No 24a - 2012

Influenza season 2011-2012

Surveillance

During the season, the occurrence of influenza-like disease (ILD) was reported to the sentinel surveillance by an average of 159 GPs weekly. ILD occurrence was also followed daily via the all-year nationwide surveillance performed by emergency call service physicians.

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 patients diagnosed with influenza at Danish intensive care departments. Mortality was assessed via vital data from the Civil Registration Registry.

ILD occurrence and mortality

ILD occurrence was low compared with previous seasons according to sentinel as well as emergency call service physician surveillance, Figure 1. An excess mortality equivalent to that of an average season was estimated throughout the period for elderly persons with circulating influenza who were above the age of 65.

Laboratory diagnosis

The SSI influenza laboratory examined a total of 1,503 samples, including 115 submitted by sentinel physicians and 1,388 samples submitted by GPs and other laboratories. A total of 514 samples tested positive to airway virus, Figure 2. Among these, 92 tested positive to influenza virus type A or B. The distribution by virus strain is presented in Table 1. For comparison, last year influenza virus was found in a total of 2,038 samples, EPI-NEWS 23/11. A total of 72 samples were tested for resistance. No oseltamivir (Tamiflu®) resistance was demonstrated. However, all influenza A virus cases were resistant to amantadin.

Intensive Care Departments

From 28 November 2011 to 9 April 2012, a total of seven patients were admitted to intensive care departments with laboratory-confirmed influenza. This number was considerably lower than the numbers observed in 2009/10 and 2010/11, when 96 and 156 influenza patients, respectively, were reported, EPI-NEWS 23/11. Six patients were diagnosed with influenza A, including one case of (H1N1)pdm09, and two of H3N2. Three cases were not subtyped. One patient was diagnosed with influenza B. Three patients died within 30 days after admission.

Vaccination coverage

Seasonal influenza vaccination was offered free of charge to specific groups from 1 October to 31 December 2011 and also to pregnant women in their second and third trimesters until 1 March 2012, EPI-NEWS 39/11.

The Danish Ministry of Health has calculated that a total of 595,103 persons accepted the offer, including 98,648 chronically ill, 29,470 early retirement pensioners, 2,250 pregnant women, 1,895 household contacts to severely immunosuppressed patients and 462,840 persons > 65 years.

Among persons > 65 years, the overall coverage was 51%; the highest coverage was observed in the Capital Region with 54% followed by the South Denmark Region: 50%, Region Central Jutland: 50%, Region North Jutland: 48% and Region Zealand: 47%. The number of influenza vaccinations given to risk group persons and the vaccination coverage among elderly persons above the age of 65 years are available at www.ssi.dk/data (Danish laguage). 

Influenza vaccine 2012-2013

The WHO has changed its recommendation for next year's seasonal vaccine to the following:

• A/California/7/2009 (H1N1)pdm09-like virus
• A/Victoria/361/2011 (H3N2)-like virus
• B/Wisconsin/1/2010-like virus.

Commentary

The 2011-12 influenza season was extraordinary as the occurrence of ILD was lower than in any other year since the initiation of the Sentinel Surveillance Programme in 1994. The number of influenza-positive samples tested at the SSI was correspondingly low and only seven patients diagnosed with influenza were admitted to Danish ICUs. Despite this, parts of Europe have seen considerable influenza activity, including Sweden and Norway. One possible explanation for the low activity may be that influenza activity peaked relatively late in the season when the weather was very mild. This was the first season after the pandemic in which influenza A(H3N2) was the dominant virus.

Surveillance of influenza virus in Denmark and Europe has shown that the correspondence between the virus strains of the vaccine and the circulating influenza A(H3N2) and B viruses was limited.

Consequently, the viral components of the influenza vaccine to be used in the upcoming 2012-13 season were updated to include surface proteins from a new A(H3N2) virus and a new influenza B virus of Yamagata lineage.

(T.G. Krause, L.K. Knudsen, S. Gubbels, Dept. of Infectious Disease Epidemiology, K. Bragstad, Microbiological Diagnostics & Virology)

13 June 2012