Welcome to Call to Decision
A
Shot of Fear
Flu Death Risk Often Exaggerated; So Is Benefit of Vaccine
Courtesy The Washington Post
Medical research often becomes news. But sometimes the news is made to appear more definitive and dramatic than the research warrants. This series dissects health news to highlight some common study interpretation problems we see as physician-researchers and show how the research community, medical journals and the media can do better.
For years, the
public health community has used fear as one strategy to promote the
flu vaccine. A vaccination poster distributed by the U.S. Centers for
Disease Control and Prevention (CDC), for example, emphasizes that
"36,000 Americans die of flu-related illnesses each year,"
implying that the vaccine could prevent many of these deaths.
When it became aware
of the vaccine shortage last October, the federal government changed
course and tried to reassure Americans that going without a shot was
no big deal. "We all need to take a deep breath. This is not an
emergency," CDC director Julie Gerberding advised the public.
Instead of urging
vaccination for everyone age 50 and older, as they had been doing,
government officials recommended shots only for people 65 and older,
and those in selected high risk groups. The public's response was
predictable: People were upset and confused. Our local television news
played a story in which a pharmacist was called "a murderer"
when his vaccine supply ran out. Ironically, the crisis mentality led
some to engage in behaviors that probably increased their risk. Frail
elderly people, some with oxygen tanks, stood in long lines in the
cold, waiting for the vaccine. Others crowded clinics and doctors'
offices, increasing their chance of exposure to flu and other
infectious agents.
With uncertainties
about this year's vaccine supply, the CDC again recommended that
highest-risk people get priority for flu shots, at least until late
October. But last year's flu season may have left people confused
about essential points: Just how risky is the flu? And just how
effective is the vaccine? The answers to these questions may surprise
readers.
How Risky Is the Flu?
First, a caveat: The
risk calculations we analyze here describe typical flu seasons only.
We don't consider here what the picture would be in the event of a
deadly flu pandemic -- a worldwide outbreak of a new, highly virulent
flu strain, the potential for which has recently drawn considerable
media attention. No one really knows how likely such an outbreak is,
but the risk profile would certainly change. A pandemic is a
fundamentally different situation: The risk of death would be
substantially higher, and untested strategies (including new
treatments, quarantine and a new vaccine) would need to be implemented
rapidly.
We deal here with
what is known about typical flu seasons, based on data that form the
basis for the federal government's flu-risk figures.
By choosing to
highlight the annual number of flu deaths, the CDC employed an
attention-grabbing tactic often used by public health and disease
advocacy groups. It's a tactic readers should be inoculated against if
they want a clear picture of the risks they face.
In fact, it is very
difficult to know how many people die from any given disease because
there is often much uncertainty in determining the cause of death.
This is particularly true for the flu. That's because it shares
symptoms with so many other diseases, and because people most likely
to die a flu-related death are also at high risk for many other causes
of death.
Flu deaths are
probably undercounted because doctors do not routinely test for the
flu, and because some deaths that should be attributed to the flu are
given other diagnoses. For example, someone who dies from a heart
attack because they are debilitated by the flu might not get counted
as a flu death. Some over-counting of flu deaths also occurs: Clearly
not all winter pneumonia deaths are caused by the flu.
According to the CDC,
90 percent of flu-related deaths occur among people age 65 years and
older. Based on this information and the age distribution of the
population, the chance of a flu-related death for people in that age
group is about one in 1,000. Another way of saying this is that
the chance of not dying from flu for those 65 and older is about 999
out of 1,000. (For context, the chance of a flu-related death is
slightly lower than the chance of dying from a fall or other
accident.)
For people younger
than 65 (including children), the chance of a flu-related death is
much smaller -- about one in 100,000. Of course, adults and children
might be concerned about flu-related problems besides death, such as
being hospitalized or just suffering with unpleasant symptoms
(typically three to seven days of fever, muscle aches, headache,
weakness, dry cough and runny nose). As you might guess, counting the
number of flu-related hospitalizations or the number of people
experiencing symptoms from the flu is even more difficult than
counting flu deaths.
How Good Is the Vaccine?
Getting a shot does
not guarantee you will not get sick from the flu or die from it.
Recently, the Cochrane Collaboration, an international group that
evaluates the evidence for various medical interventions, reviewed the
medical literature on the effectiveness of the flu vaccine in
preventing death.
