Antivirals and influenza: frequency of resistance.

Antivirals are effective in prophylaxis and therapy for influenza and are likely to be active against a new pandemic variant. They can be divided into the M2 inhibitors, adamantane and rimantadine, and the neuraminidase inhibitors (NAIs), zanamivir and oseltamivir. The former are limited in activity to type A viruses, whereas the latter are also active against type B viruses. Both classes of drugs appear similarly efficacious in prophylaxis at approximately 70-90%. However, use of M2 inhibitors (adamantanes) in therapy is limited by side effects, more common with amantadine, and also by production of antiviral resistance and lack of demonstrated prevention of complications. The NAIs prevent both types of seasonal influenza, shorten duration of illness, and reduce complications. As such, their use for seasonal influenza treatment has been increasing. They are active against A(H5N1) but oseltamivir has been most extensively stockpiled because the infection in humans may be disseminated. Resistance does emerge, but not at the same frequency as with the M2 inhibitors. Resistant viruses also appear less fit and thus less able to spread. However, as use increases, resistance needs to be carefully monitored.