The US Centers for Disease Control (CDC) has released a health advisory warning clinicians that flu vaccines will have diminished effectiveness against one strain currently circulating in the USA as it has mutated to become resistant.
Influenza A (H3N2) has been reported in almost all States. The CDC notes that previous seasons where influenza A dominated against influenza B produced higher rates of hospitalisation and increased mortality, particularly among the elderly, young children and patients already suffering from chronic medical conditions.
Since the start of October, the CDC analysed 85 influenza A samples collected from patients across the USA. Fifty-two percent had 'antigenically drifted', or genetically mutated, to render the H3N2 vaccine ineffective, meaning that the vaccine would be effective in less than half of these cases. Last year the flu vaccine's effectiveness was estimated at 60 percent.
Dr Tom Frieden, director of the CDC, said that it is now too late to produce a new flu vaccine as its production alone would take up to four months. The CDC continues to recommend treatment with the existing vaccine as it may provide partial protection against the new H3N2 strains and would still offer immunity for the older variants.
Speaking to Reuters, Professor Richard Zimmerman of the University of Pittsburgh Medical Center, agreed with the advice but added that the virus' genetic drift was 'unwelcome news' for those who care for the elderly.
In its advisory, the CDC continues to recommend antiviral medicines for hospitalised or serious patients within the first 48 hours of symptom onset.
Influenza remains one of North America's most deadly infectious diseases. The CDC estimates that around 24,000 Americans die each flu season.