Does knowing the influenza epidemic threshold has been reached influence the performance of influenza case definitions?

Abstract

Background Disease surveillance using adequate case definitions is very important. The objective of the study was to compare the performance of influenza case definitions and influenza symptoms in the first two epidemic weeks with respect to other epidemic weeks. Methods We analysed cases of acute respiratory infection detected by the network of sentinel primary care physicians of Catalonia for 10 seasons. We calculated the diagnostic odds ratio (DOR) and 95% confidence intervals (CI) for the first two epidemic weeks and for other epidemic weeks. Results A total of 4,338 samples were collected in the epidemic weeks, of which 2,446 (56.4%) were positive for influenza. The most predictive case definition for laboratory-confirmed influenza was the WHO case definition for influenza-like illness (ILI) in the first two epidemic weeks (DOR 2.10; 95% CI 1.57–2.81) and in other epidemic weeks (DOR 2.31; 95% CI 1.96–2.72). The most predictive symptom was fever. After knowing that epidemic threshold had been reached, the DOR of the ILI WHO case definition in children aged <5 years and cough and fever in this group increased (190%, 170% and 213%, respectively). Conclusions During influenza epidemics, differences in the performance of the case definition and the discriminative ability of symptoms were found according to whether it was known that the epidemic threshold had been reached or not. This suggests that sentinel physicians are stricter in selecting samples to send to the laboratory from patients who present symptoms more specific to influenza after rather than before an influenza epidemic has been declared.

SARS-CoV-2, influenza A/B and respiratory syncytial virus positivity and association with influenza-like illness and self-reported symptoms, over the 2022/23 winter season in the UK: a longitudinal surveillance cohort

“…While ILI-ECDC was more commonly reported than ILI-WHO across all ages for the three infections, only ~ 15% of reported ILI-ECDC could be explained by test positivity for SARS-CoV-2, influenza A/B, or RSV. Prior studies have found higher rates of respiratory test positivity amongst those reporting ILI (and conversely, higher rates of ILI amongst influenza-positives), yet these estimates have generally been based on patients presenting to healthcare with symptoms of respiratory infection [ 8 , 9 , 12 – 14 , 17 , 18 , 41 ]. Such samples will be skewed towards more severe cases, as individuals with milder disease are less likely to seek healthcare.…”

“…The relationship between ILI and influenza positivity remains complex, influenced by differing case definitions [ 10 , 11 ], changes in co-circulation of other viruses (notably respiratory syntactical virus (RSV) and SARS-CoV-2) across seasons [ 12 , 13 ], age-specific dynamics [ 14 ], and the non-specific nature of influenza symptoms [ 15 , 16 ]. Various studies have attempted to assess these relationships, but most have limited their scope to clinical settings, and/or focussed solely on influenza, and/or restricted to patients already reporting ILI or Acute Respiratory Illness (ARI) [ 7 – 9 , 12 – 14 , 17 , 18 ]. Similarly, influenza vaccine effectiveness evaluation typically uses disease endpoints, rather than protection from infection [ 19 ].…”

SARS-CoV-2, influenza A/B and respiratory syncytial virus positivity and association with influenza-like illness and self-reported symptoms, over the 2022/23 winter season in the UK: a longitudinal surveillance cohort

“…15,16 Various studies have attempted to assess these relationships, but most have limited their scope to clinical settings, and/or focussed solely on influenza, and/or restricted to patients already reporting ILI or Acute Respiratory Illness (ARI). [7][8][9][12][13][14]17,18 Similarly, influenza vaccine effectiveness evaluation typically uses disease endpoints, rather than protection from infection. 19 Here we use the Office of National Statistics COVID-19 Infection Survey (CIS) to investigate the relationship between respiratory infection test positivity and ILI/other self-reported symptoms.…”