Fever following immunization with influenza A (H1N1) vaccine in children: a survey-based study in the Netherlands.

Background: In November 2009, all children in the Netherlands from 6 months up to 4 years of age were indicated to receive the Influenza A (H1N1) vaccine. Fever is a common adverse event following immunization in children. Pandemrix®, an inactivated, split-virus influenza A (H1N1) vaccine, was used for this age group. A clinical study mentioned in the Summary of Product Characteristics of Pandemrix® found an increased reactogenicity after the second dose in comparison with the first dose, particularly in the rate of fever. In the Netherlands, this adverse reaction was a point of concern for the parents or caregivers of these children.

Objective: To investigate the course and height of fever following the first and second dose of Pandemrix® in children aged from 6 months up to 4 years. The secondary aim was to evaluate the use of an online survey during a vaccination campaign.

Design: Survey-based descriptive study.

Setting: Adverse drug reaction reporting database of the Netherlands Pharmacovigilance Centre (Lareb).

Participants: Parents or caregivers (n = 839) of vaccinated children who reported fever to Lareb following the first immunization with Pandemrix®. Questionnaires were sent by email to parents or caregivers of eligible children following the first and second doses of Pandremix®.

Main Outcome Measures: Time between vaccination and the occurrence of fever, the maximum measured temperature, the occurrence of other adverse events after first and second vaccination, the decision to get the second vaccination and the social implication of the fever in terms of absence from work, nursery or school, and hospitalization.

Results: Following the first vaccination against Influenza A (H1N1), the height of the fever was between 39.0 and 40.0°C in 359/639 (56.2%) of the children. In most of these children (235/639 [36.8%]), the onset of fever was between 6 and 12 hours following vaccination. 450/639 (70.4%) children recovered within 2 days. Of the 539 responders to the second questionnaire, 380 (70.5%) received the second vaccination against Influenza A (H1N1) and 213 (56.1%) of these children experienced fever again. The height of the fever was significantly lower (t-test; p = 0.001) and the duration was significantly shorter (Pearson's Chi-square; p = 0.002) in comparison with the first vaccination. The height of the fever after the first vaccination was associated with the decision to receive the second vaccination (t-test; p = 0.000). In the studied group, 342 (53.5%) parents or caregivers needed to stay home from work and 405 (63.4%) children stayed home from nursery or school due to fever following the first vaccination.

Conclusions: The results of this study can be used in future vaccination campaigns to be able to inform people in an evidence-based manner about the risks and benefits of the vaccine and to avoid unnecessary concern and negative media attention. This could contribute to improved immunization levels. A web-based survey is demonstrated to be a useful tool to quickly gather information about a current safety concern and consequently inform the public to support an ongoing vaccination campaign.