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Hand, foot, and mouth disease (HFMD), primarily caused by enterovirus A71 (EV-A71), poses significant pediatric health risks in the Asia-Pacific region, with vaccination being critical for prevention. Using longitudinal serological data from 1 585 Chinese children in a 2019–2020 prospective study (combining clinical trial and cohort samples), the present study assessed how pre-vaccination antibody titers and age influence EV-A71 vaccine response. EV-A71-specific antibodies were measured via a modified cytopathogenic effect assay, with geometric mean titers (GMTs) and fold increases (GMFIs) analyzed. At baseline, 3.9% (N = 61) of children were seropositive. After vaccination, those with high pre-existing titers exhibited higher GMTs, compared to those with low-level or undetectable titers (GMTs: 10.30 [95% CI: 9.09, 11.52] vs. 7.58 [95% CI: 6.86, 8.29] vs. 6.92 [95% CI: 6.82, 7.02]; GMFIs: 3.36 [95% CI: 1.97, 4.75] vs. 4.23 [95% CI: 3.48, 4.98] vs. 4.92 [95% CI: 4.82, 5.02]).

Generalized additive models revealed that predicted GMTs peaked (8.78) at a baseline titer of 10.20, while GMFIs declined with higher pre-vaccination titers. Vaccine-induced responses also varied with age, showing periodic increases in children under 36 months. Collectively, these findings quantify the effects of pre-existing antibodies and age on the antibody level following vaccination, highlighting the importance of tailoring immunization schedules based on both pre-existing immunity and age. Accordingly, immune-naïve children should be vaccinated as early as possible.