YFV belong to the genus Flavivirus, whereas HCV belongs to the genus Hepacivirus. Children's symptoms are usually severe. As a result, rates of hospitalization due to infection with these viruses are high. The epidemiology and disease burden of hospitalized children have rarely been described in detail to date. The objective of this study was to report the general epidemiological characteristics, clinical phenotype, length of stay (LOS), burden of disease, and potential risk factors for hospitalized children infected with JEV, DENV, YFV, or HCV in Chinese pediatric hospitals. METHODOLOGY: A cross-sectional study of epidemiology and disease burden of children hospitalized for Flaviviridae virus infections between December 2015 and December 2020 in China was performed. Face sheets of discharge medical records (FSMRs) were collected from 27 tertiary children's hospitals in the Futang Research Center of Pediatric Development and aggregated into FUTang Update medical REcords (FUTURE). Information on sociodemographic variables, clinical phenotype, and LOS as well as economic burden was included in FSMRs and compared using appropriate statistical tests. FINDINGS: The study described 490 children aged 0-15 years hospitalized for infections with Flaviviridae viruses. Japanese encephalitis (JE) cases are the highest, accounting for 92.65% of the total hospitalization cases caused by Flaviviridae virus infection. The incidence of JE peaked from July to October with a profile of a high proportion of severe cases (68.06%) and low mortality (0.44%). Rural children had a significantly higher incidence than urban children (91.63%). Most hospitalized dengue cases were reported in 2019 when dengue outbreaks occurred in many provinces of China, although only 14 dengue cases were collected during the study period. Yellow fever (YF) is still an imported disease in China. The hospitalizations for children with hepatitis C (HC) were not high, and mild chronic HC was the main clinical phenotype of patients. Among the four viral infections, JE had the highest disease burden (LOS and expenditure) for hospitalized children. CONCLUSION: First, the present study reveals that JE remains the most serious disease due to Flaviviridae virus infection and threatens children's health in China. Many pediatric patients have severe illnesses, but their mortality rate is lower, suggesting that existing treatment is effective. Both JEV vaccination and infection control of rural children should represent a focus of study. Second, although the dual risks of indigenous epidemics and imports of DENV still exist, the prevalence of DENV in children is generally manageable. Third, YFV currently shows no evidence of an epidemic in China. Finally, the proportion of children with chronic hepatitis C (CHC) is relatively large among hospitalized children diagnosed with HCV. Thus, early and effective intervention should be offered to children infected with HCV to ease the burden of CHC on public health.

Risk assessment of urban yellow fever virus transmission in Kenya: is Aedes aegypti an efficient vector?

The absence of urban yellow fever epidemics in East Africa remains a mystery amidst the proliferation of Aedes aegypti in this region. To understand the transmission dynamics of the disease, we tested urban (Mombasa, Kisumu, and Nairobi) Aedes mosquito populations in Kenya for their susceptibility to an East African yellow fever virus (YFV) genotype. Overall, 22% (n = 805) of the Ae. aegypti that were orally challenged with an infectious dose of YFV had a midgut infection, with comparable rates for Mombasa and Kisumu (χ2 = 0.35, df = 1, P = 0.55), but significantly lower rates for Nairobi (χ2 ≥ 11.08, df = 1, P ≤ 0.0009). Variations in YFV susceptibility (midgut infection) among Ae. aegypti subspecies were not associated with discernable cytochrome c oxidase subunit 1 gene haplotypes. Remarkably, no YFV dissemination or transmission was observed among the orally challenged Ae. aegypti populations. Moreover, Ae. aegypti mosquitoes that were intrathoracically inoculated with YFV failed to transmit the virus via capillary feeding. In contrast, dissemination (oral exposure) and transmission (intrathoracic inoculation) of YFV was observed among a few peri-domestic Ae. bromeliae mosquitoes (n = 129) that were assessed from these urban areas. Our study highlights an inefficient urban Ae. aegypti population, and the potential for Ae. bromeliae in sustaining an urban YFV transmission in Kenya. An assessment of urban Ae. aegypti susceptibility to other YFV genotypes, and vector potential of urban Ae. bromeliae populations in Kenya is recommended to guide cost-effective vaccination.

FluoRNT: A robust, efficient assay for the detection of neutralising antibodies against yellow fever virus 17D.

