Subjects will be recruited and divided into 3 groups:

Experimental Group (468 subjects): 1st dose : combined vaccination of COVAX+PPV23, 2nd dose: combined vaccination of COVAX+IIV4; Control Group A (468 subjects): 1st dose: COVAX only, 2nd dose: COVAX only; Control Group B (468 subjects): 1st dose: PPV23 only, 2nd dose: IIV4 only.

Blood samples will be collected 3 times:

before the 1st dose of vaccinatioin; before the 2nd dose of vaccination; 28 days after the 2nd dose of vaccination.

The immunogenicity and safety of both experimental and control groups will be analyzed.

This randomized, parallel, controlled study is designed to evaluate the immunogenicity and safety of simultaneously immunization of COVAX + PPV23 and COVAX + IIV4.

1404 subjects are divided into 3 groups, including 1 experimental group and 2 control groups (control group A and B). Each group includes 468 subjects respectively.

Experimental Group (468 subjects) will receive: 1st dose : combined vaccination of COVAX+PPV23, 2nd dose: combined vaccination of COVAX+IIV4; Control Group A (468 subjects) will receive: 1st dose: COVAX only, 2nd dose: COVAX only; Control Group B (468 subjects) will receive: 1st dose: PPV23 only, 2nd dose: IIV4 only.

Specifically, each group will be divided again. Each 468-subject group is divided again into 2 age-based subgroups: (1) 18-59 years old; (2) ≥60 years old.

Each subgroup includes 234 subjects respectively. Blood samples will be collected 3 times: (1) before the 1st dose of vaccinatioin; (2) before the 2nd dose of vaccination; (3) 28 days after the 2nd dose of vaccination.

To evaluate the immunogenicity, the investigators will detect and compare the neutralizing antibody levels, the seroconversion rates and antibody geometric mean concentrations.

The safety of all groups will be monitored as well.

Inclusion Criteria:

• ≥18 years old when enrolled;
• Participants signing the informed consent;
• Being able to participate in all planned follow-ups and accomplish all research procedures including completing a diary card and coming back for interview;
• Having not been administered any pneumonia vaccine or COVID-19 vaccine, and having not been administered an influenza vaccine in the current season before enrollment;
• ≥14 days from the most recent vaccination;
• Before enrollment, the body temperature is ≤37.0C as confirmed by medical history and clinical examination.

Exclusion Criteria:

• Having a history of COVID-19 or a positive nucleic acid test for COVID-19;
• Having a history or a family history of allergy, convulsion, epilepsy, brain disease, or mental illness;
• Being allergic to any component of vaccines, or having a history of severe allergic reaction to any vaccine;
• Having immunodeficiency disorder, during treatment of malignant tumor, having immunodeficiency induced by immunosuppressants (oral steroid hormones) or HIV, or having a family member with close contact who suffers from congenital immune disorder;
• Having injection of non-specific immunoglobulin within 1 month prior to enrollment;
• Having acute febrile illness or communicable disease;
• Having a history of confirmed thrombocytopenia or other coagulation disorders, which may cause contraindications for a subcutaneous injection;
• Having severe chronic disease or acute exacerbation of chronic disease, or having uncontrolled hypertension or diabetes;
• Having various infectious, pyogenic, or allergic skin diseases;
• Women in pregnancy or lactation or participants who plan for a baby within 3 months after vaccination should delay the vaccination;
• Having any condition that may affect trial assessment as determined by researchers.

Exclusion criteria for the second dose:

• Having any serious adverse event related to the first dose vaccination;
• After vaccination, having systemic adverse reaction or allergic reaction with a severity of grade 3 or higher as determined by researchers;
• Having a newly emerging condition that meets the exclusion criteria for the first dose;
• Having any condition that may affect trial assessment as determined by researchers.

Serum testing technicians were masked.