The National Health Board of Denmark has announced the first known case of pandemic H1N1 flu resistant to the antiviral drug Tamiflu, stressing that “there is no evidence” that the resistant virus has spread.
While the case is likely to be isolated, it raises concern over the European policy of giving low doses of Tamiflu to people who, while not sick, have been in contact with infected people. In the Danish case, involving a person who came into contact with someone else infected with the H1N1 swine flu, physicians treated her with Tamiflu as a prophylaxis to prevent her getting sick, but she developed symptoms anyway.
The subject then took Relenza, another antiviral drug, and recovered. The State Serum Institute in Copenhagen found that her virus carried a mutation giving resistance to Tamiflu, and assumes this emerged during treatment rather than having been there already.
“Such a development is no surprise from a scientific point of view,” said David Reddy, head of the pandemic taskforce at Swiss company Roche, which produces Tamiflu. Like antibiotics, antiviral drugs favour the survival of strains that resist the drug.
Tamiflu-resistant viruses have previously been seen in people receiving the drug for ordinary flu. These have never been observed to spread, and Reddy told journalists this week that the Danish case was equally unlikely to.
Despite this, H1N1 viruses that resist Tamiflu are quite capable of spreading. The normal seasonal H1N1 virus became almost entirely Tamiflu resistant over the past two years, for reasons that are not yet understood. Scientists fear the pandemic virus, also a member of the H1N1 family, might acquire Tamiflu resistance by interbreeding with these ordinary strains.
It might also evolve resistance by exposure to the drug. Denmark has had only 37 known cases of the pandemic flu so far, so it is still trying to limit the virus’s spread by giving Tamiflu to contacts of cases. Once the virus is spreading widely, such efforts to limit the spread of individual cases are abandoned as pointless.
The European Centre for Disease Control and Prevention in Stockholm, Sweden, warns there is little evidence that such delaying tactics work with flu.
Other scientists warned this month that, “it is not yet clear whether large-scale prophylaxis is justified, given the potential risks of high-level resistance developing.” Prophylactic doses are half those used for treatment, and such low doses can favor the emergence of resistant strains.
Source: Debora MacKenzie, New Scientist, June 30, 2009.