There is an urgent need for better diagnostic and analytical methods for vaccine research and infection control in virology. This has been highlighted by recently emerging viral epidemics and pandemics (Zika, SARS-CoV-2), and recurring viral outbreaks like the yellow fever outbreaks in Angola and the Democratic Republic of Congo (2016) and in Brazil (2016-2018). Current assays to determine neutralising activity against viral infections in sera are costly in time and equipment and suffer from high variability. Therefore, both basic infection research and diagnostic population screenings would benefit from improved methods to determine virus-neutralising activity in patient samples. Here we describe a robust, objective, and scalable Fluorescence Reduction Neutralisation Test (FluoRNT) for yellow fever virus, relying on flow cytometric detection of cells infected with a fluorescent Venus reporter containing variant of the yellow fever vaccine strain 17D (YF-17D-Venus). It accurately measures neutralising antibody titres in human serum samples within as little as 24 h. Samples from 32 vaccinees immunised with YF-17D were tested for neutralising activity by both a conventional focus reduction neutralisation test (FRNT) and FluoRNT. Both types of tests proved to be equally reliable for the detection of neutralising activity, however, FluoRNT is significantly more precise and reproducible with a greater dynamic range than conventional FRNT. The FluoRNT assay protocol is substantially faster, easier to control, and cheaper in per-assay costs. FluoRNT additionally reduces handling time minimising exposure of personnel to patient samples. FluoRNT thus brings a range of desirable features that can accelerate and standardise the measurement of neutralising anti-yellow fever virus antibodies. It could be used in applications ranging from vaccine testing to large cohort studies in systems virology and vaccinology. We also anticipate the potential to translate the methodology and analysis of FluoRNT to other flaviviruses such as West Nile, Dengue and Zika or to RNA viruses more generally.

An evaluation of the diagnostic performance characteristics of the Yellow Fever IgM immunochromatographic rapid diagnostic test kit from SD Biosensor in Ghana.

Yellow fever is endemic in Ghana and outbreaks occur periodically. The prodromal signs due to Yellow Fever Virus (YFV) infection are non-specific, making clinical signs unreliable as the sole criteria for diagnosis. Accurate laboratory confirmation of suspected yellow fever cases is therefore vital in surveillance programs. Reporting of ELISA IgM testing results by laboratories can delay due to late arrival of samples from the collection sites as well as limited availability of ELISA kits. In this study, the diagnostic performance characteristics of a rapid immunochromatographic Standard Q Yellow Fever IgM test kit (SD Biosensor) was evaluated for the rapid diagnosis of Yellow Fever infection in Ghana. A panel of 275 sera, comprising 81 confirmed YFV positives and 194 negatives were re-tested in this study using the Standard Q Yellow Fever IgM test kit. Using the CDC/WHO Yellow Fever IgM capture ELISA as a benchmark, the sensitivity, specificity and accuracy of the Standard Q Yellow Fever test kit were 96.3%, 97.9% and 97.5%, respectively. The false positivity rate was 5.1% and there was no cross-reactivity when the Standard Q Yellow Fever test kit was tested against dengue, malaria and hepatitis B and C positive samples. In addition, inter-reader variability and invalid rate were both zero. The results indicate that the diagnostic performance of the Standard Q Yellow Fever IgM test kit on serum or plasma is comparable to the serum IgM detection by ELISA and can be used as a point of care rapid diagnostic test kit for YFV infection in endemic areas.

Assessment of immunity against Yellow Fever virus infections in northeastern Nigeria using three serological assays.

Poor systematic surveillance for Yellow Fever virus (YFV) is primarily due to lack of affordable diagnostic facilities in resource-constrained countries. This study aimed at providing evidence-based information on immunity against Yellow Fever with a view to assessing the possibility of the recent epidemics persisting in Nigeria. Six hundred patients with febrile illness seeking malaria test in selected hospitals were tested for YFV antibody using three serological assays: ELISA IgM, microneutralization test (MNT) and plaque reduction neutralization test (PRNT). The three assays commonly detected YFV antibody (Ab) in 1.7% patients, MNT: IgM in 8.3%, IgM: PRNT in 7.1%, and MNT: PRNT in 3.2%. Immunity against YF was significantly higher in Bauchi and Borno than Adamawa and children aged 0-9 years compared to 20-29 years. YFV neutralizing antibody (nAb) strongly correlated with the vaccination status of the patients. More unvaccinated patients had nAb compared with the vaccinated. Immunity against YF among treated patients with antibiotic and/or antimalaria before sample collection inversely correlated with the untreated. YVnAb among unvaccinated indicates natural infections. Acute YFV infections were mistaken for malaria and natural infections are ongoing. Individuals aged more than or equal to 20 years should be targeted during mass vaccination campaigns. With low population immunity, repetitive YF epidemics in Nigeria is not yet over. The current policy on Yellow Fever vaccination in Nigeria still leaves a large unimmunized population at the risk of epidemics. Sufficient mass vaccination in combination with National Programme on Immunization remains key to averting YF epidemics.

