A recent headline on ScienceDaily.com announces that "Children Who Get Flu Vaccine Have Three Times Risk Of Hospitalization For Flu," and sure enough a quick Google search on that exact title produces hundreds of hits, including respectable sites such as ScienceDaily.com and medwirenews and a title at the U. S. Dept of Health & Human Services site www.healthfinder.gov: Flu Vaccine Triples Child Hospitalizations."
Wait — what?
Where is all of this coming from?
It turns out that the headlines are terribly misleading, especially for a unscientific general population that (1) easily confuses correlation with causation and (2) doesn't understand the difference between an experimental study and a quasi-experimental (or even correlational) study.
Most of the material being published out there under these headlines consists of simple regurgitations (or sometimes only pieces of) a premature and irresponsible press release issued 5/19 by The American Thoracic Society in promotion of that 'Society's 105th International Conference being held in San Diego, the press release having been timed to coincide with the specific symposium in which the related research was first being presented.
Premature? Yes. The American Thoracic Society press release concerned research not yet ready for general public consumption, concerning research not yet peer-reviewed (except minimally for conference presentation) or published in a peer-reviewed journal and involving a relatively small and special population of participants.
Irresponsible? Yes, in part for the reasons mentioned above, but also for the breath-takingly cavalier attitude with which they threw out for pop-media consumption poorly-explained research results under a heading sure to catch attention and sure to appear to have widespread and profound public health policy implications. Add to that the fact that the press release was based on an already-poorly-written abstract with questionably-worded conclusions, and the news release becomes an incredible fiasco.
Here's the original press release:
It's interesting to look at the deterioration of the information as it flowed from Joshi, et al's original conference abstract, to the press release, then on to various pop media outlets:
Original title of Joshni, et al's conference abstract:That last one of course now making it sound like the very existence of the flu vaccine is somehow leading to dramatic increases in child hospitalizations, which is utter nonsense.Flu Vaccination in Asthmatics: Does It Work?(which, given the actual description of the research actually seems a poor title)
Then to this in the press release from the American Thoracic Society:Flu Shot Not Effective in Preventing Flu-Related Hospitalizations in Asthmatic ChildrenThen to something like this in the mainstream pop-media (this one appearing on ScienceDaily.com):Children Who Get Flu Vaccine Have Three Times Risk Of Hospitalization For Flu, Study SuggestsThen further mangled by less mainstream media to things like this:Flu Vaccine Triples Child Hospitalizations
So what's really happening here? And why does the first author Dr. Avni Joshi herself remark that the findings “… do not in fact implicate it [the vaccine] as a cause of hospitalizations” ?
In part, she is acknowledging what this study was NOT: the study was not an experimental study allowing cause-effect conclusions. In other words, the study was NOT something like that outlined in the figure below:
p(hospitalization | flu vaccine) vs. p(hospitalization | no vaccine)
But again, that's NOT the study Joshi and colleagues did. Instead we have the following description from Joshi and colleagues in their conference abstract:
and this slightly elaborated description from the press release:
So, to be clear: this appears to have been a retrospective study of 263 participants already diagnosed with flu, some of whom had received a flu vaccine before their illness, some of whom had not, and some of whom were later hospitalized for their illness. The design of the study looks more like this:
and instead of giving information about the probability of hospitalization if you get a flu vaccine, the results give us information (with limited generalizability) about the probability of hospitalization if you were vaccinated but still came down with the flu. The phrasing makes this sound like some arcane, subtle distinction, but the information being communicated is dramatically different.
The pop media headlines and coverage lead one to believe that getting a flu vaccine somehow triples one's likelihood of being hospitalized for the flu. That is wrong. Instead, the research being reported implies only that if you get the flu (and have serious enough symptoms that you go to a medical clinic to be checked out) despite having self-selected into getting a flu vaccine (which means you're already part of an at-greater-risk population in general) then you're more likely to be hospitalized than someone who was also quite sick from the flu but without having had the earlier immunization.
Some of the possible explanations for such a result lie with the special patients themselves, and this is why Joshi points out that the results do not imply that the vaccine causes an increase in hospitalization rates. The special patients who get a serious case of the flu despite being vaccinated may be special (for example) in having particular health difficulties already or some other constitutional predisposition for the illness, and/or the patients may be special in having contracted particularly dangerous forms of the illness.
Children Who Get Flu Vaccine Have Three Times Risk Of Hospitalization For Flu, Study Suggests. (5/20/2009). Science Daily, accessed 5/21/2009 at http://www.sciencedaily.com/releases/2009/05/090519172045.htm
Flu Shot Not Effective in Preventing Flu-Related Hospitalizations in Asthmatic Children. (5/19/2009). New Release from the American Thoracic Society, accessed 5/20/2009 at www.thoracic.org/sections/publications/press-releases/conference/articles/2009/abstracts-and-press-releases/joshi.pdf
Heckenlively, K. (5/21/2009). Flu Vaccine Triples Child Hospitalizations, but Won’t Turn Them into Horned, Hairy Apes, say Experts! Age of Autism: Daily Web Newspaper of the Autism Epidemic, accessed 5/21/2009 at http://www.ageofautism.com/2009/05/flu-vaccine-triples-child-hospitalizations-but-wont-turn-them-into-horned-hairy-apes-say-experts.html
Joshi, A. Y., Iyer, V. N., Hartz, M. F., Volcheck, G. W., Patel, A. M., & Li, J. T. (2009). Flu vaccination in asthmatics: does it work? Presentation at The American Thoracic Society's 105th International Conference, San Diego, CA, 5/19/2009. Abstract available as part of the press release available at www.thoracic.org/sections/publications/press-releases/conference/articles/2009/abstracts-and-press-releases/joshi.pdf
Lyford, J. (5/21/2009). Flu vaccine fails to reduce hospitalization in asthmatic children. medwirenews, accessed 5/21/2009 at http://www.medwire-news.md/48/82539/Respiratory/Flu_vaccine_fails_to_reduce_hospitalization_in_asthmatic_children.html
American Thoracic Society. http://www.thoracic.org/
CDC - Influenza (Flu). http://www.cdc.gov/flu/
Centers for Disease Control & Prevention (CDC). http://www.cdc.gov/