Oseltamivir resistance in European Influenza viruses

Jan 29 2008

Preliminary surveillance into antiviral drug susceptibility against seasonal flu viruses currently circulating in Europe has revealed that some of the A (H1N1) viruses (13%) have shown resistance to the antiviral drug, oseltamivir.

These viruses however, remain sensitive to the antivirals, zanamivir and amantadine.

So far, 148 influenza A (H1N1) viruses isolated during November and December from 10 European countries have been tested and 19 samples have been found to have evidence of resistance. A very small number of samples identified by the Health Protection Agency (HPA) in the UK have shown this resistance to the drug.

Key Points

• Only a small proportion of samples with this resistance have been identified in the UK (2 out of 75 samples tested) so far. Although this is an important finding, it is too early to judge on the clinical significance until more research is completed. The HPA is working closely with colleagues at the European Centre for Disease Prevention and Control (ECDC) and the World Health Organisation (WHO).
• It must be stressed that this issue of resistance relates to just one of the three current seasonal flu strains circulating across Europe. Oseltamivir resistance has not been reported in other flu strains.
• Antivirals may still be beneficial in patients with resistant viruses.
• The source of these resistant viruses is not known at present. This emergence seems unlikely to have anything to do with antiviral use in Europe. The resistance is not explained by the affected individuals having taken any antivirals or high levels of use of oseltamivir in Europe where generally the drug is used sparingly.
• There is no evidence that this resistant strain is more virulent or transmissible than other seasonal flu strains. People who become ill with this strain of flu do not appear to become more ill than if infected with another strain of seasonal flu.
• For most people, flu is a miserable illness, lasting a week or so, but not life threatening. However, for those in at-risk groups, such as the elderly and patients with heart problems, diabetes or lung, liver or renal diseases, or those who have weak immune systems, it can be far more dangerous and can lead to more serious illnesses.
• Vaccination offers the best protection for those at risk from seasonal influenza and individuals in these groups should have been vaccinated by now. There is no need for change to the NICE recommendations for the use of oseltamivir in the UK at present.

 Background Information

These are very early findings and currently based on extremely limited data. Work is underway to develop a detailed understanding of the factors which have led to the emergence and transmission of these highly resistant viruses so that appropriate strategies can be developed for their use to treat flu symptoms.

The Health Protection Agency is undertaking sample testing for Europe and working with the WHO to develop further picture of these strains.  

There is no evidence that this oseltamivir-resistant virus is more transmissible and pathogenic than other flu strains.

International efforts are continuing to investigate this mutation and HPA is working very closely with national and internationals counterparts to establish the prevalence and geographical distribution of these resistant viruses.

Based on the evidence gathered so far there are no proposed changes to the way oseltamivir is used in the UK. However, these results highlight the need for enhanced surveillance of circulating viral isolates.

Current levels of seasonal flu are still low in the UK.

Further information is available on the European Centre for Disease Prevention and Control (ECDC) website at http://ecdc.europa.eu/