Pandemic (H1N1) 2009 influenza: surveillance systems

How the pandemic was monitored

The Health Protection Agency (HPA) has internationally respected surveillance systems for monitoring and assessing the incidence and impact of influenza in England. These systems have operated effectively through the normal 'flu season' over the last few years and have informed policy, planning and evaluation of interventions (particularly the uptake of seasonal flu vaccine). These systems are central to influenza surveillance activities in a pandemic situation and are included as part of the UK's pandemic preparedness plans. 

• Clinical surveillance through primary care using networks of sentinel General Practitioners (GPs) and the nurse-led telephone helpline NHS Direct.
• Microbiological surveillance including testing of community and hospitalised patients to assess the burden of disease, circulating influenza strains and antiviral resistance.
• Disease severity and mortality surveillance including excess mortality estimates using data on all-cause mortality from the Office of National Statistics.  

For further information please see the Seasonal Influenza pages.

In addition to the existing influenza surveillance systems, a number of systems specific to this pandemic were implemented. These systems:

• provided epidemic intelligence to inform public health action and policy decisions by assessing the severity of disease associated with this novel virus
• monitored for changes in the characteristics of the virus

A summary of the systems used is provided below.  For further information, please see the Epidemiological report of pandemic (H1N1) 2009 in the UK in which each system is described in more detail and selected results and figures are displayed. 

Pandemic Preparedness Systems ('Containment phase')

In the early stages of the pandemic, during the 'containment' phase, pandemic preparedness systems provided detailed information about the evolving epidemiology, the spectrum of clinical disease, and the transmission characteristics of the disease.

Data were collected on all suspected cases, allowing the age distribution, origin of infection, symptoms etc. to be monitored.  More detailed information was collected on just under 400 early cases, and their close contacts, as part of the First Few Hundred (FF100) Surveillance System. This enabled estimates of transmission characteristics of the novel virus to be calculated.  

“The First Few Hundred (FF100)” Project – Epidemiological Protocol (PDF, 2.1 MB)

To monitor the extent of community transmission during the early stages, a self-sampling scheme through NHS Direct was established.  In this scheme, a selection of NHS Direct callers with an influenza-like illness were sent kits to take a nose/throat sample to be sent back to the lab for testing.

'Treatment-only phase' surveillance systems

In the second phase of the pandemic, the 'treatment-only phase', additional surveillance systems were implemented to monitor the spread and the severity of the pandemic in the population.

The number of assessments, authorisations and antivirals collected through the National Pandemic Flu Service (NPFS) were monitored. This service allowed people suffering from an influenza-like illness in England to access antiviral drugs without attending their GP.  The self-sampling scheme through NHS Direct was transferred to NPFS.

Using clinical and virologica data from GP schemes and NPFS, an attempt was made to provide estimates of the number of new clinical cases of pandemic influenza in England each week.  The below documents provide methodological information on how the estimates were generated:

Synopsis of the method used to estimate the number of pandemic influenza (H1N1) 2009 cases in England in the week 21 to 27July 2009 (PDF, 51 KB)

Method used to estimate new pandemic (H1N1) 2009 influenza cases in England in the week 3 August to 9 August 2009 (PDF, 35 KB)

To assess the severity of the virus, two systems using data from hospitals were used.  The first collected the number of people admitted each week with suspected pandemic influenza and the second provided detailed information on hospitalised patients with confirmed pandemic influenza infection.  Information on cases who died with pandemic influenza was also collected to analyse which groups in the population were most at risk from complications. In addition to this, all-cause mortality surveillance was expanded to include data on age-specific mortality from the General Registry Office.

When the pandemic vaccine was introduced systems were in place to monitor the uptake of the vaccine and it's effectiveness.