A cluster of nonspecific adverse events in a military reserve unit following pandemic influenza A (H1N1) 2009 vaccination-possible stimulated reporting?

Background: On February 20, 2010, a 23 year old male Army Reservist (index case) with symptom onset 4 h after receiving inactivated monovalent pandemic 2009 (H1N1) vaccine (MIV) was hospitalized with possible Guillain-Barré syndrome (GBS). Within 1-2 days, 13 reservists from the same unit presented to the emergency department and 14 filed Vaccine Adverse Event Reporting System (VAERS) reports of nonspecific symptoms following MIV.

Objectives: To describe the spectrum of adverse events (AE) among reservists in the unit after MIV and to identify factors contributing to this cluster of reports.

Methods: We reviewed the reservists' VAERS reports and hospital records for demographics, influenza vaccination status, diagnostic results and outcome. All VAERS reports after vaccination from the same MIV lot were also screened. We conducted a survey of unit reservists to identify contributing factors for this cluster.

Results: The presumptive diagnosis of GBS in the index case was not confirmed. All other reservists demonstrated normal exam findings and laboratory investigations. VAERS reports following vaccination from the same MIV lot revealed no consistent pattern. Our survey of factors contributing to the cluster was returned by 55 reservists (response rate 28%). AEs following MIV were significantly more often reported by female and black reservists. There was a tendency for concern about the safety of the 2010-2011 seasonal influenza vaccine to be higher for reservists that reported an AE to MIV (p=0.13) or that sought medical attention for their symptoms (p=0.08).

Conclusions: This cluster represents possible stimulated reporting following receipt of inactivated pandemic 2009 (H1N1) vaccine among service personnel.