FINAL version – Sept 2016

Norwegian Institute of Public Health

National Influenza Centre, Oslo Department of Virology Norwegian Institute of Public Health P.O. Box 4404 Nydalen N-0403 Oslo, NORWAY Tel: (+47) 21 07 65 20 Fax: (+47) 21 07 64 47 e-mail: [email protected] www.fhi.no/influensa

NORWAY: National Influenza Centre Influenza Epidemiological Information prepared for the WHO Informal Meeting on Strain Composition for Inactivated Influenza Vaccines for use in the Southern Hemisphere Season 2017 Geneva, September 2016

Figure 1. Age profiles of H1N1pdm09 immunity and infection incidence, 2009 pandemic wave and 2015-16 seasonal outbreak. Prevalence of HI antibody (titre ≥ 40) to influenza A(H1N1)pdm09 virus in August 2009 and 2015, respectively, is shown as bars (% seroprevalence), and the age distribution of A(H1N1)pdm09 virus infections in the subsequent outbreaks ( incidence of laboratory verified cases per 104 population) is drawn as line plots. Whereas the youngest children remain a high-incidence group, there is a significant drop in incidence among 5-24 year olds that may well be influenced by the marked immunity attained in this age group. In strong contrast to the 2009 pandemic, school-age children and young adults are now less likely to be diagnosed with this virus than older adults and the elderly.

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FINAL version – Sept 2016 Contents Contents................................................................................................................................................................................. 2 1: The 2015-2016 influenza season, Norway ........................................................................................................................ 2 Summary.................................................................................................................................................................. 2 Incidence of influenza-like illness. .......................................................................................................................... 2 Virological surveillance. .......................................................................................................................................... 3 Pre-season influenza seroprevalence and age-distribution of viruses detected in 2015-16 season .......................... 7 Surveillance of influenza-associated hospitalisations .............................................................................................. 8 2: Influenza virus characterisations, 2015-16 season .......................................................................................................... 10 Influenza A(H1N1)pdm09 ..................................................................................................................................... 10 Influenza A(H3N2) ................................................................................................................................................ 10 Influenza B ............................................................................................................................................................ 11 Antiviral resistance monitoring ............................................................................................................................. 11 Figure 13 ................................................................................................................................................................ 18 Figure 14 ................................................................................................................................................................ 19 3: Seroepidemiology Data, August 2015 ............................................................................................................................. 20 Influenza A(H1N1)pdm09 ..................................................................................................................................... 20 Influenza A(H3N2) ................................................................................................................................................ 21 Influenza B ............................................................................................................................................................ 21 Acknowledgements. .............................................................................................................................................. 24

1: The 2015-2016 influenza season, Norway Summary For the first time since the 2012/13 winter, the 2015-2016 influenza outbreak in Norway was characterised by predominance and extensive circulation of influenza A(H1N1)pdm09 virus. Activity peaked at intermediate levels in late February. A(H1N1)pdm09 viruses mostly belonged to the newly emerging 6B.1 clade. The highest incidence was seen in the 0-4 age group and this was in accordance with the pre-season prevalence of protective antibodies to the H1N1pdm09 virus seen in this age group. An increase in influenza-related hospitalisations was also observed in this age group compared to the previous 2014/15 H3N2 season. The seroprevalence to the H1N1pdm09 vaccine virus measured during the end of the 2014/15 season was mainly unchanged from previous year, consistent with the low level of H1pdm09 viruses circulating the preceding seasons. A(H3N2) viruses was predominating during the previous season, hence a marked increase in seroprevalence was seen, and only sporadic A(H3N2) infections were seen in the 2015/16 main outbreak. Another change relative to recent seasons was the predominance among influenza B viruses of the B/Victoria/2/1987 lineage, after four winters of B/Yamagata/16/1988-lineage predominance among B virus. Several cases and a cluster of H275Y-mutation oseltamivir resistant H1N1pdm09 viruses were detected during the season, with the highest numbers in March 2016. Incidence of influenza-like illness. The incidence of influenza-like illness (ILI) in Norway is monitored through The Norwegian Syndromic Surveillance System (NorSSS). NorSSS is a new population-based automated electronic system that daily provides data from all GPs and emergency clinics in primary health care in Norway. The Norwegian Institute of Public Health (NIPH) receives data from the Norwegian Health Economics Administration (HELFO). NorSSS has been in operation since the 2014-15 season and is supported by retrospective data from the 2006-07 season and onwards. The ILI consultation rate began to rise just before Christmas but then stagnated at a sub-epidemic level during the first weeks after the Christmas/New Year school break, thus following a very common seasonal pattern in Norway (Fig.2). In week 4, the incidence resumed its increase, peaking at medium NORWAY Sept 2016 – page 2 of 24 Preliminary data – not for publication

FINAL version – Sept 2016 intensity in week 7-8, and then declining steadily until baseline level was reached in early April. No outbreaks in health care institutions were reported during the winter outbreak, but, remarkably, a few nursing home outbreaks were noted in early summer.

Figure 2 The Influenza season 2015-2016 in Norway. Proportion of patients in general practice and emergency clinics presenting with influenza-like illness (ILI), by calendar week. A selection of previous seasons is also shown. Week 52 for 2009 and 2015 represents the average value of week 53 and 52, in order to fit the format.

Virological surveillance. A network of volunteer sentinel physicians throughout the country collects specimens from patients with ILI for analysis at the National Influenza Centre. In addition, medical microbiology laboratories that perform influenza diagnostics weekly report the number of positives and the number of specimens tested and also contribute positive specimens to the NIC for further characterisation. Even though most of these laboratories are in hospitals, the majority of specimens tested for influenza virus tends to be from outpatients attending general practitioners. The week-by-week development of the outbreak is portrayed in Figure 3 and in Table 1.

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FINAL version – Sept 2016

Figure 3: Laboratory detections, Norway 2015-2016. Left hand panel: Weekly number of the different influenza viruses is displayed as stacked bars, and influenza virus positivity rates of sentinel specimens ("fyrtårn") and all lab testing, respectively, are shown as line graphs. Right hand panel: Enlarged view showing weekly number of the different influenza viruses from week 20 through 36. Lower left hand panel: Weekly numbers of influenza virus detections, with previous season numbers shown for comparison.

Ever since the end of virus circulation for the previous season around mid-summer, sporadic cases of laboratory verified influenza occurred weekly up to early November, after which a gradual increase was observed (Figure 4 and Table 2). After a brief stagnation in early January, both the number and the percentage of positive samples rose steeply from mid-January till early February, peaking at 1346 weekly virus detections in late February (week 8). This number of detections is close to the peak week of the previous H1N1-dominated season (2012/13) but lower than the preceding 2014/15 season when H3N2 virus predominated and which peaked at 1801 virus detections. Looking at influenza A and B separately, the influenza A peak occurred in weeks 5 through 8, while influenza B detections had a broad peak lasting from week 8 through week 14. Influenza A detections declined sharply to reach moderate level around week 12 and low level (32000) than for H3 (n