Title: An adaptive study of the pharmacokinetics of favipiravir in patients with severe influenza Study Design: An open label, single group assignment, adaptive study to evaluate the pharmacokinetics of favipiravir in adult patients with severe influenza.

In the first stage, participants will receive favipiravir 1600mg BID on day 1, followed by favipiravir 600mg BID for 9 days.

If the proportion of patients with a minimum observed plasma trough concentration above the MEC (20μg/ml) at all measured time points after the second dose is less than 80% then a second patient cohort will be recruited and will receive favipiravir 1800mg BID on day 1, followed by favipiravir 800mg BID for 9 days.

Intervention: The 1st stage: 1600mg BID on day 1, followed with 600mg BID for 9 days. Sample size: 15 The 2nd stage: 1800mg BID on day 1, followed with 800mg BID for 9 days. Sample size: 15 Population: Males and females aged 18 years or older admitted to hospital with a positive PCR test for influenza and a PaO2/FiO2≤300mmHg or/and on mechanical ventilation for severe lung infection on admission.

Sample size 15 or 30 severe influenza patients Research hypothesis The administration of oral favipiravir at either 1600mg/600mg BID or 1800/800mg BID will result in ≥ 80% patients achieving a minimum observed plasma trough concentration above the MEC (20μg/ml) at all measured time points after the second dose.

Phase: Phase 2a, PK, safety and feasibility study. Description of Study Agent: Favipiravir (T-705) a viral RNA-dependent RNA polymerase inhibitor.

Study Duration: 1 year Participant Duration: 38 days

Inclusion Criteria:

1. Hospitalized males or females with a positive PCR test for influenza virus infection

2. Adults aged ≥18years

3. PaO2/FiO2≤300mmHg or on mechanical ventilation

4. < 10 days since symptom onset

5. Negative pregnancy testing for childbearing age females (under 60 years)

6. Willingness to use contraception for 7 days after end of treatment

7. Informed consent

8. In addition, male subjects must:

   1. Agree not to donate sperm during the study and for 7 days following the last dose of study drug, and
   2. Agree to adhere strictly to one of the following contraceptive measures from the Screening Visit until 7 days after the last dose of study drug:

   i. abstain from sexual intercourse or ii. have a female partner using effective means of birth control as noted below or iii. use a condom with spermicide or a second barrier method by female partner.

Female subjects

a. Of child-bearing potential must agree to adhere strictly to one of the following approved contraceptive measures during the study and for 7 days after the last dose of study drug: i. abstain from sexual intercourse or ii. have a male partner incapable of fathering a child (eg, had a vasectomy at least 6 months with history of negative semen analysis prior Screening or iii. use of one of the following methods, in combination with condom and spermicide use by a male partner: nonhormonal intrauterine device (IUD); diaphragm; or hormonal contraceptives including oral contraceptives, injectable subdermal implants, hormonal IUD, or vaginal ring b. Be unable to bear children defined as one of the following: i. absence of a menstrual period for ≥12 consecutive months with FSH confirmation, ii. be 60 years of age or greater, iii. had surgical removal of uterus or removal of both ovaries, or iv. had undergone tubal ligation >6 weeks prior to Day 1 dosing

Exclusion Criteria:

1. Any condition that does not allow for safely following the protocol
2. Patient refusal to accept invasive organ support treatment if needed
3. Pregnant or breastfeeding
4. Any condition resulted to reception of renal replacement therapy
5. AST > 5 times upper of limit or Child Pugh score ≥ C
6. Serum uric acid level > 3 times upper level of normal (430 ummol/L) associated with symptoms of gout
7. Has a history of gout or is under treatment for: gout or hyperuricemia; hereditary xanthinuria; hypouricemia or xanthine calculi of the urinary tract
8. Has a history of hypersensitivity to an anti-viral nucleoside-analog drug targeting a viral RNA polymerase
9. Physician makes a decision that trial involvement is not in patients' best interest.
10. Currently or have been involved in another anti-influenza treatment trial in the last 28 days