Surveillance of Significant Adverse Events Temporally Associated with 2009 Pandemic Influenza A (H1N1) Vaccine In California

Background: Secondary to heightened public concern about the safety of the 2009 pandemic influenza A (H1N1) vaccine, the California Department of Public Health investigated post-vaccination significant adverse events (SAEs). SAEs were defined as one of the following outcomes: death, life threatening illness, prolonged hospitalization, permanent disability, congenital anomalies, or other medically important conditions.

Objectives: Identify and investigate SAEs temporally associated with receipt of the 2009 pandemic influenza A (H1N1) vaccine in California

Methods: SAE surveillance began on October 5, 2009 utilizing weekly CDC/FDA vaccine adverse event reporting system (VAERS) reports and passive reporting from local health departments/clinicians. SAEs selected for investigation were categorized as: 1) anaphylaxis; 2) pregnancy-related; 3) neurologic (excluding Guillain-Barré Syndrome); and 4) other medically significant conditions (e.g., new onset diabetes myelitis). Selected medical records were requested and reviewed to verify diagnosis. When available, specimens were requested and tested for possible infectious etiologies. Rates were compared to other vaccinated populations and/or background rates previously published in scientific literature.

Results: From October 2009 – August 2010, ~15.6 million H1N1 vaccine doses were distributed in California. One-hundred and fourteen SAEs were identified through July 2, 2010, including reports of anaphylaxis (18), spontaneous abortions (16), stillbirths (6), post-infectious encephalitis/myelitis (12), cranial nerve palsies (14), seizures (18), and deaths (4). Anaphylaxis rates were comparable to rates after seasonal influenza vaccination. Rates of pregnancy loss and neurological conditions were lower than expected background rates. Alternative possible etiologies were identified in four neurologic cases (Mycoplasma pneumoniae [1], HSV-1 [2], autoimmune disease [1]) and in four other SAEs (respiratory viruses [2], Streptococcus pneumoniae/pandemic influenza A (H1N1) [2; 1 death]).

Conclusions: SAE rates post-H1N1 vaccination in California do not appear to exceed rates observed in the general or other vaccinated populations. Broad testing for infections detected alternative etiologies in some SAEs. Additional studies to investigate and classify neurological adverse events are warranted.