Performance of Eastern Mediterranean Region laboratories in the World Health Organization external quality assessment programme for arbovirus diagnostics

Background: Arboviruses such as dengue virus, yellow fever virus, Zika virus and chikungunya virus are major threats to human health globally, including countries in the Eastern Mediterranean Region (EMR).Aims: This study aimed to assess laboratory proficiency in EMR countries for detection of dengue virus, yellow fever virus, Zika virus and chikungunya virus.Methods: A global external quality assessment programme for arbovirus diagnostics was developed and run in 2016 and 2018. National-level public health laboratories were instructed to apply the polymerase chain reaction detection method on specimen panels containing dengue virus, yellow fever virus, Zika virus and chikungunya virus.Results: Over both rounds of the programme, 100% of participating EMR laboratories correctly detected yellow fever virus and chikungunya virus, ≥ 84.6% detected dengue fever virus and ≥ 76.9% detected Zika virus. Conclusion: While participating EMR countries demonstrated good proficiency in detecting arboviruses, only half of them were enrolled in the global external quality assessment programme, providing an incomplete picture of regional capacity. Effort should be put into increasing participation in subsequent rounds.

Contexte : Les arbovirus tels que le virus de la dengue, le virus de la fièvre jaune, le virus Zika et le virus chikungunya constituent des menaces majeures pour la santé humaine dans le monde, y compris dans les pays de la Région de la Méditerranée orientale. Objectifs : La présente étude visait à évaluer l’aptitude des laboratoires dans les pays de la Région pour la détection des virus susmentionnés. Méthodes : Un programme mondial d’évaluation externe de la qualité pour le diagnostic des arbovirus a été mis au point et adopté en 2016 et 2018. Les laboratoires de santé publique nationaux ont reçu l’instruction d’appliquer la méthode de détection par amplification génique sur des panels d’échantillons de laboratoires contenant le virus de la dengue, le virus de la fièvre jaune, le virus Zika et le virus chikungunya. Résultats : Au cours des deux cycles du programme, 100 % des laboratoires participants de la Région de la Méditerranéenne orientale ont correctement détecté le virus de la fièvre jaune et le virus du chikungunya, un pourcentage supérieur ou égal à 84,6 % ont détecté le virus de la dengue et un pourcentage supérieur ou égal à 76,9 % de ces laboratoires ont détecté le virus Zika. Conclusions : Alors que les pays participants de la Région de la Méditerranée orientale ont démontré une bonne maîtrise de la détection des arbovirus, seulement la moitié d’entre eux étaient inscrits au programme mondial d’évaluation externe de la qualité, fournissant une image incomplète de la capacité régionale. Des efforts devraient être déployés pour accroître la participation aux cycles suivants.

Differences between coverage of yellow fever vaccine and the first dose of measles-containing vaccine: A desk review of global data sources.

