Yellow fever is an acute viral disease transmitted by mosquitoes in South America, Central America and Africa. It is more prevalente in males gender and the age above 15 years due to the greater exposure in the wild endemic area of yellow fever.

According to the World Health Organization (WHO), a single dose of the yellow fever vaccine is sufficient to maintain protective immunity against yellow fever for a lifetime, therefore a booster dose is not required. This issue is difficult to evaluate because there is no serological correlate of protection against yellow fever and seropositivity is defined with several cut-off points. Although studies indicate that the duration of protection after vaccination is long, many studies have demonstrated a reduction of the antibody titrer over the years. Consequently, there is more concern about people who live in endemic areas. For this reason, Brazil recommends revaccinating once at least until additional studies are performed.

It is important to know the duration of immunity induced by lower doses of YF vaccine. In our knowledge, there is a lack of clinical studies evaluating the immunity duration of the yellow fever vaccine with lower doses. This information is relevant to subsidize the routine recommendation of YF vaccine fractional dose for adults.

This is an observational study in young adult males who received the first dose of the yellow fever vaccine in 2009 when they participated in the "Dose-response study of the yellow fever vaccine 17DD produced by Bio-Manguinhos / Fiocruz.

Inclusion Criteria:

• Participated in the "Dose-response study of the yellow fever vaccine 17DD produced by Bio-Manguinhos / Fiocruz" in 2009.
• Not been revaccinated during the "Dose-response study of the yellow fever vaccine 17DD produced by Bio-Manguinhos / Fiocruz" of 2009.
• Not having received another dose of the 17DD yellow fever vaccine after participating in the 2009 "Bio-Manguinhos / Fiocruz" 17DD yellow fever vaccine dose-response study.
• Provide name, address, telephone and other information so that you can contact if necessary.
• Ability to understand and sign the TCLE.

Exclusion Criteria:

• Impossibility or refusal to collect blood

Male adults who received the first dose of the yellow fever vaccine in 2009 when they participated in the "Dose-response study of the yellow fever vaccine 17DD produced by Bio-Manguinhos / Fiocruz