Public Health: H1N1 - the disease that won't lie down and die

Recent clusters, in two hospital wards, of severely immunocompromised patients with oseltamivir-resistant H1N1 viruses have been thoroughly investigated. Although transmission of resistant virus from one patient to another is suspected, other results are reassuring. Advice on treatment options for these highly vulnerable patients has also been provided, says the World Health Organisation.

In a briefing note published 2 December, the WHO says :

- WHO has been informed of two recent clusters of patients infected with oseltamivir-resistant H1N1 viruses. Both clusters, detected in Wales, UK and North Carolina, USA, occurred in a single ward in a hospital, and both involved patients whose immune systems were severely compromised or suppressed. Transmission of resistant virus from one patient to another is suspected in both outbreaks.

- The emergence of drug-resistant influenza viruses in severely immunosuppressed or immunocompromised patients undergoing antiviral treatment is not unexpected and has been well documented during seasonal influenza. Virus replication can persist in such patients for prolonged periods of time despite antiviral treatment, creating an environment in which drug-resistant viruses can readily be selected. This phenomenon has also been observed for the pandemic (H1N1) 2009.

- The Wales outbreak, which was detected in late October, involved eight patients. All of these patients were hospitalized because of severe haematological disorders. No deaths occurred. Three of these patients remain in hospital, with one being treated in intensive care.

In the USA outbreak, which involved four severely immunocompromised patients, cases occurred in a two-week period between mid-October and early November. Three of the four cases were fatal, but the role of H1N1 infection in contributing to these deaths is uncertain.

All of the resistant viruses carried the same H275Y mutation, indicating resistance to oseltamivir but susceptibility to the second antiviral drug, zanamivir.

- Within the past two weeks, the number of documented cases of oseltamivir resistance in H1N1 viruses has risen from 57 to 96. Around one third of these cases occurred in patients whose immune systems were severely suppressed by haematological malignancy, aggressive chemotherapy for cancer, or post-transplant treatment. The clusters in the two hospital wards should be viewed in the context of these overall trends. Although all incidents of oseltamivir resistance merit investigation, no evidence suggests that events to date constitute a public health threat.

(extracted from WHO's briefing document "Oseltamivir resistance in immunocompromised hospital patients"

But the more usual form of the virus continues to spread: in Iowa, USA, the number of deaths in the state has reached 30, with more than 700 being hospitalised. But in Canada, some vaccination clinics have closed due to lack of demand. This, however, is possibly due - at least in part - to the provision of vaccination at GP surgeries.

Even so, Vancouver has seen four deaths in the past week - and 109 severe cases.

More than 170,000 doses of GlaxoSmithKlein's vaccine were recalled in Canada last week after six cases of anaphylactic shock after the use of the vaccine came to light. The problem, if it is one, appears to relate to a single batch of the vaccine - and Canadian authorities say that the number is not a significant increase over that found with more common flu vaccines.