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Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

  • The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
  • The English language content on this website is being archived for historic and reference purposes only.
  • For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

2009 H1N1 Vaccine Safety Summary

December 4, 2009 4:00 PM ET

In the December 4, 2009 Morbidity and Mortality Weekly Report (MMWR), CDC provides preliminary data on the number and types of adverse health events that have been reported to occur following vaccination with 2009 H1N1 from the first months of reports received through the U.S. Vaccine Adverse Event Reporting System (VAERS), a national surveillance system and data from the Vaccine Safety Datalink.

The VAERS database was searched from July 1, 2009 through November 24, 2009 to identify all U.S. reports received of adverse events following vaccination with 2009 H1N1 vaccines and 2009 seasonal influenza vaccines. Data from VAERS identifies adverse health events that occurred following vaccination, but VAERS data cannot be used to assess causality.

As of November 24, 2009, VAERS had received 3,783 adverse event reports following 2009 H1N1 vaccination. The vast majority (95%) of adverse events reported to VAERS after receiving the 2009 H1N1 vaccine were not serious (e.g., soreness at the vaccine injection site). Of the 3,783 total adverse event reports, 204 (5%) were reports that involved what would be considered serious health events (defined as life threatening or resulting in death, major disability, abnormal conditions at birth, hospitalization, or extension of an existing hospitalization).

The percentage of reports involving what would be considered serious health events is not substantially different between 2009 H1N1 and seasonal influenza vaccines. Additionally, no new or unusual events or pattern of adverse events have emerged.

Among the 204 reports of serious health events, there were 13 reports of death. The 13 VAERS reports that involve deaths are under review by CDC, FDA and the states where the reported deaths occurred. Preliminary findings do not indicate a common cause or pattern (such as similarities in age, gender, geographic location, illness surrounding death, or underlying medical conditions) to suggest that these deaths were associated with the vaccine. These cases are under further review pending additional medical records (e.g. autopsy reports, medical files).

VAERS received 10 reports of Guillain-Barré syndrome (GBS), for which follow-up assessments are underway. In the United States, about 80-160 cases of GBS are expected to occur each week, regardless of vaccination.

Eleven (11) reports of anaphylaxis were received by VAERS, and an additional 8 reports of possible anaphylaxis were identified. These 19 reported cases of anaphylaxis are not at a rate above the background rate for this adverse event.

Data from VAERS show that the overall adverse event reporting rate after 2009 H1N1 vaccination is higher than that for seasonal influenza vaccination. Although this might represent an actual difference in safety, increased reporting rates are expected due to the efforts to enhance reporting to VAERS and the heightened public awareness of the 2009 H1N1 vaccine.

VAERS reports continue to be monitored as more vaccine is administered. In addition, the Federal government’s robust vaccine safety monitoring program has been enhanced further to ensure 2009 H1N1 vaccine safety. Details available at "Federal Plans to Monitor Immunization Safety for the Pandemic 2009 H1N1 Influenza Vaccination Program".

As of November 24, 2009, nearly 52 million doses of 2009 H1N1 vaccine had been shipped to vaccination providers in the United States.

For information about vaccine safety can be found at Vaccine Safety.

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