Influenza viruses with reduced sensitivity to the neuraminidase inhibitor drugs in untreated young children.

The neuraminidase inhibitors are a class of antiviral drugs used for both the prophylaxis and treatment of influenza infections. Clinical trials of these inhibitors detected a low level of resistant viruses from treated individuals, although a higher frequency was detected in children (5%-6%) compared to adults (1%-4%). In addition, there have been some previous reports of NA inhibitor resistant viruses being isolated from untreated individuals. Here we report on the NA inhibitor sensitivity of over 1,000 influenza isolates collected through the World Health Organization (WHO) global influenza surveillance program. Of the total number of viruses analysed, only 2 (0.2%) strains (an A(H1N1) strain and an influenza B strain) were considered to have a significant reduction in sensitivity to at least one of the neuraminidase inhibitor drugs. Interestingly, both of these strains were isolated from untreated patients in the youngest age cohort (less than 2 years). Although the influenza B strain is unlikely to be clinically resistant, the A(H1N1) virus contained the same His274Tyr neuraminidase mutation that has been observed in resistant mutants following oseltamivir treatment. Given these results it may be important to enhance neuraminidase inhibitor susceptibility testing of viruses from patients in the less than two years cohort.