To date, a total of nine H1N1v confirmed cases have been reported amongst
patients on a hospital ward in Wales. Five of these cases are known to be
resistant to oseltamivir, one is sensitive and for three resistance status is
presently unknown.

The virus has emerged in a group of particularly vulnerable individuals in
whom the development of oseltamivir resistance is well documented.  At present
we believe the risk to the general healthy population is low. There is no
evidence that the oseltamivir resistant virus is any more virulent than any
other type of flu. The situation is being kept under review.  

Further follow-up of cases and their close contacts both on the ward and in
the community is underway to ascertain if there is evidence of onward
transmission. The virus remains sensitive to the other frontline drug Relenza®
which is being used as an alternative antiviral and patients are responding
well.

Although further epidemiological investigation is underway, it would seem
likely that transmission of oseltamivir-resistant H1N1 virus has taken
place.

The current cases of oseltamivir resistance under investigation have all
occurred in patients with haematological problems which result in
immuno-suppression either because of the disorder or the chemotherapy given to
treat the disorder.

Oseltamivir resistance to influenza viruses is well documented in
immunosuppressed individuals and can develop quickly. It is likely to be
associated with the high viral load which may occur during infection in these
patients. 

In addition, immunosuppressed people may be more susceptible to infection ie
a smaller exposure may result in infection in these patients. 

Antiviral resistance monitoring has been in place in the UK for a number of
years and this allows us to identify drug resistance early so that cases can be
investigated and managed appropriately

 

It still remains appropriate to
use oseltamivir for the treatment and prophylaxis of influenza in the UK. 
Guidance is being developed to cover the particular instance where modifications
to the current antiviral treatment policy may be required. 

Background information

As of 20th November, WHO have
reported 57 incidences of oseltamivir resistance worldwide.

There have been no documented episodes of person to person transmission of a
oseltamivir resistant pandemic influenza virus.  A small number of individual
cases have been identified, however, in whom drug resistant virus has been
isolated with no history of prior use of anti-virals..

In addition, on 19 November, WHO was notified by the US of four patients with
infection with oseltamivir-resistant pandemic H1N1 2009 influenza virus among
inpatients in the same unit at a hospital during October and November. The
possibility that these viruses have been transmitted within this hospital unit
is currently under investigation.

Generally, the pandemic H1N1 oseltamivir
resistant viruses are not considered "fit" and transmission is poor.