Monitoring the safety of influenza A (H1N1) vaccine using web-based intensive monitoring

Background: When adjuvant vaccines against the pandemic influenza A (H1N1) virus became available after an accelerated registration process, safety issues dominated the public debate. As part of the immunisation campaign, the Dutch government installed an active monitoring of possible adverse events following immunisation (AEFIs). As part of the monitoring we conducted an anonymous prospective cohort study to identify and quantify the occurrence of AEFIs related to pandemic vaccination among the population immunised in general practice.

Method: Adults aged 60 years and older or persons with a risk-elevating medical condition recommended for vaccination in general practice were eligible for participation. After receipt of the first pandemic vaccine the administrator handed over an information flyer of the web-based monitoring program. The patient could sign up for study participation online. Within one week, three weeks and three months after the first immunisation questions were asked about demographics and health, immunisations, injections site reactions and labeled reactions as well as other possible new AEFIs.

Results: In all, 3569 participants filled in the first questionnaire. Corresponding figures for the second and third questionnaires were 3395 (95.1%) and 3162 (88.6%). Mean age was 58 years (SD 15) and 50.1% was female. Main indication was 60 years or older followed by presence of pulmonary or cardiovascular disease. Of all participants, 1311 (37%) reported an AEFI. Unexpected serious reactions were not reported nor were there signals of possible new AEFIs. The occurrence of an AEFI was determined by gender, age and type of co-morbidity.

Conclusion: The web-based intensive monitoring system among patients immunised in general practice revealed AEFIs due to pandemic vaccination in one-third of participants. There were no unexpected serious adverse events in this population. This advanced methodology can be further developed to monitor real-time use and AEFIs of vaccines.