Although the vaccination is the preferred method of influenza prevention, there are some occasions on which a postexposure prophylaxis (PEP) is required. Two neuraminidase inhibitors (NAIs) may be used in chemoprophylaxis in children: oral oseltamivir, and inhaled zanamivir. Both, oseltamivir and zanamivir, are effective in treatment and in prophylaxis of influenza, and the efficacy is calculated to reach 70-90%. Oseltamivir is used more frequently, since zanamivir is licensed in older children (5 years of age and above), and children under the age of 5 years are at higher risk of influenza complications. Oseltamivir use correlated in children with higher risk of vomiting, with no increased risk of other adverse events, including those observed in adult patients (nausea, renal events, and psychiatric effects). The PEP may be indicated by individual patient's characteristics (e.g. patients in high-risk group) or epidemiological reasons, i.e. prevention of institutional outbreaks.The one research that analyzed efficacy of 3-days PEP versus 7 or 10-days and showed overall efficacy of shorter oseltamivir prophylaxis to be high and comparable to that of longer regimens. The study included several pediatric patients and made the investigators perform such an analysis in pediatric population. In this randomized controlled trial, the investigators aimed to compare efficacy, safety, and costs of 3 versus 7-days prophylaxis with oral oseltamivir in children hospitalized. The hypothesis is that 3-days duration of PEP is not less effective than 7-days PEP, and patients might gain from lower number of adverse reactions related to drug administration.

Only applies to patients hospitalized due to other than influenza reasons. If a proven contact with influenza has taken place (influenza diagnosed by signs/symptoms and positive rapid influenza diagnostic test and/or PCR), a patient may be enrolled.

Inclusion Criteria:

• patients hospitalized at the Pediatric Ward
• age: 0-18 years old
• confirmed contact with a person diagnosed with influenza
• patient's, patient's parent/tutor's informed consent

Exclusion Criteria:

• lack of an informed consent
• more than 48 hours after the first contact with influenza
• severe adverse reaction to the drug- discontinuation of the prophylaxis
• important to a parent/tutor drug intolerance (e.g. lack of tolerance of a drug's taste)
• new contact with influenza after chemoprophylaxis has finished