Protection of Rhesus monkeys against dengue virus challenge after tetravalent live attenuated dengue virus vaccination

Rhesus monkeys develop viremia after dengue virus (DENV) inoculation and have been used as an animal model to study DENV infection and DENV vaccine candidates. We evaluated, in this model, the protective efficacy of a live attenuated tetravalent DENV vaccine (TDV) candidate against parenteral challenge with parental near-wild-type DENV strains. Twenty monkeys were vaccinated with TDV at 0 and 1 month, and 20 unvaccinated monkeys served as controls. Vaccinated animals and their controls were inoculated with 10(3)-10(4) pfu of challenge virus 4.5 months after the second vaccination. Primary vaccination resulted in 95%, 100%, 70%, and 15% seroconversion to DENV serotypes 1, 2, 3, and 4 (DENV-1, -2, -3, and -4), respectively. After the second vaccination, the seropositivity rates were 100%, 100%, 90%, and 70%, respectively. Vaccination with TDV resulted in complete protection against viremia from DENV-2 challenge and in 80%, 80%, and 50% protection against challenge with DENV-1, -3, and -4, respectively. Our results suggest that the TDV can elicit protective immunity against all 4 DENV serotypes. Interference among the 4 vaccine viruses may have resulted in decreased antibody responses to DENV-3 and -4, which would require reformulation or dose optimization to minimize this interference during testing of the vaccine in humans.     

Seroepidemiological survey on dengue and Japanese encephalitis virus infections in Asian monkeys

To investigate the ecology of dengue and Japanese encephalitis (JE) viruses in the forest in Asia, a seroepidemiological survey was carried out on 358 Southeast Asian cynomolgus (Macaca iris), 33 Indian bonnet (Macaca radiata) and 37 Japanese (Macaca fuscata) monkey sera by a plaque reduction neutralization test. The results indicated that Southeast Asian monkeys were naturally infected with these viruses but the frequency of antibody to them varied considerably according to the geographical origin of the monkeys. The frequency of antibody to one or more types of dengue virus were 87.2, 49.5, 34.3, 34.2 and 14.9% in Malaysian, Vietnamese, Cambodian, Indonesian and Filipino cynomolgus monkey sera, respectively. None of the Indian bonnet monkey sera neutralized type 1 dengue virus which was the only virus type examined with this monkey species. Monkey sera collected in Japan where dengue virus infection had not been known since 1944 did not significantly neutralize dengue viruses. JE virus antibody was detected at 29.7, 9.0, 8.6, 2.7, 1.4 and 0% in Japanese, Cambodian, Vietnamese, Indonesian, Filipino and Malaysian monkey sera respectively.     

Incidence of primary dengue virus infections in Southern Vietnamese children and reactivity against other flaviviruses

Objective To study the incidence of asymptomatic primary dengue infections among children and reactivity against other flaviviruses. Methods A total of 216 children, who had no dengue‐specific IgG antibodies during a serosurvey in 2003 were re‐examined 23 months later to determine if seroconversion had occurred. Dengue‐specific IgG was demonstrated with enzyme‐linked immunosorbent assay (ELISA) and reactivity patterns against other flaviviruses were assessed by using immunofluorescence assay (IFA). Results Sixty‐six children had seroconverted for dengue virus‐specific IgG; the true annual incidence of primary dengue was thus 17.3% (95% CI: 13.8–21.4). Japanese Encephalitis virus (JEV)‐specific IgG antibodies were detected by IFA among three (4.6%) samples that showed seroconversion in the dengue ELISA, because of cross‐reactivity. Conclusion Our findings highlight the high incidence of dengue among Vietnamese children; JEV infections are rare. The true annual incidence of dengue can be estimated with a single cross‐sectional seroprevalence survey.     

Rashes occurring after immunization with a mixture of viruses in the Oka vaccine are derived from single clones of virus

Vaccination against chickenpox causes a varicella-like rash in up to 5% of healthy children and 50% of children with leukemia. The vaccine may establish latency and reactivate to cause herpes zoster, albeit more rarely than wild-type virus. All vaccine preparations are composed of a mixture of varicella-zoster virus strains that show genotypic variation at several loci. We have shown, by DNA sequencing of 40 polymorphic loci, that viruses sampled from vesicles in varicella-like and herpes zoster rashes are single clones. This finding suggests that, between the time of inoculation of the vaccine and development of rash, selection of single strains occurs. The results have general implications for the pathogenesis of varicella-zoster virus.     

In vivo enhancement of dengue virus infection in rhesus monkeys by passively transferred antibody.

Five pairs of juvenile, dengue virus-susceptible rhesus monkeys were given normal or dengue-immune human cord-blood serum injected intravenously to a final dilution of 1:300. The pool of immune human cord-blood serum had a titer of antibody to dengue type 2 virus (D2V) of 1:140 in the plaque-reduction neutralization test and a titer of human monocyte infection enhancement of greater than 1:2,000,000. Fifteen minutes after inoculation of serum, animals were infected with D2V (strain no. 16681). Daily titers of viremia were always higher in the animals that had received antiserum to D2V than in animals that had received normal cord-blood serum. Ratios of infection enhancement ranged from 2.7 to 51.4. The demonstration of antibody dependence of dengue virus infection in subhuman primates--a complex, outbred experimental host--supports the hypothesis that the severity of dengue in humans is regulated by antibody.