WHO Comment on the importance of global monitoring of variant influenza viruses

19 December 2011

Human infections with variant influenza viruses

Recently, several human infections with variant influenza viruses currently thought to be circulating in swine populations have been reported. To date, epidemiological investigations have not revealed any onward transmission of these viruses outside of small clusters. Avian influenza H5N1 viruses also continue to cause infections in humans exposed to infected birds and contaminated environments especially in countries where the virus is considered endemic in poultry. Thus far, transmission of these variant viruses from animals to humans has only resulted in sporadic human cases or small clusters among close contacts, with no evidence of community level spread. These sporadic human cases and small clusters of human infection with variant influenza viruses are expected and are not considered unusual, and do not change WHO’s current assessment of pandemic risk.

However, because influenza viruses are unpredictable, they have the possibility to change and become more transmissible among humans as shown by the emergence of the influenza pandemic H1N1 virus in 2009. For this reason, continued monitoring of the occurrence of human infections with these viruses and characterization of the viruses themselves are critically important to assess their pandemic potential.

WHO reminds Member countries that it is an obligation under the International Health Regulations (IHR) to report to WHO all human cases of infection with influenza viruses that are not currently circulating seasonally in human populations (, and to conduct epidemiological investigations around each case to identify or rule out any onward human-to-human spread which could indicate emergence of a more transmissible virus. WHO will continue to share information about variant influenza viruses with the international public health community in accordance with WHO's obligations under the IHR.

WHO also strongly recommends that all un-subtypable influenza A specimens should be immediately sent for diagnosis and further characterization to specialized laboratories or one of the six WHO Collaborating Centres for Reference & Research on Influenza (, This is important for early identification of emerging viruses with the potential to threaten global public health.