INTRODUCTION: The strategy to Eliminate Yellow Fever Epidemics (EYE) is a global initiative that includes all countries with risk of yellow fever (YF) virus transmission. Of these, 40 countries (27 in Africa and 13 in the Americas) are considered high-risk and targeted for interventions to increase coverage of YF vaccine. Even though the World Health Organization (WHO) recommends that YF vaccine be given concurrently with the first dose of measles-containing vaccine (MCV1) in YF-endemic settings, estimated coverage for MCV1 and YF vaccine have varied widely. The objective of this study was to review global data sources to assess discrepancies in YF vaccine and MCV1 coverage and identify plausible reasons for these discrepancies. METHODS: We conducted a desk review of data from 34 countries (22 in Africa, 12 in Latin America), from 2006 to 2016, with national introduction of YF vaccine and listed as high-risk by the EYE strategy. Data reviewed included procured and administered doses, immunization schedules, routine coverage estimates and reported vaccine stock-outs. In the 30 countries included in the comparitive analysis, differences greater than 3 percentage points between YF vaccine and MCV1 coverage were considered meaningful. RESULTS: In America, there were meaningful differences (7-45%) in coverage of the two vaccines in 6 (67%) of the 9 countries. In Africa, there were meaningful differences (4-27%) in coverage of the two vaccines in 9 (43%) of the 21 countries. Nine countries (26%) reported MVC1 stock-outs while sixteen countries (47%) reported YF vaccine stock-outs for three or more years during 2006-2016. CONCLUSION: In countries reporting significant differences in coverage of the two vaccines, differences may be driven by different target populations and vaccine availability. However,these were not sufficient to completely explain observed differences. Further follow-up is needed to identify possible reasons for differences in coverage rates in several countries where these could not fully be explained.

Comparative assessment of the replication efficiency of dengue, yellow fever, and chikungunya arboviruses in some insect and mammalian cell lines.

INTRODUCTION: Insect cell cultures play an essential role in understanding arboviral replication. However, the replicative efficiency of some of these viruses such as dengue (DENV), yellow fever (YFV), and chikungunya (CHIKV) in a new cellular substrate (Lulo) and in the other two recognized cell lines has not been comparatively assessed. METHODS: Vero, C6/36, and Lulo cell lines were infected with DENV, YFV, and CHIKV. The viral progeny was quantified through plaque assays and quantitative reverse transcription-polymerase chain reaction, while for DENV2, the findings were confirmed by immunofluorescence antibody assay. RESULTS: The higher DENV2 titer (from multiplicity of infection 0.001) was obtained on day four post-infection in C6/36 and on day six in Vero cells, while the Lulo cell line was almost impossible to infect under the same conditions. However, C6/36 showed the highest values of viral RNA production compared to Vero cells, while the quantification of the viral RNA in Lulo cells showed high levels of viral genomes, which had no correlation to the infectious viral particles. CONCLUSIONS: C6/36 was the most efficient cell line in the alpha and flavivirus production, followed by Vero cells. Thus, Lulo cells may be a useful substrate to study the mechanisms by which cells evade viral replication.

Comparative assessment of the replication efficiency of dengue, yellow fever, and chikungunya arboviruses in some insect and mammalian cell lines

Abstract INTRODUCTION: Insect cell cultures play an essential role in understanding arboviral replication. However, the replicative efficiency of some of these viruses such as dengue (DENV), yellow fever (YFV), and chikungunya (CHIKV) in a new cellular substrate (Lulo) and in the other two recognized cell lines has not been comparatively assessed. METHODS: Vero, C6/36, and Lulo cell lines were infected with DENV, YFV, and CHIKV. The viral progeny was quantified through plaque assays and quantitative reverse transcription-polymerase chain reaction, while for DENV2, the findings were confirmed by immunofluorescence antibody assay. RESULTS: The higher DENV2 titer (from multiplicity of infection 0.001) was obtained on day four post-infection in C6/36 and on day six in Vero cells, while the Lulo cell line was almost impossible to infect under the same conditions. However, C6/36 showed the highest values of viral RNA production compared to Vero cells, while the quantification of the viral RNA in Lulo cells showed high levels of viral genomes, which had no correlation to the infectious viral particles. CONCLUSIONS: C6/36 was the most efficient cell line in the alpha and flavivirus production, followed by Vero cells. Thus, Lulo cells may be a useful substrate to study the mechanisms by which cells evade viral replication.

Need for additional capacity and improved capability for molecular detection of yellow fever virus in European Expert Laboratories: External Quality Assessment, March 2018.

An external quality assessment of yellow fever virus (YFV) molecular detection in European laboratories was organised in rapid response to an increase in human cases in Brazil in 2018 with risk of import to Europe. Detection of YFV was assessed among 32 laboratories in 23/31 European Union (EU) and European Economic Area (EEA) countries and two laboratories in one non-EU/EEA country. Adequate capabilities were lacking in 10/23 countries; five did not participate as they lacked implemented assays.

Dengue Dynamics and Vaccine Cost-Effectiveness Analysis in the Philippines.

