People who develop flu or pneumonia may have an increased risk of suffering a heart attack or stroke shortly after infection, the results of a new study suggest.

This is the largest study ever undertaken which looked at the risk of heart attack and stroke due to specific respiratory infections.

UK researchers looked at over 1,200 adults who had suffered a first heart attack, and more than 760 who had suffered a first stroke, who also had a respiratory virus or bacterial infection between 2004 and 2014.

The study found that people with a confirmed respiratory infection were six times more likely to suffer a heart attack or stroke in the three days after infection.

It also found that while the risk of a heart attack increased for up to a week after infection, the risk of a stroke increased for up to one month.

The pneumonia bacteria and flu virus had the biggest impact on heart risk of all the respiratory infections assessed.

The researchers noted that the effect of infections on heart attack and stroke risk was greater in people under the age of 65 years compared to those aged 65 and older. They suggested that this is because flu and pneumococcal vaccine uptake tends to be higher among those aged 65 and over.

The findings suggest that being vaccinated could help to protect against heart attacks and strokes after respiratory infections.

"For most young, healthy people, the risk of heart attacks and strokes occurring after a respiratory infection is low. This research is particularly relevant for those over the age of 65, as well as people with pre-existing heart diseases, as these groups are at higher risk of heart attacks and strokes," commented lead researcher, Dr Charlotte Warren-Gash, of the London School of Hygiene & Tropical Medicine.

Respiratory infections are thought to increase the risk of heart attack and stroke by causing inflammation, which can lead to the development of blood clots. Pneumonia and flu can also have harmful effects on the heart muscle.

"Heart disease, strokes and lower respiratory infections have been the three leading causes of death globally for over 15 years, and are important public health problems that affect large numbers of people worldwide.

"As people age, having more than one medical condition becomes more common, so it is even more important to understand the links between different diseases. If we can understand who is at risk of these cardiovascular complications after respiratory infections, we can potentially intervene to prevent them, with methods such as vaccines," Dr Warren-Gash added.

Details of these findings are published in the European Respiratory Journal.