Determining Causality of Postvaccination Adverse Events Is Tricky

Without knowing the background rates of these neurologic complications among unvaccinated individuals, it is impossible to ascertain causality. An excellent data analysis conducted by Dr. Steven Black and colleagues provided very helpful estimates of the numbers of specific severe adverse events that would be expected following receipt of the 2009 H1N1 influenza vaccine.

Based on background rates, they determined that within 6 weeks of vaccination there would be 21.5 coincident cases of GBS per 10 million vaccine recipients, and 86.3 cases of optic neuritis per 10 million female vaccinees. Spontaneous abortions would occur in 16,684 of every 1 million vaccinated pregnant women, and sudden death within 1 hour of any symptom onset in 5.75 of every 10 million people vaccinated (Lancet 2009;374[9707]:2115-22).

Another important analysis was conducted by the CDC to determine whether 33 reported cases of GBS in 11- to 19-year olds within 42 days of receipt of meningococcal conjugate vaccine were causally linked. Background data from the 2000-2004 Healthcare Cost and Utilization project projected that there would be a very close 36 cases for the entire age cohort, suggesting there was no causal link. However, just 20 cases would be expected among 15- to 19-year olds, but the actual number was 26.

Although not statistically significant, this difference was enough to merit continued monitoring by the CDC, which advised that children with prior GBS not receive the vaccine (MMWR 2006;55[13]:364-6).

Finally, a population-based case-control study from France investigated cases of acute disseminated encephalomyelitis, optic neuritis, and transverse myelitis in children younger than 16 years of age between 1994 and 2003, using 12 controls per case matched for age, sex, and geographic location. Rates of hepatitis B vaccination were 24% in cases and 27% in controls, for an adjusted odds ratio of 0.74 (Neurology. 2009;73[17]:1426-7).

One might conclude from this that hepatitis B vaccine is actually protective, but the result was not statistically significant.

Dr. Pelton writes the column, "ID Consult," which regularly appears in Pediatric News, an Elsevier publication. He is chief of pediatric infectious disease and also is the coordinator of the maternal-child HIV program at Boston Medical Center. He disclosed that he has received investigator-initiated research grants from Pfizer Inc., Novartis Vaccine and Diagnostics, GlaxoSmithKline, and Intercell. He has served on advisory boards for Pfizer, Novartis, and GSK. To respond to this column, write to pdnews@elsevier.com.