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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.

Questions & Answers

2009 H1N1 Flu In The News

April 23, 2010 5:30 PM ET

2009 H1N1 Vaccine Safety Update

On April 23rd, the National Vaccine Advisory Committee (NVAC) received the fourth report from its H1N1 Vaccine Safety Risk Assessment Working Group (VSRAWG) that summarizes the review of available safety monitoring data from the Federal vaccine safety surveillance systems. This document provides a summary of the findings from this workgroup.  The working group recommended continued vaccine safety monitoring. 

What is the overall safety record of the 2009 H1N1 influenza vaccine?

The 2009 H1N1 influenza vaccine is showing a similar safety profile to seasonal flu vaccines. Over the years, hundreds of millions of Americans have received seasonal flu vaccines. The most common side effects following flu vaccinations are mild local injection site reactions such as soreness, redness, tenderness or swelling where the shot was given.

Federal scientists, the Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA), the Department of Defense (DoD), the Veterans Administration (VA), Centers for Medicare and Medicaid Services (CMS), and the Indian Health Service (IHS) have been working collaboratively to monitor 2009 H1N1 vaccine safety through twelve different vaccine safety monitoring systems.  These findings have been presented to VSRAWG throughout the 2009 H1N1 vaccination campaign.  In this recent report, VSRAWG looked primarily at three types of adverse events: Guillain-Barré syndrome (GBS), Bell’s palsy, and thrombocytopenia (a blood disorder).

What has an analysis of all vaccine safety monitoring systems found regarding the safety of 2009 H1N1 influenza vaccine?

In a report to NVAC, VSRAWG (a group of nongovernmental experts who review the vaccine safety data regularly) indicated that preliminary analyses from some of the vaccine safety monitoring systems have found some cases of Guillain-Barré syndrome (GBS), Bell’s palsy, and thrombocytopenia among some of the vaccinees.  However, these findings are not seen in all of the Federal vaccine safety monitoring systems.  Federal scientists are working collaboratively to determine whether these preliminary observations are true increases in these adverse events or a result of other factors.  For instance, additional analyses are ongoing and chart reviews are underway to verify reports.

The Federal vaccine safety system has many redundancies built in to ensure the safety of the vaccine.  Each surveillance system is monitoring a slightly different population or are targeting a different outcome.  Therefore, data from each system must be reviewed independently.  As a whole, these systems provide a collective picture of 2009 H1N1 vaccine safety.

Has Guillain-Barré syndrome (GBS) been reported after 2009 H1N1 vaccination?

CDC has been monitoring for cases of GBS following the 2009 H1N1 influenza vaccination by utilizing three vaccine safety surveillance systems, including: Vaccine Adverse Event Reporting System (VAERS), Vaccine Safety Datalink (VSD), and collaboration in 10 states to actively look for cases of GBS regardless of vaccination in the Emerging Infections Program (EIP).  These systems were designed to alert us quickly should GBS-related issues arise.

Although not statistically significant, preliminary data from EIP suggest the possibility that 2009 H1N1 flu vaccine recipients might have a rate of GBS similar to what has been seen with seasonal influenza vaccines in the past (about one in 1 million people vaccinated).  This finding is very preliminary and may be due to chance.   Further investigation is underway.

As of March 26, 2010, VAERS had received a total of 132 reports of unverified GBS after 2009 H1N1 influenza vaccination.  In the United States, about 80-160 cases of GBS occur in the general population each week, regardless of vaccination, and are frequently associated with upper respiratory and gastrointestinal infections.

The benefits of getting influenza vaccines far outweigh the risk of GBS.  Vaccination remains the most important step in preventing serious illness and death from influenza.  For more information about GBS: Fact Sheet: Guillain-Barré Syndrome (GBS).

What is Bell’s palsy and have there been any signs of it following 2009 H1N1 influenza vaccination?

Bell’s palsy is a temporary weakness or paralysis of facial muscles on one side of the face resulting from nerve dysfunction.  Various studies show Bell’s palsy rates may be higher for females especially during childbearing years or during pregnancy.  Studies have suggested Bell’s palsy is more common among people with diabetes, hypertension, influenza, a cold, or an upper respiratory infection.

Of the Federal vaccine safety monitoring systems, preliminary findings from two surveillance systems show cases of Bell’s palsy following 2009 H1N1 influenza vaccination.  Federal scientists continue to review these data and are conducting additional (more rigorous) analyses to better understand the finding.  Bell’s palsy can occur more often after infection with cold and flu viruses.  An increase seen in these surveillance systems may be due to the time of year in which the monitoring occurred.  Available data in all the other systems is also being reviewed to investigate these findings.

What is thrombocytopenia and how many reports has CDC received following 2009 H1N1 influenza vaccination?

Thrombocytopenia, or low blood platelet count, is a blood disorder that can occur for a variety of reasons. Platelets play an important role in blood clotting.  Some of the common causes of thrombocytopenia include pregnancy, bacterial infections, autoimmune diseases (i.e., lupus), or medications.

Although CDC’s vaccine safety monitoring systems did not detect an increase in thrombocytopenia post-2009 H1N1 influenza vaccination, two other Federal surveillance systems found small increases in the number of cases following 2009 H1N1 vaccination in their populations under surveillance.  These cases are also under review to determine if the findings remain after medical records are reviewed. CDC, in conjunction with other Federal officials, is continuing to monitor for this event as well as other adverse events following 2009 H1N1 vaccination.

How does 2009 H1N1 vaccine safety compare to seasonal flu vaccine safety?

The percentage of serious health events reported after 2009 H1N1 vaccination is very similar to the percentage of reports that typically follow seasonal influenza vaccines. Additionally, no unexpected or unusual events or pattern of adverse events have emerged.

How has the government monitored 2009 H1N1 vaccine safety?

The Federal government has a robust vaccine safety monitoring program that 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.

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