This observational study is designed as a cross-sectional survey of children from 6 months and 5 years old who visit the Vietnam Vaccination Center (VNVC) system in Hanoi, Danang and Ho Chi Minh City for vaccination services.

After getting the informed consent form from a parent/guardian for their child(s) to participate in the study, nasopharyngeal swab sample will be taken. His/her guardian will be interviewed face-to-face with information related to epidemiological characteristics, nutritional status, child's birth history, hospitalization history, vaccination history, recent antibiotic usage, breastfeeding time, and environmental and lifestyle factors.

The goal is to document the burden and risk factors of pneumococcal infections in the community of healthy Vietnamese children. The main questions it aims to answer are:

• What is the prevalence of pneumococcus in healthy children?
• What is the distribution of pneumococcus serotypes in the community of healthy Vietnamese children?
• What are risk factors of pneumococcus infection in the community of healthy Vietnamese children?

Inclusion Criteria:

• Children between 6 months and 5 years

Exclusion Criteria:

• Children with chronic diseases (including immunologic diseases, neoplastic disorders, renal, cardiac, hepatic, or hematologic diseases, broncho dysplasia, Down syndrome, chronic otitis media with effusion)
• Children with acute/chronic upper or lower respiratory tract infection
• Children have used antibiotics within 1 week prior to entering the study

Children from 6 months and 5 years old who visit the Vietnam Vaccination Center (VNVC) system in Hanoi, Danang and Ho Chi Minh city for vaccination services