Dengue is one of the most problematic vector-borne diseases in the Philippines, with an estimated 842,867 cases resulting in medical costs of $345 million U.S. dollars annually. In December 2015, the first dengue vaccine, known as chimeric yellow fever virus-dengue virus tetravalent dengue vaccine, was approved for use in the Philippines and is given to children 9 years of age. To estimate the cost-effectiveness of dengue vaccination in the Philippines, we developed an age-structured model of dengue transmission and vaccination. Using our model, we compared two vaccination scenarios entailing routine vaccination programs both with and without catch-up vaccination. Our results indicate that the higher the cost of vaccination, the less cost-effective the dengue vaccination program. With the current dengue vaccination program that vaccinates children 9 years of age, dengue vaccination is cost-effective for vaccination costs up to $70 from a health-care perspective and up to $75 from a societal perspective. Under a favorable scenario consisting of 1 year of catch-up vaccinations that target children 9-15 years of age, followed by regular vaccination of 9-year-old children, vaccination is cost-effective at costs up to $72 from a health-care perspective and up to $78 from a societal perspective. In general, dengue vaccination is expected to reduce the incidence of both dengue fever and dengue hemorrhagic fever /dengue shock syndrome. Our results demonstrate that even at relatively low vaccine efficacies, age-targeted vaccination may still be cost-effective provided the vaccination cost is sufficiently low.

Historical Perspective: What Constitutes Discovery (of a New Virus)?

A historic review of the discovery of new viruses leads to reminders of traditions that have evolved over 118 years. One such tradition gives credit for the discovery of a virus to the investigator(s) who not only carried out the seminal experiments but also correctly interpreted the findings (within the technological context of the day). Early on, ultrafiltration played a unique role in "proving" that an infectious agent was a virus, as did a failure to find any microscopically visible agent, failure to show replication of the agent in the absence of viable cells, thermolability of the agent, and demonstration of a specific immune response to the agent so as to rule out duplicates and close variants. More difficult was "proving" that the new virus was the etiologic agent of the disease ("proof of causation")-for good reasons this matter has been revisited several times over the years as technologies and perspectives have changed. One tradition is that the discoverers get to name their discovery, their new virus (unless some grievous convention has been broken)-the stability of these virus names has been a way to honor the discoverer(s) over the long term. Several vignettes have been chosen to illustrate several difficulties in holding to the traditions (vignettes chosen include vaccinia and variola viruses, yellow fever virus, and influenza viruses. Crimean-Congo hemorrhagic fever virus, Murray Valley encephalitis virus, human immunodeficiency virus 1, Sin Nombre virus, and Ebola virus). Each suggests lessons for the future. One way to assure that discoveries are forever linked with discoverers would be a permanent archive in one of the universal virus databases that have been constructed for other purposes. However, no current database seems ideal-perhaps members of the global community of virologists will have an ideal solution.

Live virus vaccines based on a yellow fever vaccine backbone: standardized template with key considerations for a risk/benefit assessment.

The Brighton Collaboration Viral Vector Vaccines Safety Working Group (V3SWG) was formed to evaluate the safety of live, recombinant viral vaccines incorporating genes from heterologous viruses inserted into the backbone of another virus (so-called "chimeric virus vaccines"). Many viral vector vaccines are in advanced clinical trials. The first such vaccine to be approved for marketing (to date in Australia, Thailand, Malaysia, and the Philippines) is a vaccine against the flavivirus, Japanese encephalitis (JE), which employs a licensed vaccine (yellow fever 17D) as a vector. In this vaccine, two envelope proteins (prM-E) of YF 17D virus were exchanged for the corresponding genes of JE virus, with additional attenuating mutations incorporated into the JE gene inserts. Similar vaccines have been constructed by inserting prM-E genes of dengue and West Nile into YF 17D virus and are in late stage clinical studies. The dengue vaccine is, however, more complex in that it requires a mixture of four live vectors each expressing one of the four dengue serotypes. This vaccine has been evaluated in multiple clinical trials. No significant safety concerns have been found. The Phase 3 trials met their endpoints in terms of overall reduction of confirmed dengue fever, and, most importantly a significant reduction in severe dengue and hospitalization due to dengue. However, based on results that have been published so far, efficacy in preventing serotype 2 infection is less than that for the other three serotypes. In the development of these chimeric vaccines, an important series of comparative studies of safety and efficacy were made using the parental YF 17D vaccine virus as a benchmark. In this paper, we use a standardized template describing the key characteristics of the novel flavivirus vaccine vectors, in comparison to the parental YF 17D vaccine. The template facilitates scientific discourse among key stakeholders by increasing the transparency and comparability of information. The Brighton Collaboration V3SWG template may also be useful as a guide to the evaluation of other recombinant viral vector vaccines.

