This is a clinical trial (randomized, double-blind, placebo-controlled Phase I/II) to evaluate the safety and immunogenicity of two formulations of the monovalent influenza vaccine candidate A (H5N8) (inactivated, fragmented, and adjuvanted with IB160) from the Instituto Butantan in adults and older adults. Safety will be assessed by the frequency (n, %) of participants with solicited (local and systemic) and unsolicited adverse events reported within 7 days post each vaccination; as well as the frequency of adverse reactions post causality evaluation. Immunogenicity will be assessed by seroprotection and seroconversion rates in the 21 days after the second dose.