Winter viruses can trigger a heart attack or stroke, our study shows. It’s another good reason to get a flu or COVID shot

Winter is here, along with cold days and the inevitable seasonal surge in respiratory viruses.

But it’s not only the sniffles we need to worry about. Heart attacks and strokes also tend to rise during the winter months.

In new research out this week we show one reason why.

Our study shows catching common respiratory viruses raises your short-term risk of a heart attack or stroke. In other words, common viruses, such as those that cause flu and COVID, can trigger them.

Wait, viruses can trigger heart attacks?

Traditional risk factors such as smoking, high cholesterol, high blood pressure, diabetes, obesity and lack of exercise are the main reasons for heart attacks and strokes.

And rates of heart attacks and strokes can rise in winter for a number of reasons. Factors such as low temperature, less physical activity, more time spent indoors – perhaps with indoor air pollutants – can affect blood clotting and worsen the effects of traditional risk factors.

But our new findings build on those from other researchers to show how respiratory viruses can also be a trigger.

The theory is respiratory virus infections set off a heart attack or stroke, rather than directly cause them. If traditional risk factors are like dousing a house in petrol, the viral infection is like the matchstick that ignites the flame.

For healthy, young people, a newer, well-kept house is unlikely to spontaneously combust. But an older or even abandoned house with faulty electric wiring needs just a spark to lead to a blaze.

People who are particularly vulnerable to a heart attack or stroke triggered by a respiratory virus are those with more than one of those traditional risk factors, especially older people.

What we did and what we found

Our team conducted a meta-analysis (a study of existing studies) to see which respiratory viruses play a role in triggering heart attacks and strokes, and the strength of the link. This meant studying more than 11,000 scientific papers, spanning 40 years of research.

Overall, the influenza virus and SARS-CoV-2 (the virus that causes COVID) were the main triggers.

If you catch the flu, we found the risk of a heart attack goes up almost 5.4 times and a stroke by 4.7 times compared with not being infected. The danger zone is short – within the first few days or weeks – and tapers off with time after being infected.

Catching COVID can also trigger heart attacks and strokes, but there haven’t been enough studies to say exactly what the increased risk is.

We also found an increased risk of heart attacks or strokes with other viruses, including respiratory syncytial virus (RSV), enterovirus and cytomegalovirus. But the links are not as strong, probably because these viruses are less commonly detected or tested for.

What’s going on?

Over a person’s lifetime, our bodies wear and tear and the inside wall of our blood vessels becomes rough. Fatty build-ups (plaques) stick easily to these rough areas, inevitably accumulating and causing tight spaces.

Generally, blood can still pass through, and these build-ups don’t cause issues. Think of this as dousing the house in petrol, but it’s not yet alight.

So how does a viral infection act like a matchstick to ignite the flame? Through a cascading process of inflammation.

High levels of inflammation that follow a viral infection can crack open a plaque. The body activates blood clotting to fix the crack but this clot could inadvertently block a blood vessel completely, causing a heart attack or stroke.

Some studies have found fragments of the COVID virus inside the blood clots that cause heart attacks – further evidence to back our findings.

We don’t know whether younger, healthier people are also at increased risk of a heart attack or stroke after infection with a respiratory virus.

That’s because people in the studies we analysed were almost always older adults with at least one of those traditional risk factors, so were already vulnerable.

The bad news is we will all be vulnerable eventually, just by getting older.

What can we do about it?

The triggers we identified are mostly preventable by vaccination.

There is good evidence from clinical trials the flu vaccine can reduce the risk of a heart attack or stroke, especially if someone already has heart problems.

We aren’t clear exactly how this works. But the theory is that avoiding common infections, or having less severe symptoms, reduces the chances of setting off the inflammatory chain reaction.

COVID vaccination could also indirectly protect against heart attacks and strokes. But the evidence is still emerging.

Heart attacks and strokes are among Australia’s biggest killers. If vaccinations could help reduce even a small fraction of people having a heart attack or stroke, this could bring substantial benefit to their lives, the community, our stressed health system and the economy.

What should I do?

At-risk groups should get vaccinated against flu and COVID. Pregnant women, and people over 60 with medical problems, should receive RSV vaccination to reduce their risk of severe disease.

So if you are older or have predisposing medical conditions, check Australia’s National Immunisation Program to see if you are eligible for a free vaccine.

For younger people, a healthy lifestyle with regular exercise and balanced diet will set you up for life. Consider checking your heart age (a measure of your risk of heart disease), getting an annual flu vaccine and discuss COVID boosters with your GP.