ResearchBlogging.orgIt’s a bold claim, that being vaccinated for Influenza will protect you against having a heart attack or Myocardial Infarction (MI). Well according to a study published last month in the Canadian Medical Association Journal that could well be the case.

The study, “Influenza vaccination, pneumococcal vaccination and risk
of acute myocardial infarction: matched case–control study
“, included 78,706 individuals over the age of 40 in an attempt to determine whether and how much of an effect the vaccine had on MI. The results showed that the vaccine was associated with an almost 20% reduction in risk of MI compared to the unvaccinated population.

My first thought when I read this was that those individuals who get vaccinated might engage in other activities that would lend themselves to reducing risk of MI, healthy eating habits, regular exercise etc. The so called “healthy user” effect. This study attempted to control for this sort of confounding factor by using matched controls with similar risk factors. In particular they performed two further analyses that I think comfortably undermine this interpretation of the results.

First they compared the timing of the receipt of the vaccine, ie early or late in the ‘flu season. Second they looked at subjects who had been vaccinated with the pneumococcal vaccine as a comparison. In the first case there was a greater reduction in risk for those who had received the vaccine early in the season, 21% compared to only 12%. In the second case they found no protective effect for those who had received the pneumococcal vaccine. It seems unlikely that the subjects of the study would vary their healthy habits in precisely the ways they would have to in order to see these results as being independent of the vaccine itself.

An interesting aspect of this kind of epidemiology though is that simply looking at the raw numbers there is a greater incidence of MI in the vaccinated group compared to the unvaccinated group. This is an artefact of how the vaccines are administered clinically. Those patients who have greater cardiovascular risk are also the patients who are more likely to be recommended for vaccination. It’s like noticing that people who buy antiperspirant tend to have sweatier armpits than those who don’t*. Those who are prone to sweaty armpits will likely tend to be those who will buy and use antiperspirant, to accurately gauge effectiveness you would have to control for this factor.

Tying this into the anti-vax focus of this week, one of the claims I’ve seen is that the flu vaccine is useless as it is based on strains that were around the previous season (Here via IAS). This is distressingly simplistic thinking. This paper shows that the vaccine is far from useless, in fact the most benefit was found within ‘flu seasons. If there was no effect of the vaccine on the current ‘flu season strains then the study would not have shown the protective effect that it did. Once again the anti-vax crowd (and by extension the altmed crowd as there tends to be overlap) has shown their inability to grasp the nuance of the situation.

There is a gradation of effect when a vaccine does not exactly match the wild strain, the interaction of antigens and antibodies is more complicated than you might suppose. Changes in the antigen (mutation of the virus over the ‘flu season) mean that antibodies may bind less tightly and therefore have a reduced effect but that is not the same as no effect.

To sum up, health is a complicated thing. Many factors are inter-related and in order to get the whole picture we sometimes have look at things from an unusual perspective. Vaccinating for the ‘flu can reduce your risk of heart attack, who knew? But, we should also be wary that we are keeping things within the realm of plausibility. Infection does cause inflammation which can plausibly effect heart function**. This does not mean it’s valid to blame the Wi-fi at your local school for your vague aches and pains.***


*To make up an example.

** See



Siriwardena AN, Gwini SM, & Coupland CA (2010). Influenza vaccination, pneumococcal vaccination and risk of acute myocardial infarction: matched case-control study. CMAJ : Canadian Medical Association journal = journal de l’Association medicale canadienne, 182 (15), 1617-23 PMID: 20855479

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