Influenza

The currently available antiviral drugs rimantadine and amantadine are effective only for influenza A viruses. Another class of influenza antiviral drugs is the neuraminidase inhibitors, which selectively inhibit both influenza A and B viruses. Recent studies have found the neuraminidase inhibitors zanamivir and oseltamivir to be 67-82% effective in preventing laboratory-confirmed infection when administered as prophylaxis during the influenza season. As treatment, they reduce the duration of illness by 1-1.5 days when started within 36-48 h of illness onset. The reported adverse effects of these drugs are minimal, and unlike amantadine and rimantadine, the drugs do not appear to affect the central nervous system. Poor oral bioavailability and rapid renal clearance limit the use of zanamivir to inhalation and concern has been raised about its use in asthmatics. The sialic acid analogue, GS4071, has been shown to be a potent inhibitor of neuraminidase activity and is shown to be effective in controlling influenza, and its prodrug form--GS4104 (oseltamivir) can be given orally. Direct comparison of zanamivir and oseltamivir, their use for prophylaxis and treatment in high-risk groups, and evaluation of their cost effectiveness are all required before they enter routine clinical practice.