The neutralizing antibody response to 17D vaccines has been evaluated in several uncontrolled studies since the late 1930s. In 24 studies conducted world-wide between 1962 and 1997 using 17D vaccines involving a total of 2, 529 adults and 991 infants and children, the seroconversion rate was greater than 91% in all but two studies and never lower than 81%. There were no significant age-related differences in immunogenicity.¹

Five of these 24 studies were conducted in the US between 1962 and 1993 and included 208 adults who received YF-VAX vaccine. The seroconversion rate was 81% in one study involving 32 subjects and 97% to 100% in the other four studies.^11-15

In 2001, YF-VAX vaccine was used as a control in a double-blind, randomized comparison trial with another 17D-204 vaccine, conducted at nine centers in the US. YF-VAX vaccine was administered to 725 adults ≥18 years old with a mean age of 38 years. Three hundred twelve of these subjects who received YF-VAX vaccine were evaluated serologically, and 99. 3% of them seroconverted with a mean LNI of 2. 21. The LNI was slightly higher among males compared to females and slightly lower among Hispanic and African-American subjects compared to others, but these differences were not significant with respect to the protective effect of the vaccine. There was no difference in mean LNI for subjects < 40 years old compared to subjects ≥40 years old. Due to the small number of subjects (1. 7%) with prior flavivirus immunity, it was not possible to draw conclusions about the role of this factor in the immune response.¹⁶

Results of one clinical trial involving 33 HIV-positive adults residing in the US indicate that the seroconversion rate to 17D-204 vaccine may be reduced in these patients.¹⁷

In pregnancy or in immunosuppressed individuals the seroconversion rate after administration of yellow fever vaccine may be significantly reduced.¹⁸

Existing data suggest that the small percentage of immunologically normal subjects who fail to develop an immune response to an initial vaccination may do so upon re-vaccination.¹⁹

In two separate clinical trials of 17D-204 vaccines, 90% of subjects seroconverted within 10 days after vaccination, ²⁰ and 100% of subjects seroconverted within 14 days.¹¹ Thus, International Health regulations stipulate that the vaccination certificate for yellow fever is valid 10 days after administration of YF-VAX vaccine.²¹

REFERENCES

1. Monath TP. Yellow Fever. Plotkin SA, Orenstein WA (eds. ). Vaccines. 3^rd Edition, WB Saunders Company. 1999;815-879.

2. Recommendations of the Advisory Committee on Immunization Practices (ACIP). Yellow Fever Vaccine. 2002. MMWR 2002;51(RR17):1-10.

3. Teichmann D, et al. A haemorrhagic fever from the Côte dIvoire. 1999. Lancet 354:1608.

4. ACIP. Fatal yellow fever in a traveler returning from Venezuela, 1999. MMWR 49:(14);303-305, 2000.

5. McFarland JM, et al. Imported yellow fever in a United States citizen. Clin Infect Dis 25:1143-1147, 1997.

6. Centers for Disease Control and Prevention (CDC) Fatal yellow fever in a traveler returning from the Amazonas, Brazil, 2002. MMWR 2002;51(15):324-325.

7. World Health Organization (WHO). Imported case of yellow fever, Belgium. Weekly Epidemiological Record 2001;76:357, 365.

8. Barros MLB, Boecken G. Jungle yellow fever in the central Amazon. Lancet 1996;348:969-970.

9. Mason RA, et al. Yellow fever vaccine: Direct challenge of monkeys given graded doses of 17D vaccine. Appl Microbiol 1973;25(4):539-544.

10. Requirements for yellow fever vaccine. WHO Technical Report Series 594:23-49, 1976.

11. Wisseman CL, et al. Immunological studies with Group B arthropod-borne viruses. Am J Trop Med Hyg 1962;11:550-561.

12. Dukes C, et al. Safety and Immunogenicity of Simultaneous Administration of Typhim Vi (TV), YF-VAX (YV), and Menomune (MV). [abstract]. American Society for Microbiology. The 36^th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC):1996; September 15-18:159.

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