Strains of influenza virus that infect humans are becoming increasingly resistant to a widely-used class of flu drugs, according to a study published on the website of medical journal The Lancet.

The results suggest that the drugs - called the adamantanes or M2 channel blockers and used in flu prevention and treatment for 30 years - should not form part of the flu drug and vaccine stockpiling currently being undertaken by national governments and the World Health Organisation in preparation for a flu pandemic.

Governments around the world are amassing stocks of flu drugs and vaccines in anticipation of the next major flu outbreak. Usually occurring every two to four decades, the last, relatively minor pandemic took place in the late 1960s, leading to speculation that the next could occur at any time. Most of the betting is that the next strain to make the leap to pandemic status will be H5N1, the avian flu that has already led to a number of deaths in Asia.

The Lancet study suggests that resistance to the two approved M2 inhibitors - amantadine and rimantidine - has been rising at an alarming rate over the last decade. Circulating strains of flu isolated from Asia last year show resistance rates of between 15% and 74%, according to the report. Moreover, since 2003, nearly all H5N1 strains are resistant to the drugs.

The findings suggest that stockpiling efforts should focus on vaccines and a newer, more expensive, class of influenza drug, the neuraminidase inhibitors, represented by Roche’s Tamiflu (oseltamivir) and GlaxoSmithKline’s Relenza (zanamivir), both of which have been the subject of major orders from the US Government [[20/09/05d]] [[16/09/05a]]. However, nearly all stockpiling efforts rely on the M2 blockers - at least in part - so the new findings raise serious questions about the effectiveness of these plans.