Proposed working definition of an active case of influenza: research report 2

Report

Background: NHS organisations lack a consistent operational definition of an “active case” of influenza. Analysis of 161 Freedom of Information responses from NHS organisations across the United Kingdom identified substantial heterogeneity in case definitions and widespread absence of reproducible criteria. Under the prespecified classification, only 2 responses were operationally adequate, while 159 of 161 (98.8%) failed to meet this standard. None incorporated all four evidential domains proposed in this framework: clinical illness, temporal relevance, influenza-specific laboratory evidence, and viral burden or activity.

Aim: To propose a working definition of active influenza suitable for NHS Trust clinical, surveillance, and analytical use, derived from the deficiencies identified in current practice.

Framework: The proposed approach distinguishes laboratory detection of influenza from evidence of current active disease. Classification is based on four complementary domains: (1) a current influenza-compatible acute clinical illness; (2) a defined temporal relationship between symptom onset, specimen collection, and ongoing illness; (3) influenza-specific detection using a validated assay; and (4) evidence of viral burden or activity, where technically available and appropriately validated. Molecular measures such as Ct/Cq/Cn or quantitative viral load should be interpreted in the context of assay platform, specimen type, sampling quality, and timing rather than through a universal Ct threshold. Serial measurements may reduce uncertainty concerning changes in viral burden but do not, in themselves, establish infectiousness.

Implications: The framework provides a reproducible distinction between suspected active influenza, laboratory-confirmed influenza infection, an active influenza case, and potentially infectious influenza. It also proposes a minimum reporting dataset that incorporates clinical features, symptom onset, specimen information, influenza-specific results, viral burden measures where available, serial trends, clinical status, and assigned case state. The framework is an analytical working definition derived from an FOI dataset rather than a consensus guideline, and influenza-specific validation of viral-burden measures, thresholds, and sampling schedules remains necessary.

Authors: Jefferson, T; Heneghan, C

• Publication status: Published
• Publisher: Trust the Evidence
• Publication date: 2026-09-19
• Language: English