an interval of 21 days or more between the doses. A total of 12,396 volunteers participated in the study between 21 July and 16 December 2020. All participants received at least one dose of the vaccine or placebo. Of this total, 9,823 participants received both doses. Further detail published by Sinovac showed an efficacy against all symptomatic cases, against cases that require medical treatment, and against severe, hospitalized and fatal cases. In the placebo group (N=4870), there were 168 COVID cases, 30 cases requiring medical attention, and 10 severe cases including one death. In the vaccine group (N=4953), there were 85 COVID cases, 5 cases requiring medical attention, and no severe cases or deaths. On 1 April 2021, a preliminary report from a phase III clinical trial in Chile revealed that CoronaVac is safe and induces humoral and cell-mediated immunity in adults (18–59 years old) and the elderly (60 years or older) similar to previous phase II trials conducted in China with the same age groups and immunization schedule consisting of two doses with a 14-day interval. The side effects were mild and local, mainly limited to pain at the injection site, which was more common in adults. Seroconversion rates for adults 14–28 days after the second dose were 95.6% for the IgG specific against the S1-RBD (receptor binding domain of the S1 subunit of the spike protein) and 96% for neutralizing anti-S1-RBD IgG. For the elderly, seroconversion rates were 100% 14 days and 87.5% 28 days after the second dose for the S1-RBD specific IgG, 90% 14 days, and 100% 28 days after the second dose for neutralizing anti-S1-RBD IgG. As found in studies in animals, seroconversion rates for IgG specific against the N (nucleocapsid) protein were weak for both groups, although CoronaVac contains significant amounts of the N protein. A robust increase of T helper cells (CD4+) secreting interferon gamma was detected 14 days after both doses in response to stimulation with peptides of the S protein and of other viral particles, but the response to S protein peptides was reduced in the elderly due to a natural reduction of activated CD4+ T cells in this age group, as found in studies of other vaccines. The immune response of cytotoxic T cell (CD8+) was not as robust. The observed CD4+ T cell response is considered a balanced immune response capable of viral clearance and is similar to that