Immunogenicity and safety of a SARS-CoV-2 inactivated vaccine (CoronaVac) co-administered with an inactivated quadrivalent influenza vaccine: A randomized, open-label, controlled study in healthy adults aged 18 to 59 years in China

•This is the first report of the co-administration of CoronaVac with IIV4 in adults aged 18–59 years.•The occurrence of adverse reactions (AR) in all groups was no more than 20% and 98.9% of the ARs were mild or moderate.•Both concomitant or separate administration of CoronaVac and IIV4 could induce sufficient immunogenicity.•A slight interference with the immune response to CoronaVac was observed in participants in C2 subgroup.•At least two doses of CoronaVac are needed to fulfill the primary immunization of inactivated COVID-19 vaccine.