Yellow fever risk assessment in the Central African Republic.

BACKGROUND: Starting in 2008, the Central African Republic (CAR) experienced an unprecedented number of reported yellow fever (YF) cases. A risk assessment of YF virus (YFV) activity was conducted to estimate potential disease risk and vaccine needs. METHODS: A multistage cluster sampling design was used to sample humans, non-human primates, and mosquitoes in distinct ecologic zones. Humans and non-human primates were tested for YFV-specific antibodies; mosquitoes were tested for YFV RNA. RESULTS: Overall, 13.3% (125/938) of humans were found to have naturally-acquired YFV antibodies. Antibody levels were higher in zones in the southern and south central regions of CAR. All sampled non-human primates (n=56) were known YFV reservoirs; one tested positive for YFV antibodies. Several known YF vectors were identified including Aedes africanus, Ae. aegypti, Ae. luteocephalus, and Ae. simpsoni. Several more urban locations were found to have elevated Breateau and Container indices for Ae. aegypti. CONCLUSIONS: A country-wide assessment of YF risk found YFV to be endemic in CAR. The potential for future YF cases and outbreaks, however, varied by ecologic zone. Improved vaccination coverage through mass campaign and childhood immunization was recommended to mitigate the YF risk.

Serologic assessment of yellow fever immunity in the rural population of a yellow fever-endemic area in Central Brazil.

INTRODUCTION: The yellow fever epidemic that occurred in 1972/73 in Central Brazil surprised the majority of the population unprotected. A clinical-epidemiological survey conducted at that time in the rural area of 19 municipalities found that the highest (13.8%) number of disease cases were present in the municipality of Luziânia, State of Goiás. METHODS: Thirty-eight years later, a new seroepidemiological survey was conducted with the aim of assessing the degree of immune protection of the rural population of Luziânia, following the continuous attempts of public health services to obtain vaccination coverage in the region. A total of 383 volunteers, aged between 5 and 89 years and with predominant rural labor activities (75.5%), were interviewed. The presence of antibodies against the yellow fever was also investigated in these individuals, by using plaque reduction neutralization test, and correlated to information regarding residency, occupation, epidemiological data and immunity against the yellow fever virus. RESULTS: We found a high (97.6%) frequency of protective titers (>1:10) of neutralizing antibodies against the yellow fever virus; the frequency of titers of 1:640 or higher was 23.2%, indicating wide immune protection against the disease in the study population. The presence of protective immunity was correlated to increasing age. CONCLUSIONS: This study reinforces the importance of surveys to address the immune state of a population at risk for yellow fever infection and to the surveillance of actions to control the disease in endemic areas.

Serologic assessment of yellow fever immunity in the rural population of a yellow fever-endemic area in Central Brazil

Introduction The yellow fever epidemic that occurred in 1972/73 in Central Brazil surprised the majority of the population unprotected. A clinical-epidemiological survey conducted at that time in the rural area of 19 municipalities found that the highest (13.8%) number of disease cases were present in the municipality of Luziânia, State of Goiás. Methods Thirty-eight years later, a new seroepidemiological survey was conducted with the aim of assessing the degree of immune protection of the rural population of Luziânia, following the continuous attempts of public health services to obtain vaccination coverage in the region. A total of 383 volunteers, aged between 5 and 89 years and with predominant rural labor activities (75.5%), were interviewed. The presence of antibodies against the yellow fever was also investigated in these individuals, by using plaque reduction neutralization test, and correlated to information regarding residency, occupation, epidemiological data and immunity against the yellow fever virus. Results We found a high (97.6%) frequency of protective titers (>1:10) of neutralizing antibodies against the yellow fever virus; the frequency of titers of 1:640 or higher was 23.2%, indicating wide immune protection against the disease in the study population. The presence of protective immunity was correlated to increasing age. Conclusions This study reinforces the importance of surveys to address the immune state of a population at risk for yellow fever infection and to the surveillance of actions to control the disease in endemic areas. .