Both influenza A(H1N1) and B viruses have been detected since November, and during weeks 4-5 a very few A(H3N2) viruses were also seen. The numbers of both A and B virus detections peaked simultaneously to the clinical activity. A clear majority has been A(H1N1) virus, but last weeks the B virus for were in majority. A high proportion of the A(H1N1) viruses in Norway are resistant to the antiviral drug oseltamivir.

Clinical surveillance: Incidence of influenza-like illness under the epidemic threshold

Figure 1. Influenza-like illness (ILI), Norway 2007-2008: ILI consultation rate in sentinel practices.

Subsequent to a slightly elevated, sub-epidemic, incidence of influenza-like illness during the first few weeks of January, the activity rose towards a modest peak in week 8. The nation-wide level is now under the designated epidemic threshold (which is defined as 1.4 per cent of sentinel consultations).

Laboratory-confirmed cases of influenza: 
Still falling number of virus detections, influenza B in majority   

Figure 2a. Recorded influenza virus detections in Norway 2007-2008.
Data for the last week(s) may be incomplete

The virological surveillance of influenza is based on weekly reports from all laboratories in the country that perform influenza diagnostics, as well as the diagnostics on influenza sentinel specimens, performed in the National Influenza Centre in the Department of Virology, Norwegian Institute of Public Health. 

From week 43 till present, cases of influenza have been detected in all parts of Norway. Numbers remained low until mid-January, but then rose rapidly until reaching a peak in week 8 (also clinical incidence peak). Since then numbers have been falling, both for influenza A and B.  Both influenza A(H1N1) and B viruses have been detected, and in addition, a total of 9 A(H3) viruses were detected. Whereas a clear majority of viruses has been A(H1N1), an increasing proportion has been influenza B, which was the majority virus the last weeks. This pattern has also been seen in other European countries.

A number of A(H1N1) and B viruses have been forwarded to the WHO Collaborating Centre (WHO CC) in the UK for further antigenic analysis, and some have also been subjected to sequence analysis in the National Influenza Centre. Sequence analyses show that the A(H1N1) viruses are closely related to the present vaccine strain, A/Solomon Islands/3/2006(H1N1). They furthermore share the K140E substitution in the HA1 subunit of the H1 haemagglutinin. Correspondingly, antigenic analyses performed in the WHO CC show that the majority is also fairly well antigenically matched to the vaccine strain, as well as to H1N1 viruses that circulated in limited numbers in Norway last winter. Sequenced B viruses seem to resemble closely a strain that has been recommended for the Southern Hemisphere vaccine for the 2008 season, B/Florida/4/2006, and this is in agreement with antigenic characterisation results performed in the WHO CC.

Statistics: Clinical surveillance

Per cent influenza-like illness numbers listed by health region

Region east (ØST): Østfold, Akershus, Oslo, Hedmark og Oppland
Region south (SØR): Buskerud, Vestfold, Telemark, Aust-Agder og Vest-Agder
Region west (VEST): Rogaland, Hordaland og Sogn og Fjordane
Region mid-Norway (MIDT): Møre og Romsdal, Sør-Trøndelag og Nord-Trøndelag
Region north (NORD): Nordland, Troms, Finnmark og Svalbard.

In the notification system for influenza-like illness (ILI), 201 designated sentinel practices report weekly the numbers of ILI, as well as the total consultations. The data are summarised as consultation rate, in per cent, at Health Region and national levels.

Virus surveillance

Data for the most recent week(s) may be incomplete, and data for preceding weeks may have been updated.