FluSurvey (influenza-like illness rate)
Last updated on Friday, 18 September 2026 at 04:12pm
Summary
- Topic
- Influenza
- Category
- API name
influenza_cases_surveyILIRateByWeek
Rationale
This metric helps monitor influenza-like illness (ILI) in the community. It includes people who may not use healthcare services. This can provide a broader picture of respiratory illness than other systems which rely on healthcare use.
Definition
The weekly rate of FluSurvey participants living in England who report symptoms that meet the ILI case definition and the weekly rate of FluSurvey participants living in England who report both symptoms that meet the ILI case definition and subsequent use of healthcare services.
The rate is shown per 1,000 participants who completed the weekly symptoms survey.
Methodology
FluSurvey (https://flusurvey.net/) is an online survey used to monitor respiratory illness in the UK. Participants complete regular surveys about:
• respiratory symptoms
• social contacts
• use of healthcare services
For this metric, only surveys from participants living in England are included.
A participant is counted as having ILI if their reported symptoms meet the European Centre for Disease Prevention and Control (ECDC) case definition. The definition requires a sudden onset of symptoms and:
• at least one general symptom, such as fever or chills, malaise, headache or muscle pain
• at least one respiratory symptom, such as cough, sore throat or shortness of breath
Read the ECDC case definition.
These symptoms can also be caused by respiratory infections other than influenza.
The weekly ILI rate is calculated as:
number of participants who meet the ILI case definition ÷ number of participants who completed the weekly symptoms survey × 1,000
Each participant is counted once for each reporting week.
Caveats
ILI is based on reported symptoms. It does not confirm that a participant has influenza.
Other respiratory infections, including COVID-19, can cause similar symptoms.
FluSurvey participants may not be representative of the population of England.
Participation is voluntary. The number and characteristics of people completing the survey can change from week to week. Changes in the weekly rate should therefore be interpreted alongside the number of participants completing the survey.