Background: Each Belgian winter season is characterized by a wave of influenza like and respiratory symptoms. Especially, the elderly people are more vulnerable to be infected by influenza, but also RSV. The recent COVID-19 pandemic and eventually a next wave, will increase the prevalence of influenza like and respiratory symptoms.

Method: A multicentre non-commercial cohort study will be conducted in nursing home staff and residents during the Winter season 2020-2021.

Objectives: Primary objective is the difference in incidence of influenza like and respiratory symptoms between cases (cases have evidence of past infection with SARS-CoV-2, referred to as Covid +) and controls (controls have no evidence of previous infection and are referred to as Covid -). The primary outcome analysis as well as the secondary outcome analyses will use two strata: nursing home staff and nursing home residents. The secondary objectives are the difference in incidence of COVID-19, influenza, RSV infections confirmed by PCR between cases and controls, to define a correlate of protection in the covid + group against re-infection with SARS-CoV-2 based on the study of the pre-existing antibody profile (antigen specificity, antibody type and antibody level) at the time of re-exposure. A multiplex assay will be used to assess the antibody profile. Finally, to study the COVID-19 disease severity (7 point WHO ordinal scale, this includes a.o. hospitalisation, mechanical ventilation need and ICU admission, mortality) based on the presence/absence of pre-existing antibodies and the pre-existing antibody profile. For other respiratory infections we will study the need for hospitalization and mortality.

Inclusion Criteria:

• Being a staff member or resident from te participating nursing homes
• The subject should be aged 18 years or older, have a Belgian National Number and be insured by a Belgian sickness fund.
• All subjects that are cognitively capable to give consent themselves to participate in the study.

Exclusion Criteria:

• Insufficient knowledge of the Dutch or French language.
• Residents with a previous diagnosis of dementia having a mini-mental state examination (MMSE) below 18/30.
• Participants which life-expectancy is less than the time of the study.
• Staff members not expected to continue working at the nursing home during the winter season.
• Participants for whom veins are not accessible for simple periphery blood puncture.