YF-VAX Rx

In 24 uncontrolled studies conducted world-wide between 1962 and 1997 evaluating neutralizing antibody responses to 17D strain vaccines among a total of 2529 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. 

Five of these 24 studies were conducted in the US between 1962 and 1993 and included 208 adults who received YF-Vax. The seroconversion rate was 81% in one study involving 32 participants, and 97–100% in the other 4 studies. 

In 2001, YF-Vax was used as a control in a double-blind, randomized comparison trial with another 17D-204 strain vaccine, conducted at 9 centers in the US.

• YF-Vax was administered to 725 adults ≥18 years old with a mean age of 38 years.
• Three hundred twelve of these participants who received YF-Vax were evaluated serologically, and 99.3% of them seroconverted with a mean log10 neutralization index (LNI, measured by a plaque reduction assay) of 2.21.
• The LNI was slightly higher among males compared to females and slightly lower among Hispanic and African-American participants compared to others, but these differences were not associated with differences in protective effect of the vaccine. 

For most healthy individuals, a single dose of yellow fever vaccine provides long-lasting protection. In controlled studies where the immune response to vaccination was evaluated, the small percentage of immunologically normal individuals who failed to develop an immune response to an initial vaccination typically did so upon re-vaccination. In two separate clinical trials of 17D-204 strain vaccines, 90% of participants seroconverted within 10 days after vaccination, and 100% of participants seroconverted within 14 days. Therefore, International Health regulations stipulate that the vaccination certificate for yellow fever is valid 10 days after administration of YF-Vax.