In a previous study by the researchers' group, the researchers' investigate the duration of yellow fever post-vaccination immunity in vaccinated children between 9 and 23 months of age. However, in this study, samples of children in the pre-vaccine period, also known as unvaccinated children samples (NV) have not been investigated. It is believed that to seek evidence about the immune status in the medium and long term after vaccination against yellow fever is necessary to investigate paired samples of children not vaccinated (NV), with re-evaluation 30-45 days after primary vaccination. The proposed study is to consolidate aspects of humoral (neutralizing antibodies) and cellular (phenotypic and functional parameters of T cells and memory B) by means of complementary longitudinal investigation children, 9-23 months old, unvaccinated (NV) and 30-45 days after primary vaccination.

A sample of 60 children aged 9 to 23 months paired in the pre-vaccine period (NV) and 30-45 post-primary vaccination days will be selected at health facilities in the metropolitan region of Belo Horizonte.

Children whose mothers consent to participate in research, have collected blood and elements of past medical history recorded. Analyses of humoral and cellular biomarkers will be used in a comparative study with other post-vaccination periods previously analyzed in children during the first phase in study funded by the Ministry of Health.

specific objectives:

1. Estimate and compare the proportion of positivity and the geometric mean plasma titers of neutralizing antibodies against yellow fever;
2. Assess lymphocyte frequency T and B memory induced in vitro by the vaccine antigen 17DD substrains, and...
3. Quantify the lymphocytes T CD8 + producers of intracytoplasmic cytokines induced in vitro by the vaccine antigen 17DD substrains in unvaccinated children and primed aged between 9 and 23 months, categorized according to the vaccination time 30-45 days.

Inclusion Criteria:

Will be eligible for the study:

• children 9-23 months of age;
• both sexes;
• children who will be vaccinated against yellow fever, whose parents or guardians sign the consent form.

Exclusion Criteria:

• children who received another vaccine within 30 days before or after the yellow fever vaccine;
• children which have permanent or temporary immunosuppression, including those treated with corticosteroids at doses higher than 2 mg / kg, and those with disease autoimmune diseases, transient or permanent immunosuppression induced diseases (cancer, AIDS, etc.) or treatment (immunosuppressive drugs, radiotherapy, etc.). Corticosteroids (such as prednisone, dexamethasone) for less than two weeks, or by inhalation or topical use, do not indicate exclusion from the study, but must be registered in the questionnaire (Annex II);
• children with hemoglobinopathies;
• children with a history of blood transfusion or treatment with hyperimmune serum up to 90 days prior to blood collection.

The study provides immunoassays paired before and after anti-yellow fever vaccination in children of both sexes, of any color, class or social group. This will be necessary 60 paired samples (pre-vaccine period and 30-45 days after primary vaccination) of children aged 9 to 23 months, living in areas where YF vaccine makes up the basic immunization calendar and will receive the vaccine in the routine of UBRs in the first two years of life. This study will be conducted with samples collected from children living in Ribeirão das Neves - Minas Gerais. It will be collected 7 mL of whole blood collected in Heparin Sodium. It will be allowed to make two attempts to collect only. These samples will be used for the realization of the humoral and cellular immunity study.