The CDC and FDA provide weekly updates on vaccine safety monitoring activities in an effort to put the data that is publicly available through the Vaccine Adverse Event Reporting System (VAERS; and CDC’s website, WONDER ( in context. The following information summarizes adverse event reports to VAERS after the administration of 2009 H1N1 monovalent influenza vaccine (either nasal spray or shot).

Swine Flu Shot
An adverse event is a health problem that is reported after someone gets a vaccine or medicine. Note that persons may experience adverse events shortly after vaccination which may or may not be caused by the vaccine. While VAERS is an important system for helping to find potential signs, VAERS is primarily used to detect signals that may require further investigation, but is not able to determine if an adverse event was caused by vaccination.

VAERS Summary:

  • As of January 8, 2010, over 110 million doses of 2009 H1N1 vaccine had been shipped to vaccination providers in the United States, although the precise number of vaccines administered is unknown.
  • As of January 8, 2010, VAERS had received 7814 adverse event reports following 2009 monovalent H1N1 vaccination.
  • The vast majority (94%) of adverse events reported to VAERS after receiving the 2009 monovalent H1N1 vaccine are classified as “non-serious” (e.g., soreness at the vaccine injection site).
  • Of the 7814 reports, 477 (6%) were reports that 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)*.
  • The percentage of reports involving what would be considered serious health events is not different between 2009 H1N1 and seasonal influenza vaccines. Additionally, no new or unusual events or pattern of adverse events have emerged.
  • VAERS reports continue to be monitored as more vaccine is administered.
  • Among the 477 reports of serious health events, there were 33 reports of death.
    • As with all reports of serious adverse events and deaths, the 33 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 has received 46 reports of Guillian-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.

VAERS Limitations

When reviewing data from VAERS, 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. These changes are reflected in the weekly updates of VAERS data in the WONDER database. VAERS data in WONDER should be used with caution because numbers and conditions are often updated. Events reported in VAERS should not be viewed as evidence that the vaccine directly caused the event. Data does not infer causality. Further investigation is warranted.
  • 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.