How Relative Risks are relatively misleading
Atrial Fibrillation (afib) offers a good illustration of relative risk’s limitations, and is one of the case studies we present in our book. Afib is an aberrant heart rhythm common in the elderly that puts people at higher risk of having a stroke. Blood thinners such as Warfarin (Coumadin) are considered standard of care in treating afib patients because they reduce the risk of stroke by 50% compared to using aspirin. Newer blood thinners such as Xaralto, Pradaxa, and Eliquis have flooded the market and airwaves as alternatives to Warfarin that may even reduce strokes further. But 50% is a relative risk. What is the absolute benefit of these drugs over aspirin in preventing strokes, and what is the absolute rate of complications, something that no ads, and no articles we have found in the press, ever mention? In fact, if 1000 people with afib take Warfarin instead of aspirin, only 6 of them will prevent a meaningful stroke in a year, which means that 994/1000 people achieve no benefit. More concerning, 6/1000 people who take Warfarin will bleed in their brain, something just as dangerous as a stroke. And 40-70 of them will develop a major bleed that may send them to the hospital or threaten their lives. Hearing about a 50% risk reduction of stroke (a relative risk) would convince any afib patient of the need to take a medicine like Warfarin. But when people learn about the absolute benefit and risks of Warfarin and other similar drugs, they are now able to assess the efficacy of the drug more sensibly and can make an informed decision.
A more recent example of relative risk and its power to mislead patients and the media relates to high dose flu shots. An elderly person chastised me recently for not giving high dose flu shots this year in a retirement community where I serve as medical director; she accused me of causing people harm by providing only the more traditional standard dose flu shot. She had reason for her concern. The news media declared that the new high dose flu shot is clearly superior than more standard dose vaccine. Wrote the New York Times: “Now a study, published recently in The New England Journal of Medicine, finds that Fluzone High-Dose does indeed prevent influenza in older adults, reducing cases of the flu by 24 percent compared with the standard version.” (“A Better Flu Shot,” NYT, 9/3/14). But 24% is a relative risk. What is the absolute risk reduction? In fact, looking at the study itself (NEJM, 8/14/14) only 5/1000 fewer people developed flu with a high dose vs. standard dose shot (1.4% incidence vs 1.9%), a tiny difference likely ascribed to chance. Interestingly, and a fact ignored by the press, a large number of vaccine recipients developed serious side effects (8.3% vs 9%), perhaps the most salient outcome of the study. Thus, few elderly people developed flu in either arm of the study (with a negligible difference between each arm), and many people developed serious side effects. But all we heard about was a 24% risk reduction.
Relative risk, then, is a misleading statistic that actually often can lead us to make erroneous conclusions. In our book, we present all information in the language of absolute risk/benefit, and we implore the media to do the same. Until we move down a road where the medical statistics we share with patients are meaningful, then it is virtually impossible to have a constructive conversation about risks and benefits.