Sep 17, 2010 (CIDRAP News) – The 2009 H1N1 influenza vaccine generated more adverse-event reports than recent seasonal flu vaccines, but this was probably due in part to heavy publicity, and the vaccine's safety profile appears similar to that of seasonal vaccines, according to a new analysis by federal scientists.
The researchers, from the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA), found that serious adverse events (AEs)—particularly Guillain-Barre syndrome (GBS), anaphylaxis, and death—were rare, at less than two of each per-million people vaccinated, according to their early-release report in Vaccine.
The authors examined all reports submitted to the Vaccine Adverse Event Reporting System (VAERS) for the H1N1 vaccine from October 2009 through January 2010. They compared the H1N1 reports with seasonal vaccine adverse event reports for the past five seasons, including 2009-10.
"The AE profile after 2009-H1N1 vaccine in VAERS was consistent with that of seasonal influenza vaccines," their report says. "The reporting rate was higher after 2009-H1N1 than seasonal influenza vaccines, which may, in part, be a reflection of stimulated reporting."
VAERS, administered by the FDA and CDC, accepts AE reports after immunization with all US-licensed vaccines. It is not designed to determine if AEs are caused by vaccines, but it detects signals of possible problems, which can then be tested using other data sources, the article notes.
Using data from the CDC's National 2009 H1N1 Flu Survey, the authors estimated that 82.4 million doses of H1N1 vaccines were administered in the 4-month study period. That included 64.6 million doses of inactivated vaccine and 17.5 million doses of live attenuated (intranasal) vaccine.
The group also estimated that 125.5 million doses of seasonal flu vaccines were administered from August 2009 through January 2010, including 117.6 million doses of inactivated vaccine and 7.9 million doses of live vaccine.
A total of 10,085 AE reports were submitted in connection with the H1N1 vaccine, of which 726 (7.2%) were classified as serious. For the seasonal vaccine, there were 6,469 reports, 540 (8.3%) of which were listed as serious.
The total rates of AEs within the four different age-groups examined were 2 to 3 times higher for the H1N1 vaccine than for the seasonal vaccine, signaling significant differences, the report says.
The rate of serious events reported for the pandemic vaccine was 8.8 per million doses, which was significantly higher than the 4.1 per million doses reported for the seasonal vaccine (P<.01). Serious events were defined as death, hospitalization, life-threatening illness, persistent or significant disability, or congenital anomaly.
In comparing reports for the pandemic vaccine with those for the past five seasonal vaccines, the researchers found few differences in the percentages of all AEs classified as serious.
Of 176 possible GBS reports, the authors verified 99 (56%). Ninety-three patients had GBS onset within the "biologically plausible window" of 42 days after vaccination. The GBS reporting rates were 0.42 per million vaccinees for those younger than 25 and 1.75 per million for those 25 and older, both of which are below the expected population background rates, based on a CDC analysis of published reports.
VAERS reports related to the H1N1 vaccine included 48 deaths. Cardiovascular conditions were listed as the cause of death in 22 cases, and 45 of the 48 victims had underlying medical conditions and risk factors for cardiovascular disease. There were no anaphylaxis deaths.
A review of the 48 deaths revealed no patterns suggesting that they were caused by the vaccine, the report says.
While the number of AE reports and the reporting rate were higher for the H1N1 vaccine than for the 2009-10 seasonal vaccine, the researchers write, "These findings, however, should be interpreted in light of the publicity around the 2009 H1N1 vaccine and efforts to increase reporting to VAERS. Heightened public awareness and stimulated reporting likely enhanced reporting to VAERS.
"Furthermore, although 2009-H1N1 was licensed similarly to seasonal influenza vaccines, it was likely perceived as a 'new' vaccine by the public and susceptible to the known tendency (i.e., the Weber effect) for adverse events to be reported more frequently following newly licensed products."
The report adds that many efforts were made to boost AE reports for the pandemic vaccine, including providing an information card to vaccinees that included VAERS reporting information. As a result, the VAERS Web site received three times as many visits in the 2009-10 season as in past seasons.
The authors also comment that the fairly consistent reporting of AEs for the H1N1 vaccine compared with the seasonal vaccine among all age-groups and for both serious and nonserious events "argues against an association between [the] vaccine and a particular adverse outcome."
They also note that VAERS was just one piece of a large pandemic vaccine safety monitoring effort, and several other systems, such as the Vaccine Safety Datalink, will be better equipped to assess potential links between the pandemic vaccine and adverse reactions.
Vellozzi C, Border KR, Haber P, et al. Adverse events following influenza A (H1N1) 2009 monovalent vaccines reported to the vaccine adverse events reporting system, United States, October 1, 2009–January 31, 2010. Vaccine 2010 (published online Sep 16) [Abstract]
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