On 24 April 2009, the World Health Organization (WHO) first reported the emergence of a new Influenza virus in the United States of America and Mexico . This novel Influenza virus was identified as a new subtype A(H1N1) resulting from a reassortment of avian, human and swine Influenza viruses. This virus, which was later on referred to as A(H1N1)2009, rapidly demonstrated its capacity to transmit among humans . On 11 June 2009, the WHO decided to raise the pandemic alert phase to its maximum level (phase 6) .
Influenza in humans is caused by one of three types of influenza viruses A, B and rarely C, which all belong to the orthomyxoviridae family, and have two glycoproteins on the surface of the virions, the hemagglutinin (HA) and the neuraminidase (NA). These proteins elicit antibody responses to the Influenza virus, and so far sixteen different HAs (H1 to H16) and 9 different NAs (N1 to N9) have been recognised. Several combinations of HA and NA proteins are possible, each combination representing a different subtype (for example A/H1N1, A/H3N2, A/H1N2 ...). An important characteristic of Influenza viruses is their ability to evolve continuously to escape the immune response. The mechanisms behind this evolution are either antigenic drift (point mutations in the HA and NA genes) or antigenic shift (reassortment between different Influenza viruses).
For a virus to cause a pandemic, two major criteria must be met: the virus must be novel (an-tigenic shift), as this means that a large proportion of the population is susceptible to infection, and the virus must be transmissible from person to person [4,5]. In this context, the initial reporting of this novel transmissible Influenza variant to the WHO and the continuous surveillance of this virus is of the highest importance and allows to monitor virus evolution and reduce the public health risk for the population worldwide .
In Belgium, the surveillance is ensured by the National Centre for Influenza (NIC) http://www.wiv-isp.be/flu. According to the WHO [7,8], the responsibilities of the NIC during the pandemic period are: 1) to provide laboratory diagnosis for monitoring the geographical spread of the pandemic in the country; 2) as the pandemic intensifies and becomes widespread, to adjust virological surveillance in order to monitor the progress of the pandemic in the country; 3) to maintain adequate virological surveillance to assist WHO in monitoring for example antigenic and genetic changes in the pandemic virus, pathogenicity, and antiviral susceptibility.