This is a multicenter, prospective, observational, community-based study designed to evaluate the incidence of respiratory syncytial virus (RSV) infection among community-dwelling elderly individuals in China, as well as their cohabiting older adults and children, over a 24-month period. During the study, home visits will be conducted to collect nasopharyngeal (NP) swabs from all community residents aged ≥50 years who report symptoms of acute respiratory infection (ARI). Pathogen detection will be performed using both point-of-care testing (POCT) and centralized laboratory analysis.

The study will cover about 10 communities representing diverse geographical locations, economic conditions, and population densities.

Inclusion Criteria:

• CRE-ARI group:

  1. Male or female adults aged ≥50 years (calculated based on date of birth and date of visit).

  2. Presence one of the following conditions of ARI case definition :

     • At least two respiratory symptoms/signs within 48 hours prior to self-reporting OR
     • At least one respiratory symptom/sign + one systemic symptom/sign within 48 hours prior to self-reporting

  3. Provision of written informed consent by the participant or their legal guardian, in accordance with local EC (Ethics Committee) requirements.

• CAI group:

  1. In the household, include at least one patient meeting the inclusion criteria (index case[s]) of CRE-ARI group.
  2. Male or female adults aged ≥50 years or children aged<5 years (calculated based on date of birth and date of visit).
  3. In the household, apart from the index case(s), there are two or more members who meet the criteria (2) AND are willing to participate, regardless of whether these household members exhibit ARI symptoms.
  4. Provision of written informed consent by the participant or their legal guardian, in accordance with local EC (Ethics Committee) requirements.

Exclusion Criteria:

1. Enrollment in the CRE-ARI group within the preceding 14 days.
2. Enrollment in the CRE-ARI group has been more than 4 times during the previous 12 months (applicable only to the CRE-ARI group enrollment).
3. For CAI group, if a household has been enrolled into CAI group within the previous 12 months.
4. Residence outside the designated study catchment area.
5. Unwilling to provide consent/assent (if applicable) to participate.
6. Unwilling to provide biological samples.
7. Any other situation deemed inappropriate for participation by the investigator, including potential risks to the subject upon enrollment or factors that may affect the interpretation of study results, such as clinical judgment, safety concerns, or the integrity of the study.

The study will cover approximately 10 communities. All individuals aged 50 years and older residing in these communities will be considered potential participants. Those who self-report symptoms of acute respiratory infection (ARI) will be enrolled. In addition, if their cohabitants meet the inclusion criteria, they will be enrolled as a family unit.

After thesis defense the study may be published internationally to be available for the public.