Following the 2009 influenza A(H1N1) pandemic and difficulties experienced in establishing new surveillance systems, or undertaking ad hoc surveys in a crisis authorities in the UK established routine surveillance for severe influenza disease appearing in hospital.(1) This is called the UK Severe Influenza Surveillance System (USISS).
It is a sentinel network of 23 Trusts, coordinated by the Health Protection Agency and the Department of Health in England to monitor hospitalisations due to confirmed seasonal influenza in England. A more detailed protocol and reporting details are available on the internet and this season’s results can be seen here.
This article presents the system and the results of the first season of operation in 2010/11. The system defined a case as a person hospitalised with confirmed influenza of any type. Weekly aggregate numbers of hospitalised influenza cases, broken down by influenza type and level of care, were submitted by the participating Trusts. Cases in 2010/11 were compared to cases during the 2009 pandemic (from the 1st April 2009 to the 6th January 2010) in hospitals with available surveillance data for both time periods. (from the 1st April 2009 to the 6th January 2010). Both during the 2009 pandemic and during the 2010/11 season, reports were submitted both prospectively and retrospectively. In this paper hospitalisation rates were calculated and compared for 19 Acute Trusts that submitted data during both time periods.
The start of the system in the autumn of 2010 was well timed as the system was in place to record an unexpected resurgence in seasonal A(H1N1)2009pdm influenza activity in England in December 2010 with reports of severe disease.(2,3) The cases over the period 4th October 2010 to 13th February 2011 were mostly due to influenza A(H1N1)2009pdm.
Over 1000 cases of influenza A(H1N1)2009pdm occurred over this period compared to just over 400 at the same hospitals over the 2009/10 pandemic period. During the 2009 pandemic, the crude overall rate of hospitalisation at these Trusts for influenza A(H1N1)2009pdm was 0.8/100,000 population. During the 2010/11 season, the crude overall rate of hospitalisation for influenza A(H1N1)2009pdm at the same Trusts was 2.1/100,000. As in the 2009 pandemic, there was a trend of decreasing hospitalisation rates with age, from 5.3/100,000 population for cases aged 0 to 4 years to 1.1/100,000 population for cases aged over 64 years. This compared to hospitalisation rates of 2.6/100,000 population for cases aged 0 to 4 years and 0.1/100,000 population for cases aged over 64 years during the 2009 pandemic.
The authors also found that, in 2010/11, the lowest hospitalisation rate was 0.6/100,000 population in cases aged 5 to 14 years, which is a big difference when compared to the 2009 pandemic period, when the hospitalisation rate for this age group was the second highest at 1.7/100,000 population.
Overall, all age groups were 2.6 (95% CI: 2.3 to 2.9) times more likely to be hospitalised with influenza A(H1N1)2009pdm infection during the 2010/11 season than during the 2009 pandemic. The rate ratio for hospitalisation increased with age, the exception being those aged 5 to 14 years, who were significantly less likely to be hospitalised than during the 2009 pandemic.
In addition, the decrease in hospitalisation rates in cases aged 5 to 14 years during the 2010/11 season was confirmed by reports from laboratory data which showed that the proportion of positive influenza A(H1N1)2009pdm specimens for the 5 to 14 year age group decreased from 43.3% during the 2009 pandemic to 17.6% in 2010/11. Moreover, the observed median age of cases of influenza A(H1N1)2009pdm in 2010/11 - the first season of operation of this new sentinel surveillance system - was 35 years, compared with 20 years during the 2009 pandemic.
In conclusion, these data indicate an upward shift in the age-distribution of influenza A(H1N1)2009pdm during the 2010/11 influenza season as compared to the pandemic season 2009/10.
ECDC Comment (31 January 2012):
A repeated observation across countries in the EU and world-wide during the 2009 pandemic were weaknesses in surveillance for severe influenza disease, that is disease due to confirmed influenza and appearing in hospital or resulting in deaths. Exceptions were where such surveillance was already in place, such as paediatric influenza deaths reporting or the Emerging Infections Programme in the USA.(4) This example is one of the early routine systems to be established in Europe after the World Health Organization (WHO) and for the European Centre for Disease Prevention and Control (ECDC), highlighted the importance of establishing systems to monitor severe influenza in the wake of the 2009 influenza pandemic. The Health Protection Agency has successfully established a sentinel surveillance system for severe influenza in 2010/11, detecting a rise in influenza cases mirroring other surveillance indicators.(2,3) Systems like this one, based on laboratory confirmed influenza are proving equally or more suited to Europe than the syndromic ‘SARI’ approach recommended by WHO for resource poor settings. Following work with a number of member states ECDC has recently issued a consultation document on this kind of development and comments are welcomed on this.
The observation of a worse second winter following a pandemic is not unheard of by any means. It indicates the difficulties of the transition period from a pandemic to the enw seasonal influenza. A number of countries recorded similar phenomenon in earlier pandemics. This is documented in the UK paper and has been noted before by ECDC, especially in the 1968-70 and 2009 pandemics. (2,5)
1) Nguyen-Van-Tam J, Openshaw PJM, Hashim A, Gadd Eet al & Influenza Clinical Information Network Thorax 2010 65: 645-651 doi: 10.1136/thx.2010.135210 http://thorax.bmj.com/content/65/7/645.full.pdf
2) ECDC Risk Assessment Seasonal influenza 2010–2011 in Europe (EU/EEA countries) January 2011 http://ecdc.europa.eu/en/publications/Publications/110125_RA_Seasonal_Influenza_EU-EEA_2010-2011.pdf
3) Bolotin S , Pebody R , White PJ , et al. Sentinel Surveillance System for Severe Influenza in England - a Shift in Age Distribution of Hospitalised Cases in the Post-Pandemic Period PLoS ONE 7(1): e30279. 2012
4) US Centers for Disease Control and Prevention Fluview http://www.cdc.gov/flu/weekly/summary.htm
5) Rizzo C, Bella A, Viboud C, Simonsen L, Miller MA, Rota MC, et al. Trends for influenza-related deaths during pandemic and epidemic seasons, Italy, 1969–2001. Emerg Infect Dis. 2007 13 No. 5. http://www.cdc.gov/eid/content/13/5/694.htm