No 23 - 2011

Influenza season 2010-2011

Influenza season 2010-2011

The 2010-2011 influenza season was the first after the pandemic with the new influenza virus A (H1N1) 2009, which spread globally from the spring of 2009 to 2010, EPI-NEWS 23/10.


During the season, the occurrence of influenza-like disease (ILD) was reported to the sentinel surveillance by an average of 188 GPs on a weekly basis. ILD occurrence was also followed daily via the all-year nationwide surveillance performed by emergency call service physicians. The occurrence and any changes in the influenza virus were monitored via samples submitted to the SSI Influenza Laboratory. The disease burden and its load on the healthcare system were assessed via reporting of the number of relevant patients at Danish intensive care units. Mortality was assessed via vital data from the Civil Registration Registry.

ILD occurrence and mortality

Both sentinel and emergency call service surveillance recorded ILD from December to March, the normal seasonal influenza period. The occurrence peaked in weeks 3-6 when the share of ILD patients seen by the sentinel physicians reached the modelled epidemic threshold value. The ILD incidence recorded by the emergency call service physicians was lower than that registered during the 2009/2010 pandemic, but higher than that observed in the 2008/2009 season. ILD occurred more frequently in children and younger adults. In contrast to previous influenza seasons, the 2010/11 season saw no excess mortality in periods with influenza.

Laboratory diagnosis

The SSI influenza laboratory examined a total of 4,146 samples, including 391 submitted by sentinel physicians and 1,121 surveillance samples submitted by other Danish laboratories. A total of 2,038 samples tested positive for influenza virus types A or B. Positive findings with influenza virus strain typing are shown in Table 1.


The first new type A (H1N1) 2009 virus in the world with resistance to oseltamivir was detected in Denmark in 2009. In the 2010-11 season, the SSI Influenza Laboratory tested a total of 1,089 samples and found resistance to oseltamivir in 26 patients. No signs were found of persistent infection with resistant influenza virus.

Intensive Care Units

From 13 December 2010 to 11 April 2011, a total of 156 patients were reportedly admitted to intensive care units with laboratory-confirmed influenza: 69 females and 87 males. Patients had an average age of 52 years (1 week-83 years). The highest number of admissions occurred in week 4, Figure 1, when 10.2% of the bed capacity was occupied by influenza patients compared with a maximum of 4.5% during the 2009/10 pandemic. A total of 115 (74%) of patients were diagnosed with influenza A, including 45 subtyped as (H1N1) 2009. A total of 41 (26%) patients had an influenza B infection. A total of 14 patients received ECMO (Extra Corporal Membrane Oxygenation) and 26 patients died. Among the 156 patients, 24 were previously healthy, 93 had one or more underlying diseases and in 39 cases no information on underlying diseases was provided.

Vaccination coverage

Seasonal influenza vaccination was offered free of charge to selected groups from 1 October to 31 December 2010, EPI-NEWS 39/10. The Danish Ministry of Interior and Health calculated that a total of 567,986 persons accepted the offer, including 89,774 chronically ill, 31,070 early retirement pensioners, 2,746 pregnant women, 1,186 household contacts to severely immunosuppressed patients and 443,210 persons >65 years. Among persons >65 years, the overall coverage was 50%; the highest observed in the Capital Region with 53% followed by the South Denmark Region: 51%, Region Central Jutland: 49%, Region North Jutland: 48% and Region Zealand: 46%.

Influenza vaccine 2011-2012

The WHO recommends that the composition of the next seasonal vaccine should be identical to that of the previous seasonal vaccine:

 - A/California/7/2009 (H1N1)-like virus
 - A/Perth/16/2009 (H3N2)-like virus
 - B/Brisbane/60/2008-like virus


In the 2010/11 season, influenza virus circulated in the period expected for seasonal influenza. Not unexpectedly, influenza virus type A (H1N1) 2009 dominated, but type B virus and to a limited extent type A (H3N2) virus circulated simultaneously. All three types of influenza virus were covered by the year's vaccine. Particularly children and young adults were affected, as was the case during the pandemic, EPI-NEWS 23/10, but older age-groups were also at risk.

No general excess mortality was observed, but the period saw more intensive care unit admissions than during the 2009/2010 pandemic, and the average age of these patients was higher. The majority of intensive care patients belonged to a risk group, and 26% of patients were infected with type B influenza virus. Similar conditions were described in other European countries, e.g. Great Britain. Despite a comparatively mild influenza season, disease occurrence and seriousness draw attention to the importance of influenza vaccination of risk group persons, e.g. to improve the coverage among >65-year old persons which was far below the 75% objective.

(S. Glismann, L.K. Knudsen, S. Gubbels, K. Mølbak, Dept. of Epidemiology, K. Bragstad, L.P. Nielsen, Influenza laboratory) 

Individually notifiable diseases and selected laboratory diagnosed infections (pdf)

8 June 2011