Comparison of Safety of Different Vaccine Boosters Following Two-Dose Inactivated Vaccines: A Parallel Controlled Prospective Study

A vaccine booster to maintain high antibody levels and provide effective protection against COVID-19 has been recommended. However, little is known about the safety of a booster for different vaccines. We conducted a parallel controlled prospective study to compare the safety of a booster usingfour common vaccines in China. In total, 320 eligible participants who had received two doses of an inactivated vaccine were equally allocated to receive a booster of the same vaccine (Group A), a different inactivated vaccine (Group B), an adenovirus type-5 vectored vaccine (Group C), or a protein subunit vaccine (Group D). A higher risk of adverse reactions, observed up to 28 days after injection, was found in Groups C and D, compared to Group A, with odds ratios (OR) of 11.63 (95% confidence interval (CI): 4.22–32.05) and 4.38 (1.53–12.56), respectively. Recipients in Group C were more likely to report ≥two reactions (OR = 29.18, 95% CI: 3.70–229.82), and had a higher risk of injection site pain, dizziness, and fatigue. A gender and age disparity in the risk of adverse reactions was identified. Despite the majority of reactions being mild, heterologous booster strategies do increase the risk of adverse reactions, relative to homologous boosters, in subjects who have had two doses of inactive vaccine.