Influenza virus and heart attacks

Charlotte -  Well we've been looking at Influenza virus and heart attacks.  We're interested in looking at whether flu can actually trigger heart attacks in some vulnerable people because some of the people who have heart attacks don't have any risk factors and it's not clear why they have their heart attack.

Chris -   You mean, people who have a heart attack usually have some kind of antecedent problem like high blood pressure or high cholesterol and you're seeing people who have heart attacks have none of those risk factors but they do have flu potentially.

Charlotte -   Well possibly.  It's still a grey area.  We're really not sure whether there's a strong link or not, which is why we've been doing this research.

Chris -   So how did you do it, what did you do?

Charlotte -   We used the big general practice research database that contains records of over 5 million patients in the UK.  These are all anonymised records but we could see when people went to the GP with a flu-like illness and we used linked data from a heart attack registry database also in the UK, and that gave us detailed information on people who had heart attacks.  And so, by linking these two datasets together, we pulled out a group of people who had both the heart attack and had been to the GP with a flu-like illness.  And then we began our analysis, and our analysis was looking at something called the self-control case series.  So what that means is you take people who have had heart attacks and you look at whether they were more likely to have their heart attack in the time period just after visiting the GP with a flu-like illness compared to other times in their lives.

Chris -   That's neat.  So in other words, each person is their own control, aren't they because if they're not having a heart attack when they haven't had a flu-like illness, then they're controlling from the time when they have had a flu-like illness.

Charlotte -   Yes, that's right and it makes this type of method quite efficient because you don't need to find control patients to compare them to because when you look at different cases and controls, sometimes there can be other differences as well so you can introduce bias that way.  But here, as you say, everyone was acting as their own control.

Chris -   And - the million dollar question, what did you find?

Charlotte -   Well, what we found was that in the first few days after having a flu-like illness or a respiratory infection, people were nearly four times more likely to have their MI (myocardial infarction - heart attack) than in other time periods.  We wanted to look really though whether this was a specific effect of the flu virus or whether it was more of a general effect of any old respiratory infection.

Chris -   Did you know they actually were flu cases though?

Charlotte -   Well, no.  This is one of the problems with using this type of data.  It's GP data and you rarely have - these people have rarely been tested for flu, so we had to use various proxy measures for what we thought might be flu such as the codes that GPs use to classify the illnesses or looking at the amount of flu that was actually circulating in the community.  When we use those different proxy measures, we actually got very similar results when you compared those illnesses more likely to be due to flu compared to other respiratory illnesses.  So to summarize, this didn't seem to be a specific effect of flu.  It seems that any sort of respiratory infection that we found in our study could actually have this triggering effect in vulnerable groups and it seem to be the elderly that this was happening in, mostly people over 80.

Chris -   How big is the effect?

Charlotte -   It's basically a four-fold increase that we saw in the first few days immediately following a respiratory infection and the effect gradually tapered off over time, so about a month after your infection, there was really no increased risk at all.  But having said that obviously, most heart attacks are not going to be triggered by respiratory infection and most respiratory infections are not going to trigger a heart attack.  So this really was just a very small proportion of people, but quite a significant effect in those particularly vulnerable groups.

Chris -   How do you think that having an upper respiratory infection might cause someone to then go on and have a heart attack in that risk window?

Charlotte -   I think it's a bit unclear at the moment. Flu is an acute inflammatory event - and when your body is subjected to acute inflammation, particularly something like flu that can affect all parts of your body it's not just the lung, it can cause muscle aches and fever.  So it can cause symptoms affecting the whole body and it's suggested that it can cause a systemic inflammatory response, so an inflammation all around the body that might then also include the coronary arteries and might in that way, trigger a heart attack.  But I think it's still unclear and there's quite a lot of research going on about potential biological mechanisms at the moment.

Chris -   Given you found this then what should we do about maybe raising awareness of the link or is there anything we should do?  Is the effect so trivial that it's not actually worth worrying about?

Charlotte -   No, I think it is important particularly because we have a good flu vaccine to help prevent flu and this vaccine is quite underutilised particularly in people with some chronic conditions, and the recommendation is that people with chronic diseases such as chronic heart disease should have the flu vaccine every year, so I think it's important to raise awareness in those groups, and amongst their GPs as well so that they do encourage people to come forward for the flu vaccination each year.

Chris -   What about also telling GPs, "Look, there is this association that I've found between respiratory infection and then heart attack afterwards.  So if you see someone with symptoms that might be an evolving heart attack, maybe have slightly higher index of suspicion, maybe do something about it more than just writing it off as indigestion"?

Charlotte -   Yes, I think that definitely true.  We wouldn't just take this one study in isolation as evidence.  We need to gather evidence from a number of sources.  But absolutely, I hope that this study will contribute to the evidence available for GPs and other practitioners.

Chris -   Is that what you're going to do?  You're now going to say, "Let's go and look at some of the individual viruses and see if there are any particular viruses that are more of a risk than the others"?

Charlotte -   Well that would be nice to do.  I think it's important though to make sure that this does have some practical applications and I think flu is really one of the respiratory viruses for which there is an effective prevention, so that's why we focused on flu at the moment.  But yes, potentially in the future, we could look at other viruses as well.