Harmonizing influenza primary-care surveillance in the United Kingdom: piloting two methods to assess the timing and intensity of the seasonal epidemic across several general practice-based surveillance schemes

Abstract

General Practitioner consultation rates for influenza-like illness (ILI) are monitored through several geographically distinct schemes in the UK, providing early warning to government and health services of community circulation and intensity of activity each winter. Following on from the 2009 pandemic, there has been a harmonization initiative to allow comparison across the distinct existing surveillance schemes each season. The moving epidemic method (MEM), proposed by the European Centre for Disease Prevention and Control for standardizing reporting of ILI rates, was piloted in 2011/12 and 2012/13 along with the previously proposed UK method of empirical percentiles. The MEM resulted in thresholds that were lower than traditional thresholds but more appropriate as indicators of the start of influenza virus circulation. The intensity of the influenza season assessed with the MEM was similar to that reported through the percentile approach. The MEM pre-epidemic threshold has now been adopted for reporting by each country of the UK. Further work will continue to assess intensity of activity and apply standardized methods to other influenza-related data sources.

Fig. 1.

Illustration of (a) the moving epidemic method approach and (b) the percentile approach. RCGP, Royal College of General Practitioners; ILI, influenza-like illness.

Fig. 2.

Weekly Royal College of General Practitioners (RCGP) influenza-like illness (ILI) GP consultation rates per 100 000 population compared to confirmed influenza A and B hospitalizations* and proportion of samples positive for influenza† (England), 2011/12 and 2012/13. [* A sentinel network (USISS sentinel hospital network) of acute NHS trusts has been established in England to report weekly laboratory-confirmed influenza cases hospitalized at all levels of hospital care. † The Respiratory Datamart System is a laboratory-based surveillance scheme, including both positive and negative results, and has been established in England to monitor the activity of influenza and other respiratory viruses.]

Fig. 3.

Weekly influenza-like illness (ILI) GP consultation rates per 100 000 population by scheme and their corresponding moving epidemic method (MEM) category and percentile category, 2012/13, UK. (a) MEM categories; (b) percentile categories. RCGP, Royal College of General Practitioners; ILI, influenza-like illness.

Fig. 4.

Weekly influenza-like illness (ILI) GP consultation rates per 100 000 population by scheme* with corresponding thresholds and percentile category, 2011/12 and 2012/13†. - - - -, traditional threshold; ·····, moving epidemic method pre-epidemic threshold. (* Data was not available for Scotland for the 2011/12 season due to general practice software changes. † The weeks are colour-coded as follows: blue, <50th percentile; green, 50–75th percentile; yellow, 75–85th percentile; orange, 85–95th percentile; red, 95–99th percentile. Red dot indicates week of peak sentinel positivity with corresponding % samples positive for influenza. Few samples were received in 2011/12 and so % positive is not shown. Fewer than ten samples were received weekly though the Welsh scheme in 2012/13 and so % positive is not shown.)