Study of BK1803 in Healthy Infants

Inclusion Criteria: Subjects meeting all of the following inclusion criteria at the time of the first vaccination with the study drug will be eligible for study participation. 1. Japanese healthy infants aged \>= 2 months and \< 7 months at the time of the first vaccination with the study drug. However, although subjects meeting the following definition of "subjects who should receive a vaccination with care" may participate in the study, whether or not to enroll them in the study should be decided carefully by the (sub)investigator. 2. Subjects from whose legal guardians (persons with parental rights) written consent to study participation has been obtained. Subjects Who Should Receive A Vaccination With Care 4) applies only to the Main Cohort. 1. Subjects who clearly have underlying diseases, such as cardiovascular disease, kidney disease, liver disease, blood disease, respiratory disease, or a developmental disorder 2. Subjects who have developed a pyrexia within 2 days after receiving a vaccination in the past 3. Subjects with a past history of convulsions 4. (Main Cohort only) Subjects with the gestational age \<37 weeks, or who weighed less than 2500 g at birth Exclusion Criteria: Subjects meeting any of the following exclusion criteria at the time of the first vaccination with the study drug will be excluded from study participation. However, (15) applies only to the Safety Lead-in Cohort. 1. Subjects who have received diagnoses of immunodeficiencies in the past, or who are currently receiving treatments that cause immunosuppression 2. Subjects with close relatives (up to third degree of kinship) who have a congenital immunodeficiency 3. Subjects who might experience a serious allergy in response to a food, drug product (especially any component of the study drug), etc. 4. Subjects who have had Haemophilus Influenzae type b (Hib) infections, pertussis, diphtheria, tetanus, acute poliomyelitis, or hepatitis B in the past 5. Subjects who have received a vaccination against Hib, pertussis, diphtheria, tetanus, polio, or hepatitis B in the past 6. Subjects who have received hepatitis B immunoglobulin (HBIG) for the prevention of vertical infection in the past 7. Subjects who were born to a mother who had received a vaccination against Hib, pertussis, diphtheria, tetanus, polio, or hepatitis B (including a combination vaccine such as diphtheria-pertussis-tetanus vaccine) during pregnancy 8. Subjects who are receiving anticoagulant therapy or who have thrombocytopenia or coagulation disorder 9. Subjects with fibrodysplasia ossificans progressiva 10. Subjects who have received live vaccines within 27 days before the first vaccination with the study drug 11. Subjects who have received inactivated vaccines/toxoids or mRNA vaccines within 6 days before the first vaccination with the study drug 12. Subjects who have received blood transfusions, immunosuppressants (except for topical drugs), or immunoglobulin preparations in the past 13. Subjects who have received corticosteroids (except for topical drugs) at a prednisolone dose level equivalent of 2 mg/kg/day or higher in the past 14. Subjects who have participated in another clinical study and received another study drug within 12 weeks before obtaining informed consent 15. (Safety Lead-in Cohort only) Subjects with the gestational age \<37 weeks, or who weighed less than 2500 g at birth 16. Subjects otherwise judged to be unsuitable for participation in this study by the (sub)investigator