Rationale: Respiratory tract infections (RTI) are a major cause of morbidity in young children in high- income countries and the major cause of mortality in developing countries. Causative bacteria and viruses are regular residents of the nasopharynx of asymptomatic individuals (colonization) and live there together with other presumed harmless commensals, without causing disease. These non-pathological infections/colonization episodes are important for transmission, intermediate step to disease and boost immune responses. The investigators recently validated the use of minimally-invasive nasal sampling methods that can be done at home for the study of host and microbial parameters in adults and children. In this study the investigators will focus on the daily microbial and immunological composition of the nasopharynx during health in relation to symptoms.

Primary objective: Associate acquisition of pneumococcal colonisation with levels of pre-existing polysaccharide specific memory B cells.

Secondary objectives include: Validate the use of synthetic absorptive matrices (SAM) for detection of respiratory pathogens versus nasopharyngeal swabs (NPS) and saliva; Assess dynamics of URT infection/colonisation and examine its relationship with symptoms, host responses and microbiota; Measure transmission between children and parents and immune responses in parents.

Study design: Prospective community-based cohort study.total of 45 children, aged 1-5 years old attending daycare or (pre-)school, will be included, including a pilot of 10 children to assess tolerability. If there are insufficient pneumococcal acquisitions in the study to assess the primary outcome, additional children can be recruited in groups of 3 or 4 children (up to 10). For a subset of participating children, both parents will be asked to self-collect daily saliva during the study.

Primary study parameters: Frequency of systemic polysaccharide specific B cells in children that become colonised during the study versus children that do not become colonised Secondary study parameters: Dynamics of respiratory bacteria and viruses during URT infection/colonisation. Presence and load for bacteria and viruses in children in SAM versus saliva and NPS. Local microbiota and immune profiles and association with infection/colonisation and symptomology. For a subset of parents, daily presence and load of bacteria and viruses as well as host immune factors measured in saliva.

Inclusion Criteria:

• Written informed consent obtained from all legal representatives, for example both parents.
• Child aged 1-5 years of age attending day care, peuterspeelzaal or school at least 2 (half) days a week.
• Parents ability and willingness to adhere to protocol-specified procedures, including availability of a freezer at home to store samples. This does not include donation of saliva by parents themselves, which is related to a secondary endpoint.

Exclusion Criteria:

• History of respiratory tract infections requiring hospitalization
• Current use of antibiotics, or antibiotics use in past four weeks
• Use of immune-altering medication (such as steroids, including inhaled corticosteroid)
• No history of severe concomitant disease (severe congenital heart disease, bronchopulmonary dysplasia, prematurity <32 weeks, cystic fibrosis, sickle cell disease, congenital or acquired immunodeficiency disorders, cardiovascular disease, neuromuscular disorders, oncology patients or major congenital anomalies)

Children aged 1-6 years old, as these are a main population at risk for severe respiratory tract infections, and a main source of transmission. The study team will recruit families from the area of North/South-Holland as this will facilitate the home visits.

If any samples or data are shared to perform analyses as described in the study protocol, either inside or outside EU, only pseudonymized material is shared to ensure participant privacy.

This can be done upon request and following the signing of MTA/DTA.

Individual datasets might be placed on repositories with restricted access or fully anonymized (such as Zenodo or flowrepository.org) upon publication.