A Prospective Epidemiological Cohort Study Evaluating Occurrences of Influenza-like Illness.

Inclusion Criteria: 1. Written informed consent. 2. Healthy male or female subjects, as determined by medical history and medical examination (as needed). 3. Between the ages of 18 and 55 years, inclusive. 4. Subject who has fully been vaccinated with licensed severe Acute Respiratory Syndrome SARS-CoV-2 (COVID-19) vaccine(s) according to national recommendations for the corresponding population group. 5. Reliable and willing to make themselves available for the duration of the study, and willing and able to follow study procedures. 6. Ability and technical possibility for completing an electronic Diary (eDiary) and electronic Flu-PRO® questionnaire (ePRO). Exclusion Criteria: 1. Previous influenza vaccination within 6 months before the day of enrollment or planned to receive during the study duration. 2. Any known or suspected immunodeficient conditions. 3. Current history of significant uncontrolled medical illness such as diabetes, hypertension, heart, renal or hepatic diseases, as judged by the Investigator. 4. Known infection with human immunodeficiency virus (HIV), hepatitis C virus (HCV), or hepatitis B virus (HBV) to the subject's knowledge. 5. Administration of any investigational or non-registered drug or vaccine within 3 months prior to enrollment, or planned administration of any such product during the whole study period. 6. History of receiving blood, blood components or immunoglobulins within 3 months prior to the day of enrollment, or planned to receive such product during the whole study period. 7. Behavioral or cognitive impairment, or psychiatric disease that, in the opinion of the Investigator, may interfere with the subject's ability to participate in the study. 8. Past (stopped less than 6 months before enrolment) or current history of alcohol or drug abuse, or current smoking habit above 10 cigarettes per day, or current vaping. 9. Treatment that can affect immune response such as systemic or high dose inhaled corticosteroids (\>800μg/day beclomethasone or equivalent; occasional inhaled corticosteroids for asthma therapy are allowed), radiation treatment, cytotoxic drugs, or current or recent (within 30 days before study entry) chronic or prolonged (\>10 days) use of systemic non-steroidal antiinflammatory drugs, interferon, immunomodulators, allergy shots, as judged by the Investigator. 10. Individuals with history of any illness that, in the opinion of the Investigator, might interfere with the results of the study or pose additional risk to the subjects due to participation in the study.