Antivirals

OSELTAMIVIR(Tamiflu^®)

Oseltamivir is currently indicated for the treatment of influenza in patients over a year of age. A treatment course consists of 75mg caps bd for 5 days, and prophylaxis is one cap daily. It comes in powder for oral suspension for children. It is not licensed for infants under a year of age, nor for pregnant women, as there are no data in these groups.

Oseltamivir generally appears to be a safe drug - the most frequently reported side effects are transient nausea, vomiting, abdominal pain and headaches. The gastrointestinal reactions may be reduced if the drug is taken with food. Recent reports from Japan of neurological symptoms are believed to have been due to the influenza infection itself rather than the drug. There is evidence of viruses developing resistance to this drug, including in one strain of the H5N1 virus. However, in general resistant strains of influenza A viruses appear to be less effective at transmission.

The information below is sourced from an excellent summary of the use of Oseltamivir in pandemic influenza:
Ref: Ward P et al Oseltamivir (Tamiflu^®) and its potential for use in an influenza pandemic. JAC (2005) 55 suppl. S1, i5-i21

Clinical trials show that early usage (within 48 hours) leads to earlier resolution of the illness by 1 -2 days and a reduction in secondary complications by 30 - 60% in seasonal influenza. The data also show that the earlier the drug is given the greater the benefit.

As the pandemic virus does not exist yet, the efficacy of Oseltamivir is unknown. However, Oseltamivir has been shown to be effective against the H5N1 virus in the laboratory. In some humans infected with H5N1, the drug has been without effect, possibly as it has been given too late. The possibility remains that higher doses and/or longer durations of therapy may be warranted.

Used as prophylaxis it reduces the incidence of laboratory-confirmed seasonal influenza by up to 92%, reduces spreading and reduces complications. However it does not reduced the ability to form antibodies i.e. while using prophylaxis a person can still develop immunity to the virus if exposed. [This is because the drug does not prevent infection, but does prevent the offspring of the virus that have infected a cell form leaving it. By blocking the neuraminidase, the drug leads to clumping of the viral progeny on the sialic acid receptor]

A further important note from this paper is the comment that treatment of influenza is not an effective stratagem for preventing transmission of influenza in close contact scenarios, therefore control of an influenza outbreak is not possible solely by treating ill cases as they develop. This is because by the time treatment is started, the cases will already have been symptomatic and spreading virus for some time.

Refer to the datasheet from: http://www.medsafe.govt.nz/profs/Datasheet/t/Tamiflucapsusp.htm

Private market Usage

Oseltamivir is available as prescription-only. Currently, there are no stocks for private market sale in New Zealand and there are not expected to be any further stocks until the middle of 2006.

With many countries now stockpiling and the private market growing as the communications around a possible pandemic spread, Oseltamivir is in high demand internationally and the company Roche has re-prioritised supply to the Northern hemisphere for use during that influenza season. Other production facilities are currently being built around the world to match demand.