Phase | Data type | Participants | Measurement |
Baseline/enrolment (years 1 and 2 finished, year 3 ongoing) | Face-to-face questionnaire interview* | All | Basic demographics, education-level attained, underlying medical conditions, seasonal influenza vaccination receipt, potential risk factors for influenza, maternal history, contact information including telephone numbers and home address |
Active follow-up twice weekly (ongoing) | Phone call/WeChat Questionnaire survey† | All participants with any illness | Any illness since last contact? Symptoms; timing and characteristics of acute respiratory illness; health-seeking behaviour and medication taken |
Combined nasal and throat swab | Participants reporting an acute respiratory illness‡ | Real-time reverse transcription polymerase chain reaction for influenza subtype/lineage | |
Paired sera | Participants enrolled before winter influenza season start | Seroconversion of antibodies against the vaccine and circulating strains determined by haemagglutination inhibition assay | |
Data linkage (planned) | Maternal and child records | All | Pregnancy outcome; gestational age at birth; birth weight |
Hospital medical records | Participants hospitalised during study | Prescriptions, duration and cost of hospitalisation, clinical outcome |
*See Annex 1.
†See Annex 2.
‡An acute respiratory illness refers to onset of at least one respiratory symptom (cough, sore throat, stuffy nose, chest pain or difficulty breathing) and at least one systemic symptom (feeling feverish or having a measured fever ≥38°C, chills or headache) or at least two respiratory symptoms.