title: Age-related differences in the accuracy of web query-based predictions of influenza-like illness.
abstract: BACKGROUND
  Web queries are now widely used for modeling, nowcasting and forecasting influenza-like illness (ILI). However, given that ILI attack rates vary significantly across ages, in terms of both magnitude and timing, little is known about whether the association between ILI morbidity and ILI-related queries is comparable across different age-groups. The present study aimed to investigate features of the association between ILI morbidity and ILI-related query volume from the perspective of age.
  
  
  METHODS
  Since Google Flu Trends is unavailable in Italy, Google Trends was used to identify entry terms that correlated highly with official ILI surveillance data. All-age and age-class-specific modeling was performed by means of linear models with generalized least-square estimation. Hold-out validation was used to quantify prediction accuracy. For purposes of comparison, predictions generated by exponential smoothing were computed.
  
  
  RESULTS
  Five search terms showed high correlation coefficients of > .6. In comparison with exponential smoothing, the all-age query-based model correctly predicted the peak time and yielded a higher correlation coefficient with observed ILI morbidity (.978 vs. .929). However, query-based prediction of ILI morbidity was associated with a greater error. Age-class-specific query-based models varied significantly in terms of prediction accuracy. In the 0-4 and 25-44-year age-groups, these did well and outperformed exponential smoothing predictions; in the 15-24 and ≥ 65-year age-classes, however, the query-based models were inaccurate and highly overestimated peak height. In all but one age-class, peak timing predicted by the query-based models coincided with observed timing.
  
  
  CONCLUSIONS
  The accuracy of web query-based models in predicting ILI morbidity rates could differ among ages. Greater age-specific detail may be useful in flu query-based studies in order to account for age-specific features of the epidemiology of ILI.
authors.affiliation: Department of Health Sciences, University of Genoa, Genoa, Italy.
authors.forename: Alexander
authors.initials: A
authors.lastname: Domnich
authors.affiliation: Department of Health Sciences, University of Genoa, Genoa, Italy; Inter-University Centre of Research on Influenza and other Transmissible Infections (CIRI-IT), Genoa, Italy.
authors.forename: Donatella
authors.initials: D
authors.lastname: Panatto
authors.affiliation: Department of Health Sciences, University of Genoa, Genoa, Italy.
authors.forename: Alessio
authors.initials: A
authors.lastname: Signori
authors.affiliation: Department of Health Sciences, University of Genoa, Genoa, Italy; Inter-University Centre of Research on Influenza and other Transmissible Infections (CIRI-IT), Genoa, Italy.
authors.forename: Piero Luigi
authors.initials: PL
authors.lastname: Lai
authors.affiliation: Department of Health Sciences, University of Genoa, Genoa, Italy; Inter-University Centre of Research on Influenza and other Transmissible Infections (CIRI-IT), Genoa, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Gasparini
authors.affiliation: Department of Health Sciences, University of Genoa, Genoa, Italy; Inter-University Centre of Research on Influenza and other Transmissible Infections (CIRI-IT), Genoa, Italy.
authors.forename: Daniela
authors.initials: D
authors.lastname: Amicizia
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007251:Influenza, Human / Q000453:epidemiology*; D020407:Internet; D007558:Italy / Q000453:epidemiology; D008875:Middle Aged; D009017:Morbidity; D015203:Reproducibility of Results; D057386:Search Engine; D013997:Time Factors; D055815:Young Adult