The immune response of the mRNA vaccine CS-2034 as a heterologous booster before and after breakthrough infection: A follow-up study.

BACKGROUND

The emergence of highly mutated SARS-CoV-2 variants challenges vaccine protection. Evaluating long-term immune protection from boosters is crucial. The experiment compares neutralizing antibody dynamics and ACE2-RBD binding inhibition induced by mRNA and inactivated vaccine boosters, and assesses the impact of breakthrough infections and age on immune responses.

METHODS

This study is based on a previous randomized, double-blind, parallel controlled trial. Healthy participants aged ≥18 who had received ≥3 inactivated COVID-19 vaccine doses. Volunteers (32 aged 18-59 and 32 ≥ 60) were stratified by age and vaccine type (24 mRNA, 8 inactivated in each age group). Pregnant women, those with severe chronic diseases or a history of allergies are excluded. Neutralizing antibodies were measured via microneutralization assay, and ACE2-RBD binding inhibition by high-sensitivity electrochemiluminescence. The study assessed changes in antibody levels and ACE2-RBD binding inhibition pre- and post-infection across age groups to evaluate vaccine efficacy and durability. (Registed number: ChiCTR2200064575).

RESULTS

The neutralizing antibody titers from mRNA vaccine showing stronger and more consistent responses against the original strain and variants like XBB and BA.5. After breakthrough infections, mRNA-vaccinated individuals show a significant and lasting boost in antibodies, while the increase in inactivated vaccine recipients is modest. By age, mRNA-vaccinated people over 60 experience a faster antibody decline post-vaccination, whereas the inactivated vaccine group sees an initial rise at three months before declining. For ACE2-RBD binding inhibition, mRNA vaccines maintain higher early-phase rates, while inactivated vaccines show a delayed rise at three months followed by decline.

CONCLUSIONS

mRNA vaccines induce stronger neutralizing antibodies and broader immunity than inactivated vaccines. Breakthrough infections boost antibody levels more in mRNA recipients than in inactivated vaccine recipients. Antibody decline is faster with mRNA vaccines in people over 60, while inactivated vaccines show a modest but weaker antibody increase against variants in the elderly.