The Norwegian Institute of Public Health has informed World Health Organization of a mutation detected in three H1N1 viruses. The viruses were isolated from the first two fatal cases of pandemic influenza in the country and one patient with severe illness.
Norwegian scientists have analysed samples from more than 70 patients with clinical illness and no further instances of this mutation have been detected. This finding suggests that the mutation is not widespread in the country.
The virus with this mutation remains sensitive to the antiviral drugs, oseltamivir and zanamivir, and studies show that currently available pandemic vaccines confer protection.
Worldwide, laboratory monitoring of influenza viruses has detected a similar mutation in viruses from several other countries, with the earliest detection occurring in April. In addition to Norway, the mutation has been observed in Brazil, China, Japan, Mexico, Ukraine, and the US.
Although information on all these cases is incomplete, several viruses showing the same mutation were detected in fatal cases, and the mutation has also been detected in some mild cases. Worldwide, viruses from numerous fatal cases have not shown the mutation. The public health significance of this finding is thus unclear.
The mutations appear to occur sporadically and spontaneously. To date, no links between the small number of patients infected with the mutated virus have been found and the mutation does not appear to spread.
The significance of the mutation is being assessed by scientists in the WHO network of influenza laboratories. Changes in viruses at the genetic level need to be constantly monitored. However, the significance of these changes is difficult to assess. Many mutations do not alter any important features of the virus or the illness it causes. For this reason, WHO also uses clinical and epidemiological data when making risk assessments.
Although further investigation is under way, no evidence currently suggests that these mutations are leading to an unusual increase in the number of H1N1 infections or a greater number of severe or fatal cases.
Laboratories in the WHO Global Influenza Surveillance Network closely monitor influenza viruses worldwide and will remain vigilant for any further changes in the virus that may have public health significance.
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