Summary of “Adverse Events following Influenza A (H1N1) 2009 monovalent vaccines reported to the Vaccine Adverse Event Reporting System, United States, October 1, 2009 – January 31, 2010”

The U.S. Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) conducted a comprehensive review of the Vaccine Adverse Event Reporting System (VAERS) to identify potential rare events or unusual adverse event (AE) patterns after 2009 H1N1 influenza vaccination. The article, “Adverse events following influenza A (H1N1) 2009 monovalent vaccines reported to the Vaccine Adverse Event Reporting System, United States, October 1, 2009-January 31, 2010,” is available online in the October 2010 issue of Vaccine.

This CDC-FDA study concluded that the safety profile of 2009-H1N1 influenza vaccines was consistent with that of seasonal influenza vaccines.  While the rate of reporting adverse events to VAERS was higher for all age groups following 2009-H1N1 influenza vaccine than seasonal flu vaccine, this was believed to be due in part to heightened public awareness and stimulated reporting. The proportion of serious reports received following 2009-H1N1 vaccines was generally consistent with the pattern of reporting over the last 5 years of VAERS data for seasonal influenza vaccines. This study, using VAERS data contributed to a large federal vaccine safety monitoring effort.

VAERS Summary:

• After 82 million doses of 2009-H1N1 influenza vaccinations given by January 31, 2010, VAERS had received 10,085 adverse event reports following 2009 monovalent H1N1 vaccination by March 15, 2010.
• The vast majority (93%) of adverse events reported to VAERS after receiving the 2009 monovalent H1N1 influenza vaccine were classified as non-serious (e.g., soreness at the vaccine injection site).
• Of the 10,085 adverse event reports, 726 (7.2%) were classified as “serious” health events (defined as life threatening or resulting in death, major disability, abnormal conditions at birth, hospitalization, or extension of an existing hospitalization).
• Sufficient information was available in the VAERS reports and medical records to classify 559 of 570 non-fatal serious reports after 2009-H1N1 vaccination received by January 31, 2010. The most frequent type of non-fatal serious reports following inactivated and live, attenuated 2009-H1N1 influenza vaccines were neurologic (34%) and respiratory conditions (31%), respectively.
• Among the 726 reports of serious health events, there were 48 reports of death.  Review of these deaths revealed no pattern that would suggest a causal association between the vaccine and deaths; there were no deaths due to anaphylaxis. 
• The most common reported cause of death was cardiovascular conditions (22 reports). Among individuals with this reported cause of death, 94% had underlying medical conditions and risk factors for cardiovascular disease.
• During the study period for this publication, VAERS had received 176 possible GBS reports – 99 (56%) reports were verified. Of the verified GBS reports, 93 had symptom onset within 42 days of 2009-H1N1 vaccination. VAERS had received 278 reports of possible anaphylaxis after 2009- H1N1 influenza vaccination and verified 117.  Death, GBS and anaphylaxis reports to VAERS after 2009-H1N1 vaccine were rare (each < 2 per million doses administered).

Facts about VAERS:

• VAERS, jointly administered by CDC's Immunization Safety Office and FDA, is a national reporting surveillance system that accepts adverse event reports following receipt of any U.S.-licensed vaccine.
• VAERS receives information from different sources (vaccine recipients, parents, other family members, doctors, other healthcare providers, and the vaccine manufacturer) across the United States that choose to report an adverse event occurring after vaccination. VAERS is designed to identify potential adverse events that warrant additional study.
• Serious adverse event and other selected reports are reviewed by medical officers, nurses, and trained staff at both FDA and CDC. VAERS receives reports of many events that occur following immunization. It serves as an early warning system that can detect patterns in reports and determine whether further investigation is necessary.
• An adverse event is a health problem that is reported after someone gets a vaccine or medicine. It may or may not have been caused by the vaccine or medicine. Some of these events may occur by chance or during the period following vaccination, while others may actually be caused by vaccination.

VAERS Limitations:

• When reviewing data from VAERS, please keep in mind what the system is designed to do and what it is unable to do:
  • VAERS is a national reporting system, in which reports are submitted voluntarily by people who think an adverse event occurred after vaccination.  VAERS does not solicit reports in any systematic way from all people who have been vaccinated.  Reports can be submitted by anyone, including healthcare providers, patients, or family members. Because of this feature, VAERS reports may and often do include incorrect and incomplete information.  VAERS reports often lead to more complete follow-up and review of medical records.
  • VAERS staff follow-up on all serious and other selected adverse event reports and obtain additional medical, laboratory, and/or autopsy records when available. As a result of the follow-up/review process, coding terms (e.g., serious or non-serious) for individual VAERS reports may change based on the information received. VAERS data on WONDER should be used with caution as numbers and conditions do not reflect data collected during follow-up. Events reported in VAERS should not be viewed as evidence that the vaccine directly caused the event. Reports do not infer causality; further investigation is required.
  • Underreporting, or failure to report events, is also one of the main limitations of VAERS. Serious medical events are more likely to be reported than minor events. 
  • Most importantly, VAERS cannot determine cause-and-effect. VAERS accepts all reports without regard to whether or not the event was caused by the vaccine. The report of an adverse event to VAERS does not mean that a vaccine caused the event. It only indicates that the event occurred sometime after administration of the vaccine. Proof that the event was caused by the vaccine is NOT required in order for VAERS to accept the report.  
  • No reports are deleted from VAERS.  Therefore, it is possible to have more than one VAERS report on an individual case (e.g., a physician and a patient may have filed separate reports for the same case).
  • For all reports of serious adverse events, VAERS staff seeks follow-up medical records on each case, and medical officers review them closely to determine if any additional action or studies may be needed.
  • The most reliable information about vaccine side effects can be found in the manufacturers’ vaccine package insert, vaccine information statements (VISs), or the Advisory Committee on Immunizations Practices' (ACIP's) statements on vaccines.