Focus on Flu

by Anthony Fiore, MD, MPH , medical epidemiologist for the Influenza Division of the CDC.

Like everything having to do with 2009 H1N1, there has been lots of attention on 2009 H1N1 vaccine safety – and now that the vaccine has been available for a while, it’s a good time to check in with the vaccine safety experts responsible for monitoring the reports of health problems (“adverse events” is the official term) after getting the H1N1 vaccine. These initial reports are very reassuring.

On December 4, CDC and FDA released the first of several planned comprehensive studies of safety information available so far. The bottom line: the types and frequency of health problems reported for the 2009 H1N1 flu vaccines are about the same as those we typically see among people who get seasonal influenza vaccines. The safety information we have so far is based on the first 46 million doses of 2009 H1N1 vaccines that were distributed in the United States through November 24, and the number of reports is quite low considering how many doses have gone out.

The studies have found:

• Up to November 24, 2009, the U.S. Vaccine Adverse Event Reporting System (VAERS) had received 3,783 adverse event reports following 2009 H1N1 vaccination.
• Nearly all (95%) of adverse events reported to VAERS after receiving the 2009 H1N1 vaccine were typical short term side effects (e.g., soreness at the vaccine injection site).
• Of the 3,783 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 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. VAERS reports continue to be monitored as more vaccine is administered.
• 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 might have just been due to chance or bad luck – for example, 1 death that occurred after vaccination was due to a motor vehicle accident. These cases are under further review pending additional medical records (e.g., autopsy reports, medical files).
• VAERS received 10 reports of Guillian-Barré syndrome (GBS), and follow-up assessments are underway. An additional 2 reports describing neurologic events are also under review as possible GBS. Remember that GBS can occur for a variety of reasons. In the United States, about 80-160 cases of GBS are expected to occur each week, regardless of vaccination. So it’s not clear that any of these cases of GBS were due to the vaccine.

So what does this mean?

The most reasonable interpretation is that 2009 H1N1 vaccines are not substantially any more or less safe than the seasonal flu shot or nasal spray vaccines available every year. The number of reports is quite low considering that nearly 50 million doses have gone out.

How is the vaccine safety information gathered?

The CDC and FDA review vaccine safety results from two different monitoring systems that have been used for nearly two decades. First, data from reports received through the VAERS are analyzed. VAERS is a voluntary reporting system established in 1990 through which anyone can report an event that happened after vaccination. Approximately 30,000 VAERS reports are filed each year.

Experts also analyze data that comes through the Vaccine Safety Datalink (VSD) which includes 8 large managed-care organizations that provide care for about 9.5 million people around the country. Reports received through VAERS can be thought of as a way to look for signals that might need to be investigated further, and many reports of serious adverse events are reviewed quite thoroughly, including looking at medical records.

Experts suspected that more people than usual would report things that happened after getting the vaccine, and CDC and FDA had encouraged reporting this season because they did not want to miss any early signals. This might explain why there are more VAERS reports per persons vaccinated for the 2009 H1N1 vaccines than for seasonal vaccines. But the very low number of serious events, and the lack of any signal in the VSD- which is our “gold standard” system for comparisons – is reassuring.

There is also another group working to independently evaluate 2009 H1N1 vaccine safety called the National Vaccine Risk Assessment Working Group. Members include people outside of the Federal government who are experts in internal medicine, pediatrics, immunology, or vaccine safety. The group is meeting every two weeks and will provide reports to the public after considering data from the many safety monitoring systems. To keep following the vaccine safety story, check back every week or two with the Vaccine Safety page at CDC.