Surveillance for Adverse Events Following Pandemic H1N1 Immunization

Influenza vaccines are continuously modified to adjust to the virus antigenic shifts or drifts, and its safety profile may vary. While generally considered safe, influenza vaccines have been associated in the past with increases in cases of Guillain-Barré syndrome (1976) or with oculorespiratory syndrome (2001). The emergence of a novel strain of H1N1 influenza virus (pH1N1) has prompted health departments worldwide to prepare for mass vaccination campaigns with a new H1N1 pandemic vaccine. Following recommendations of the World Health Organization (WHO), Canada immunized its population with a dose-sparing adjuvanted vaccine. While the adjuvant developed by GlaxoSmithKline (GSK) has been administered to over 39 000 people, only a few hundred will have been vaccinated with the H1N1 formulation in clinical trials before the mass campaign was launched. With this small number, adverse events occurring at a rate < 1% will not be detected by these clinical trials.

Considering that most cases of pH1N1 to date have been relatively mild, it will be imperative to rapidly detect adverse events serious enough to reconsider the use of the vaccine. Passive surveillance is collecting notifications of adverse events but the sensitivity of this system is not high and its timeliness is not necessarily optimal.

In Canada, healthcare workers (HCW) are among those who were offered the new pandemic vaccine in priority. Because they were offered the vaccine early in the campaign and because they constitute a well-defined group in good general health, this group of people may be well suited for monitoring the safety of the pH1N1 vaccine.

The main objective of this project was to estimate in HCW vaccinated against pH1N1 the frequency of adverse events of sufficient severity to cause work absenteeism or medical consultation.

The active surveillance was done in HCW from 3 sites (Canadian hospitals) participating in the Pandemic Influenza Research Network (PCIRN): (1) Halifax (Nova Scotia), (2) Quebec City (Quebec): 3 hospitals, (3) Toronto