Impact of Prior Exposure to Anisakis simplex Antigens on Immune Response During SARS-CoV-2 Vaccination.

Parasitic infections can induce immunomodulatory effects that may influence host responses to vaccination. Chronic exposure to Anisakis antigens could modify T-cell and antibody profiles, potentially affecting SARS-CoV-2 vaccine effectiveness in endemic populations. We aimed to determine whether prior exposure to Anisakis alters SARS-CoV-2 vaccine-induced immunity. Forty healthy volunteers were evaluated at five time points before and after receiving either the Pfizer-BioNTech or Oxford-AstraZeneca COVID-19 vaccines. Serum levels of anti-Anisakis and anti-SARS-CoV-2 antibodies were quantified by ELISA, whereas T- and B-cell subsets and apoptosis rates were analysed by flow cytometry. SARS-CoV-2 vaccination induced significant changes in anti-Anisakis antibody levels, particularly IgG and IgA, which increased after the second dose and persisted throughout follow-up. Individuals seropositive for anti-Anisakis antibodies exhibited higher post-vaccination anti-SARS-CoV-2 IgG levels, and anti-Anisakis IgA positivity correlated with stronger anti-SARS-CoV-2 IgA responses at specific time points. Positive associations were also observed between anti-Anisakis antibody levels and circulating T- and B-cell subsets. Prior sensitization to Anisakis antigens appears to influence both the magnitude and quality of the adaptive immune response elicited by SARS-CoV-2 vaccination, affecting antibody profiles and lymphocyte subset dynamics. Conversely, COVID-19 vaccination can alter humoral reactivity to Anisakis antigens, indicating bidirectional immunomodulation between parasite exposure and vaccine-induced immunity.