When I openly pose the post-vaccine fever scenario, most people consider it more likely than not that the vaccine caused the fever. That is, most people think the chance that the vaccine caused the fever is > 50%. Almost every medical student in a group of 20 that I was teaching a few weeks ago thought that. After all, fever is a well-recognized side effect of vaccine injections
and your child was perfectly fine until a few days after the injection. Who wouldn’t reasonably conclude that the vaccine likely
caused the fever?
Notice that by focusing responses on 1-49%, I set the question up to give you a better chance at being correct than they were but still, nobody got the question right. Don't worry. You are in good company. For now, just remember the feeling you had when you made your choice.
Back in the 80’s, investigators were concerned that
combining mumps, measles, and rubella vaccines into one (MMR) might increase the risk of side effects, so
some doctors in Finland did an ingenious study to determine what was not just
temporally associated with vaccine injection, but caused by vaccine injection.
They studied 581 pairs of twins and randomly gave the MMR injection
to one of the twins and a placebo injection to the other. Then, so as to not
deprive the children that got a placebo injection of the vaccine's proven benefits, three weeks
later, they gave each twin the opposite of what they had earlier received. Parents, who were blinded (they didn’t know when the kids were getting the placebo or the MMR injection), were instructed to vigilantly check
for fever and a variety of other potential side effects after each injection.
In the first 6 days after placebo injection, 17% of children
had a fever. That’s
the background frequency of fever in vaccine-age children, and it's pretty high, huh? On
the other hand, in the first 6 days after MMR injection, 17.2% of children got
a fever. The difference - 0.2% - is
what can be causally attributed to the active ingredients in the vaccine.
Conclusion: >99% of fevers that occur in the first week after MMR injection have
nothing to do with the vaccine at all.
However, we are very likely to
erroneously attribute the fever to the vaccine. We do this because we have a
powerful intuition that leads us to identify a causal relationship when 2
events that could possibly be
causally related follow each other in time. We are mistaking what is merely
possible for what is probable. Unfortunately, children develop
mysterious illnesses like MS, autism, epilepsy, hepatitis, arthritis, etc. with
regular frequency. Sometimes, those illnesses will appear fairly soon after
getting a vaccination, and that’s when that intuition of ours can do some serious
damage. That same intuition does damage when mysterious conditions like these disappear -as they often spontaneously do- after interventions that have no effect on the disease like prayer, acupuncture, chiropractic neck manipulations, homeopathic remedies, etc. Only randomized controlled trials can sort this stuff out.
Imagine once again your son who got a fever early after the
MMR injection, only now, he is a healthy 18-year-old man who was spared the
ravages of mumps, measles, and rubella. At any point in his upbringing, would
you have ever had cause to stop and be thankful for the vaccine? How could you?
You’d have had no way of knowing whether your child would have contracted one
of these illness had he not been vaccinated!
See what’s happening here? The way that we experience
life offers us no way to identify the situation where a vaccine prevents a
horrible illness or death. On the other hand, we have a tendency to erroneously
attribute adverse events to vaccines when they follow each other in time. Remember how you felt about your child's fever after MMR vaccine? Parents considering whether to vaccinate their children can anticipate the
regret that they will experience if their child does develop an illness like
autism afterwards, but they cannot anticipate the relief that they cannot
experience when their child is spared a vaccine-preventable illness. Anti-vaxxers are much more likely to anticipate the former type of regret than the latter because they tend to overestimate the risks of vaccines and underestimate their benefits. Our intuitions erroneously set vaccines up to be unattractive.
And so it is with physicians and oral anticoagulants (OAC’s). Bleeding events are
fairly common. Whenever somebody experiences a bleeding event on an OAC, they and
the doctors tasked with treating the bleeding blame the OAC. The reality is
that most bleeding events that happen while on an OAC would have happened
otherwise: just as only 0.2% of fevers early after the MMR injection were caused by it, only 0.2-0.3% of bleeds /yr are caused by the OAC. But physicians can
anticipate the regret that they will experience when they prescribe an OAC to a
person with atrial fibrillation (AF) who then develops a bleeding event. The finger of blame will
point to the OAC and the physician who prescribed it. Unfortunately, nobody
ever returns to the prescribing physician to pat them on the back and thank
them for the stroke that they and the OAC prevented because, just like the case
of mumps that your son avoided, there is no way to recognize a stroke that
would otherwise have happened. Doctors' intuitions erroneously set OACs up to be unattractive.
Just as there is evidence that anti-vaxxers overestimate the risk of vaccines, so is there evidence that physicians overestimate the risks of OACs, exaggerating their anticipated regret over prescribing one.
Our ancestors evolved on an African Savannah with no
pressures to select for intuitions that address the types of complicated
primary prevention questions posed by vaccination and OAC use in AF. It seems that
we did evolve intuitions that lead us to readily identify patterns and infer
causal relations where there often isn't one. On the whole, this rudimentary
heuristic has done well to protect us from certain kinds of dangers, but, as Sam Harris has
written, “we have flown the perch built for us by evolution”. If we are to make
advances with complex questions, we simply have to recognize when our
intuitions lead us astray, and they regularly do, whether we are specialized
physicians or lay people. None of us are spared the consequences of irrationality. It
is a struggle we all must recognize and participate in.
If you are faced with complex decisions like vaccinating or taking an OAC, I hope that you are most heavily weighing the evidence from RCTs. If you aren't, then I hope that you are seeking the involvement of people who are. Otherwise, you're just being irrational.
If you are faced with complex decisions like vaccinating or taking an OAC, I hope that you are most heavily weighing the evidence from RCTs. If you aren't, then I hope that you are seeking the involvement of people who are. Otherwise, you're just being irrational.
