Introduction
coronavirus disease 2019
BREAKTHROUGH INFECTION, CORONAVIRUS DISEASE 2019
Breakthrough infection cases of coronavirus disease 2019 (COVID-19) are a continuing issue in the clinical arenaThe administration of a fourth vaccine dose (additional booster) was recommended by the WHO for individuals aged ≥60 years, immunocompromised patients, and healthcare personnel after they have received the third doseImmunity in those who had complete primary immunization was shown to be more efficiently restored by a heterologous booster than homologous boosters in clinical trials. In comparison to a third homologous dose of CoronaVac, administration of a recombinant adenoviral vectored vaccine, mRNA vaccine, or recombinant adenoviral-vectored ChAdOx1 nCoV-19 vaccine increased humoral and cellular immune responsesHere, we present the safety and immunogenicity results following heterologous booster immunization with Ad5-nCoV or homologous boosters with different inactivated SARS-CoV-2 vaccines (CoronaVac or Covilo) after three-dose priming with inactivated vaccine in healthy participants aged 18–80 years in a randomized, double-blind, parallel-controlled phase 4 trial. We further explored the potential benefits of anti-Omicron BA.4/5 protection, dynamic responses, and durations of antibody levels in individuals 3 months after the booster, as well as the relevant factors influencing the antibody responses.