Can syndromic thresholds provide early warning of national influenza outbreaks?
BACKGROUND
Influenza incidence thresholds are used to help predict the likely impact of influenza and inform health professionals and the public of current activity. We evaluate the potential of syndromic data (calls to a UK health helpline NHS Direct) to provide early warning of national influenza outbreaks.


METHODS
Time series of NHS Direct calls concerning 'cold/flu' and fever syndromes for England and Wales were compared against influenza-like-illness clinical incidence data and laboratory reports of influenza. Poisson regression models were used to derive NHS Direct thresholds. The early warning potential of thresholds was evaluated retrospectively for 2002-06 and prospectively for winter 2006-07.


RESULTS
NHS Direct 'cold/flu' and fever calls generally rose and peaked at the same time as clinical and laboratory influenza data. We derived a national 'cold/flu' threshold of 1.2% of total calls and a fever (5-14 years) threshold of 9%. An initial lower fever threshold of 7.7% was discarded as it produced false alarms. Thresholds provided 2 weeks advanced warning of seasonal influenza activity during three of the four winters studied retrospectively, and 6 days advance warning during prospective evaluation.


CONCLUSIONS
Syndromic thresholds based on NHS Direct data provide advance warning of influenza circulating in the community. We recommend that age-group specific thresholds be developed for other clinical influenza surveillance systems in the UK and elsewhere.
Cooper
D L
DL
Regional Surveillance Unit, West Midlands Health Protection Agency, 9th Floor, Ladywood House, 45 Stephenson Street, Birmingham B2 4DY, UK. duncan.cooper@hpa.org.uk
Verlander
N Q
NQ
Elliot
A J
AJ
Joseph
C A
CA
Smith
G E
GE
D000293:Adolescent; D000328:Adult; D000368:Aged; D000465:Algorithms; D002648:Child; D002675:Child, Preschool; D003139:Common Cold / Q000453:epidemiology; D004196:Disease Outbreaks; D004739:England / Q000453:epidemiology; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053124:Influenza A Virus, H5N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology* / Q000503:physiopathology; D008875:Middle Aged; D016012:Poisson Distribution; D011159:Population Surveillance / Q000379:methods*; D011634:Public Health; D012189:Retrospective Studies; D013222:State Medicine; D013577:Syndrome; D014852:Wales / Q000453:epidemiology; D055815:Young Adult