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Background: Lower respiratory tract infections remain a major cause of morbidity and mortality in infants worldwide. Newborns possess an immature immune system but acquire passive immunity through maternal antibodies transferred via the placenta (IgG) and breast milk (IgA). Maternal vaccination may enhance this protection. This study aimed to quantify antibody levels against respiratory viruses in serum and breast milk from lactating women. Methods: Serum and breast milk samples were collected from 26 lactating mothers. Antibody levels were measured using an indirect enzyme-linked immunosorbent assay (ELISA) targeting seven viral antigens: influenza A (A/Thailand, A/California), influenza B (B/Phuket, B/Austria), SARS-CoV-2 (Spike and receptor-binding domain, RBD) and RSV F pre-fusion protein. Antibody isotypes IgG, IgA and IgM were analysed. Results: Virus-specific IgG and IgA antibodies were detected in all samples. Breast milk showed the highest levels of IgA, whereas serum contained higher IgG levels. A moderate positive correlation was observed between serum and milk IgG. No correlation was found between serum IgG and milk IgA, but both levels were elevated. Conclusions: Breast milk and serum contain relatively high levels of antibodies against the tested respiratory viruses. The elevated levels of serum IgG and milk IgA indicate a coordinated defence between systemic and mucosal immunity in response to infections. The levels and correlation of specific isotypes point to the source of the antibodies: milk IgG probably originates from the blood, whereas milk IgA is produced locally.