Unfortunately, the
evidence on how well the vaccine works to prevent death in the elderly
is limited. Few of the existing studies are randomized trials --
considered the gold standard for medical evidence. Instead, most data
are from observational studies -- studies in which scientists simply
count up outcomes (here, the number of deaths that occur among people
who did or did not get the vaccine).
But drawing
conclusions about cause and effect from such observations is fraught
with problems.
For example, a 2003
study published in the New England Journal of Medicine observed that
the flu vaccine was associated with a 50 percent reduction in the
overall death rate (that is, death from heart disease, stroke, cancer
and all other causes combined). To attribute an effect of this
magnitude solely to the flu vaccine is ludicrous: Flu-related deaths
make up less than 2 percent of all deaths. If the claim were accurate,
the vaccine's power would dwarf that of any other medical
intervention. There is, however, a much more likely explanation:
People who choose to get a flu shot are much healthier -- and
therefore already at much lower risk of death -- than people who do
not.
Only five randomized
trials have examined the effectiveness of the flu vaccine. In these
studies, patients were randomly assigned -- a selection technique
equivalent to the flip of a coin -- to get either a flu vaccine or a
placebo injection. But none of these studies looked at whether the
vaccine prevents death. Instead, the scientists measured who developed
a flu-like illness. For a summary of the findings of these studies,
see "How Well Does the Vaccine Work in the Elderly?" below.
In the absence of
good randomized trial data, it is still possible to gauge the
effectiveness of vaccination by looking at time trends in flu vaccine
rates compared with flu-related deaths in the elderly. As more people
get vaccinated, you would expect the flu-related death rate to decline
-- if the vaccine is effective. But, as the graph below, titled
"A Windening Gap," shows, despite a dramatic increase in
vaccination among the elderly, deaths from the flu and pneumonia have
hardly budged. (The calculations have taken into account the aging of
the population.)
For younger adults,
flu-related death is so rare that it has not been reliably studied:
Doing so would require a trial of millions of people.
Of course, the flu
shot may have benefits besides reducing the chance of death. Many may
get flu shots simply to avoid getting sick. The Cochrane Collaboration
identified more than 20 randomized trials addressing this question.
The overall chance of developing "clinical" flu (we'll
explain in a minute) was 19 percent in those chosen, again by chance,
to receive the recommended flu vaccine vs. 23 percent in the control
groups.
The careful reader
may notice that these percentages are substantially higher than those
reported for the elderly. (See "How Well Does the Vaccine Work in
the Elderly?") This is because clinical flu is defined as a set
of non-specific symptoms including fever, cough and muscle aches --
symptoms shared by many non-flu illnesses like the common cold. These
non-flu illnesses may be especially common in younger adults because
of their exposure to other people, particularly children. To try to
isolate the effect of the vaccine, scientists sometimes use laboratory
tests to confirm the activity of flu virus in the blood. Using this
measure, the chance of flu in the vaccine group is 2 percent vs. 7
percent in the control group.
Studies have also
measured another outcome: how vaccination affects days lost from work.
On average, there are about 0.16 fewer days lost from work per person
vaccinated. Another way of saying this is that about 5 percent of
those vaccinated avoid missing about three days of work because of the
flu. (That is, 0.16 days divided by the 5 percent who benefited from
vaccination equals 3.2 days.) The other 95 percent vaccinated got no
benefit.
Take-Home Messages
To promote vaccine
use, many in the public health community have overstated the risk of
flu-related death and the effectiveness of the vaccine in preventing
it. While the flu vaccine may have some important benefit (less
flu-related illness), we really do not know whether it reduces the
risk of death. For younger individuals -- for whom the chance of
flu-related death is extremely small -- any death-protection benefit
can only be very modest (and it is unlikely we will ever reliably know
whether it even exists). However, we do know that the vaccine reduces
the risk of being sick and time lost from work. But because the effect
is small, individuals will have to judge for themselves whether it's
worth the bother.
We are not
suggesting that Americans forgo flu vaccines. We simply want to help
people make informed decisions.
For many people,
getting the vaccine is a reasonable choice. And many may reasonably
choose not to get it. (Consequently, the use of flu vaccination rates
by Medicare and others to measure health care quality probably does
not make sense.)
Regardless, public
health officials should not exaggerate risks or benefits to promote
vaccination. Exaggeration carries a price: Not only do some people get
scared and engage in behaviors that increase their risk (like waiting
in a crowded clinic for a flu shot). They may also grow cynical and
end up ignoring health messages that really matter.