Yellow fever vaccine. IV. Reactogenicity and antibody response in volunteers inoculated with a vaccine free from contaminating avian leukosis viruses

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.

Defective adenoviruses as novel vaccines for the Flaviviridae

BACKGROUND

Vaccines against many flaviviruses, such as Japanese encephalitis virus (JEV), yellow fever virus (YFV) and tick-borne encephalitis virus (TBEV), have been successfully used for many years. Other diseases such as dengue fever (DF) and hepatitis C are still major public health problems as no licensed vaccines are in use.

OBJECTIVES

To review studies on the use of defective recombinant adenoviruses (Rads) as experimental flavivirus vaccines and comment on their use to prevent infections with other members of the Flaviviridae such as hepatitis C virus.

STUDY DESIGN

Recombinant adenoviruses, defective in their replication strategy, contain deletions in the E1 and E3 regions of the genome to increase the amount of foreign genetic material that can be inserted. The expression of foreign genes, inserted into these regions, can be driven by the adenovirus's own promoter, or by an additional viral promoters.

CONCLUSIONS

Rads have been successfully used to raise protective immunity in experimental models of infection with several viruses. They can elicit both humoral and cell-mediated immunity and can be given parenterally or by oral administration. In addition, their hepatotropism makes them suitable for tackling diseases such as hepatitis C. Careful design of the vaccine vectors is advised to ensure their efficacy and safety, and as hepatitis C is a persistent infection, it may be advisable to design Rads containing genes encoding for non-structural proteins in preference to structural proteins.

Yellow fever vaccine: direct challenge of monkeys given graded doses of 17D vaccine

In rhesus monkeys a wide dosage range of 17D yellow fever (YF) vaccine extending to a level even below that recommended for vaccination of man elicited an immune response providing solid protection to challenge with virulent YF virus. Forty-three of 45 monkeys vaccinated with 10(2.3) or greater weanling mouse mean lethal doses of 17D vaccine were resistant to challenge 20 weeks later with virulent Asibi strain YF virus. Monkeys given graded doses of lesser amounts of vaccine were progressively more susceptible to challenge. With a vaccine dose >/= 10(2.3) weanling mouse mean lethal doses, plaque neutralization (PN) seroconversion rates were 90% or greater, whereas hemagglutination-inhibiting (HI) and complement-fixing (CF) seroconversion rates were unrelated to vaccine dosage and were generally in the range of 20 to 80%. Ninety-six percent (51 of 54) of immune monkeys had PN titers >/=0.7 log(10) (fivefold) neutralization index as compared to approximately 55 to 65% who showed HI or CF titers >/=2 log(2) (fourfold) neutralization index. After challenge with Asibi strain YF virus, antibody titers of all three tests increaed equally. In rhesus monkeys PN antibody titers were well correlated with YF immunity, whereas HI and CF antibody titers were not.

Yellow fever vaccine. V. Antibody response in maonkeys inoculated with graded doses of the 17D vaccine

A dosage equal to or greater than approximately 3.4 Dex (decimal exponent, log(10)) weanling mouse intracerebral 50% lethal dose (LD(50)) was sufficient to elicit a yellow fever antibody response, as determined by the plaque neutralization (PN) test, in better than 90% of vaccinated rhesus monkeys. Lower dosages were progressively less effective in terms of PN titers and the PN and hemagglutination-inhibition serological conversion rates observed. A dose of between 3.4 and 4.2 Dex weanling mouse intracerebral LD(50), or one-tenth to one times the dosage recommended for man, provided an optimal antibody response in monkeys. In rhesus monkeys, in contrast to the findings for man, pre-existing yellow fever antibody did not interfere with the antibody response to yellow fever vaccine. The PN test was felt to be a more sensitive and specific indicator of yellow fever antibody in rhesus monkeys after vaccination than the hemagglutination inhibition or complement fixation tests.