This study is clinical trial (intervention study with external comparison group) to test
vaccination with reduced dose (half dose) of ChAdOx1 nCoV-19 (AZD1222), in a 2-dose
schedule with an interval of 8 weeks, including all adults aged 18 to 49 years from Viana
city - Espírito Santo (ES)/Brazil, on the incidence of new cases over 12 months following
treatment, compared to an external group from same state and adjusted for
socio-demographic and epidemiological variables. Cellular and humoral immunity will be
studied in a representative sub-sample compared to an external reference group (cohort of
health workers) vaccinated with standard dose. The outcomes will be evaluated at 28 days
after the second dose and follow-up after 6 months and 12 months. The main outcome will
be to decrease in 60% the incidence of new cases over 6 months after receiving the
vaccine. The clinical epidemiological variables will be obtained from e-SUS VS, e-SUS
notifica and datasus: number of cases, number of deaths with specific ICD for covid-19,
number of hospital admissions for covid-19; number of ICU admissions for the treatment of
SARS, total number of tests (RT-PCR) performed and positive. The cellular and humoral
immune response will be assessed by viral neutralization assay (neutralizing antibody
test), serological assay by chemiluminescence, determination of specific IgM and IgG
profiles, measurement of systemic soluble factors (chemokines, cytokines and growth
factors), stimulation antigen-specific peripheral blood mononuclear cells in vitro and
investigation of memory T and B lymphocytes and intracytoplasmic cytokines. It is
estimated to include 29,637 people in the study, to reach 85% vaccination cover of
individuals aged 18-49 years. The subsample size immunogenicity test is 600 individuals
from the eligible population, estimating losses of 15% and study power of 90%, alpha
error of 1%. The hypothesis of the study is that the reduction in the incidence of
covid-19 and the cellular and humoral immune response achieved with a half dose will be
similar to the reduction expected with the standard dose.