Coronaviruses and influenza viruses are respiratory viruses with a history of causing pandemics in humans. Vaccines are one of the best ways of limiting and containing their spread. However, these viruses frequently mutate and diverge into new, emergent variants. Because of this, vaccines based on the earlier variants are often significantly less effective at inducing protective immunity against them. This necessitates the updating of vaccines with the new variants, often annually, to ensure humoral and cellular immunity are protective against circulating variants. In the studies presented in this dissertation, we sought to analyze humoral and cell mediated immunity induced by vaccines against SARS-CoV-2 Omicron variants and avian influenza viruses. We also explored how SARS-CoV-2 breakthrough infections affect antiviral immunity. To accomplish this, we analyzed antibodies present in the serum samples taken from healthcare workers and AIV at-risk groups with ELISA and microneutralization tests. We also analyzed cell samples with activation induced marker assays combined with flow cytometry. We show that three vaccinations with Spikevax and Comirnaty mRNA vaccines induced high titers of long-lasting S1-specific antibodies with a good neutralizing capacity against early Omicron subvariants BA.1, BA.2, and BA.5. The fourth and fifth vaccine doses based on later Omicron subvariants were shown to induce high titers of S1-specific antibodies, that declined more slowly against later Omicron subvariants BQ.1.1, XBB.1.5 and JN.1. Even though vaccine induced antibodies were considered neutralizing and thus protective, the neutralization capacity of the antibodies against the newer Omicron subvariants was substantially reduced compared to older Omicron subvariants. The combination of breakthrough infection and vaccinations induced the highest neutralization capacity. Additionally, we showed that effective CD4+ and CD8+ T-cell responses are likewise induced. We showed that an inactivated subunit vaccine based on A/Astrakhan/3212/2020 (H5N8)-like strain induced antibodies with high neutralization capacity, and robust CD4+ T-cell activation against circulating H5N1 viruses of clade 2.3.4.4b. The results of this dissertation show that mRNA and inactivated subunit vaccines are adaptable platforms capable of inducing efficient humoral and cellular immune responses against emergent SARS-CoV-2 variants and AIV strains.