Int J Epidemiology: Evidence of bias in estimates of influenza vaccine effectiveness in seniors

[Hannah Bell]

http://www.ncbi.nlm.nih.gov/pubmed/16368725?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&linkpos=1&log$=relatedarticles&logdbfrom=pubmed

Int J Epidemiol. 2006 Apr;35(2):337-44. Epub 2005 Dec 20. Links

Jackson LA, Jackson ML, Nelson JC, Neuzil KM, Weiss NS.
Center for Health Studies, Group Health Cooperative, Seattle, WA 98101, USA. [e-mail]

BACKGROUND: Numerous observational studies have reported that seniors who receive influenza vaccine are at substantially lower risk of death and hospitalization during the influenza season than unvaccinated seniors. These estimates could be influenced by differences in underlying health status between the vaccinated and unvaccinated groups. Since a protective effect of vaccination should be specific to influenza season, evaluation of non-influenza periods could indicate the possible contribution of bias to the estimates observed during influenza season.

METHODS: We evaluated a cohort of 72,527 persons 65 years of age and older followed during an 8 year period and assessed the risk of death from any cause, or hospitalization for pneumonia or influenza, in relation to influenza vaccination, in periods before, during, and after influenza seasons. Secondary models adjusted for covariates defined primarily by diagnosis codes assigned to medical encounters.

RESULTS: The relative risk of death for vaccinated persons compared with unvaccinated persons was 0.39 <95% confidence interval (95% CI), 0.33-0.47> before influenza season, 0.56 (0.52-0.61) during influenza season, and 0.74 (0.67-0.80) after influenza season.

The relative risk of pneumonia hospitalization was 0.72 (0.59-0.89) before, 0.82 (0.75-0.89) during, and 0.95 (0.85-1.07) after influenza season.

Adjustment for diagnosis code variables resulted in estimates that were further from the null, in all time periods.

CONCLUSIONS: The reductions in risk before influenza season indicate preferential receipt of vaccine by relatively healthy seniors. Adjustment for diagnosis code variables did not control for this bias. In this study, the magnitude of the bias demonstrated by the associations before the influenza season was sufficient to account entirely for the associations observed during influenza season.

[BrklynLiberal]

1. Gee. I wonder if the drug companies sponsored the studies that showed

the bias.....duh.

[aquart]

2. I read it, but I have absolutely no idea what that thing said.

I do know if I take the shot I don't get sick. If I don't get the shot, I get laid up for three months while everyone else is fine and dandy in two weeks. Is that what it says?

[Hannah Bell]

3. It says: they studied (via medical records) a group of people who vaccinated

v. a group who didn't, 8 years of records.

The vaccination group was significantly less likely to die of any cause, even outside of flu seasons. That includes accidents, brain cancer, murder etc.

OTOH, the difference in hospitalizations for pneumonia was smaller.

Suggestive of a healthy/wealthy user effect: in the community, healthier/wealthier folks more likely to get the shot & less likely to die - not because of the shots, but because the healthy/wealthy are less likely to die.

But less difference in the rates at which either group is hospitalized for pneumonia, a complication commonly occurring w/ flu. Suggesting effects specific to flu are less than the effects of healthy/wealthy self-selection.

Take the shots if you think they work for you. I have no problem with it. But your personal experience is not definitive or generalizable.

[Hannah Bell]

4. edit: "effects specific to flu vaccination"