Abstract
BackgroundConsultation rates of influenza-like illness (ILI) in an outpatient setting have been regarded as a good indicator of influenza virus activity in the community. As ILI-like symptoms may be caused by etiologies other than influenza, and influenza virus activity in the tropics and subtropics is less predictable than in temperate regions, the correlation between of ILI and influenza virus activity in tropical and subtropical regions is less well defined.Methodology and Principal FindingsIn this study, we used wavelet analysis to investigate the relationship between seasonality of influenza virus activity and consultation rates of ILI reported separately by General Out-patient Clinics (GOPC) and General Practitioners (GP). During the periods 1998–2000 and 2002–2003, influenza virus activity exhibited both annual and semiannual cycles, with one peak in the winter and another in late spring or early summer. But during 2001 and 2004–2006, only annual cycles could be clearly identified. ILI consultation rates in both GOPC and GP settings share a similar non-stationary seasonal pattern. We found high coherence between ILI in GOPC and influenza virus activity for the annual cycle, but this was only significant (p<0.05) during the periods 1998–1999 and 2002–2006. For the semiannual cycle high coherence (p<0.05) was also found significant during the period 1998–1999 and year 2003 when two peaks of influenza were evident. Similarly, ILI in GP setting is also associated with influenza virus activity for both the annual and semiannual cycles. On average, oscillation of ILI in GP and of ILI in GOPC preceded influenza virus isolation by approximately four and two weeks, respectively.ConclusionsOur findings suggest that consultation rates of ILI precede the oscillations of laboratory surveillance by at least two weeks and can be used as a predictor for influenza epidemics in Hong Kong. The validity of our model for other tropical regions needs to be explored.
Highlights
Influenza has been associated with a heavy burden on morbidity and mortality, both in the temperate and subtropical/tropical regions [1,2,3,4]
Laboratory surveillance was implemented at the same time, in which respiratory specimens were collected from influenza-like illness (ILI) patients in General Practitioners (GP) and General Out-patient Clinics (GOPC) settings as well as patients hospitalized with acute respiratory diseases
It is interesting that the emergence of a semiannual cycle in Hong Kong coincided with significant antigenic change in influenza A(H3N2) viruses
Summary
Influenza has been associated with a heavy burden on morbidity and mortality, both in the temperate and subtropical/tropical regions [1,2,3,4]. Non-specific symptoms of influenza infection reduce the reliability for early detection of influenza epidemics based on increased consultation rates of influenza-like illness (ILI). We used wavelet analysis to investigate the relationship between seasonality of influenza virus activity and consultation rates of ILI reported separately by General Out-patient Clinics (GOPC) and General Practitioners (GP). We found high coherence between ILI in GOPC and influenza virus activity for the annual cycle, but this was only significant (p,0.05) during the periods 1998–1999 and 2002–2006. ILI in GP setting is associated with influenza virus activity for both the annual and semiannual cycles. Our findings suggest that consultation rates of ILI precede the oscillations of laboratory surveillance by at least two weeks and can be used as a predictor for influenza epidemics in Hong Kong. The validity of our model for other tropical regions needs to be explored
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