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Flu jab "may cut" stroke risk by a quarter

A recent report has revealed that people who have the seasonal flu jab are 24% less likely to suffer a stroke, according to The Daily Telegraph.

The report is based on the results of a large study, which used the GP database for England and Wales to access data on almost 50,000 people who had suffered a stroke or transient ischaemic attack (TIA or a so-called “mini” stroke) over an eight-year period. Researchers then matched them to a person of the same age and gender who had attended the GP around the same time (these people are known as the “controls”). They then compared how likely they were to have been given the seasonal flu vaccine before the date of the stroke or TIA.

They found that slightly more controls had received the flu vaccine before the date: 50.8%, compared to 50.6% of people who had had a stroke or TIA. This meant that, overall, having the flu vaccine reduced the risk of a person having a stroke by about a quarter (there was no link with TIAs).

The research benefits from a large quantity of reliable data, with a number of health and lifestyle factors that may have influenced the results also adjusted.

It is plausible that a link exists between the protection against flu strains the vaccine provides and the risk of having a stroke.

The researchers summarise by saying they “reinforce current recommendations for annual influenza vaccination” with “potential added benefit for stroke prevention”.

Where did the story come from?

The study was carried out by researchers from the University of Lincoln and University of Nottingham, and was funded by the National Institute for Health Research.

The study was published in the peer reviewed journal Vaccine.

The Daily Telegraph and The Independent’s reporting of the study was accurate.

What kind of research was this?

This was a case-control study, which aimed to see if the influenza or pneumococcal vaccination might prevent a stroke. Some previous research studies have suggested that respiratory infections, such as the flu, may be associated with a stroke.

They cite a number of studies that have found there is an increased likelihood of respiratory symptoms in the weeks prior to having a stroke. However, other observational studies failed to find any significant link.

Due to this inconsistent evidence, the researchers aimed to investigate this further themselves, using data for thousands of people stored on the UK General Practice research database.

They identified people who had suffered a stroke or TIA and a matched group of people without, examining if they had been given the seasonal flu jab or the pneumococcal vaccine.

The pneumococcal vaccine is part of the child vaccination programme. It is also offered as a one-off jab to all adults over the age of 65 and to younger adults that have a heightened risk of infection (such as those with weakened immune systems). The vaccine protects them against infection with the bacteria Streptococcus pneumonia, which can cause pneumonia and other severe infections.

What did the research involve?

The researchers used the General Practice Research Database (now called the Clinical Practice Research Datalink, CPRD), which contains anonymised data for over 5% of the population of England and Wales. The database codes for vaccinations, diseases and health behaviours using a validated coding system. They used the eight-year period 2001-9 to identify adults coded for stroke or TIA (“case”). Each case was randomly matched to a control of the same age and gender who attended a general practice at the same time. They excluded cases and controls with a previous diagnosis of stroke or TIA.

They looked for vaccinations recorded prior to the ”index” date, when the stoke or TIA was recorded. Because the influenza vaccine is seasonal, the researchers looked to see if the vaccine was given in the same seasonal year (September 1 to August 31 the following year) and whether given “early” (between September 1 and November 15) or “late” (November 16 to February 28), and the time elapsed since last influenza vaccination (defined as 0 to 3, 3 to 6, 6 to 12, or more than 12 months before the index date). Pneumococcal vaccination was defined as vaccination at any time before the index date, as it is given as a one-off vaccine.

The researchers looked at the odds for cases and controls given either or both vaccines.

They adjusted their analyses for potential confounders, including cardiovascular risk factors, current medications, number of cormorbid medical illnesses, lifestyle factors (such as whether a person smokes) and the number of GP consultations and home visit requests. 

What were the basic results?

The researchers identified 47,011 cases (comprising 26,784 cases of stroke and 20,227 cases of TIA), with the same number of matched controls.

Very slightly more controls than cases had received the flu vaccine in the same season as the index date: 50.8% of controls compared to 50.6% of cases. After adjusting for the measured confounders, this meant that having an influenza vaccine within the same season as the index date was associated with a 24% reduction in the risk of a stroke (odds ratio [OR] 0.76, 95% confidence interval [CI] 0.72 to 0.80).

Further adjustment for body mass index (BMI), cholesterol and blood pressure slightly reduced the risk association, such that the risk reduction was only 19%, but still remained statistically significant (OR 0.81, 95% CI 0.77 to 0.85).

Reduction in the risk of a stroke was greatest when the vaccine had been given within three months of the index date (22% reduction), reducing to 11% reduced risk when the vaccine had been given between three and 12 months of the index date.

However, the flu vaccine only seemed to protect against a stroke if given early in the flu season: September to mid-November (26% risk reduction, OR 0.74, 95% CI 0.70 to 0.78). Giving the vaccine late in the flu season (mid-November to February) did not result in a significantly reduced risk.

Flu vaccine was not significantly associated with being at risk of TIA. Neither was pneumococcal vaccination significantly associated with being at risk of a stroke or TIA.

How did the researchers interpret the results?

The researchers conclude that: “Influenza vaccination was associated with a 24% reduction in risk of having a stroke, but not TIA. Pneumococcal vaccination was not associated with reduced risk of a stroke or TIA. This has important implications for the potential benefits of an influenza vaccine.”

Conclusion

This research finds that, overall, having the flu vaccine reduced the risk of a person experiencing a stroke by about 25%. The reduction in risk seemed to be greatest within the first three months of vaccination, but remained for up to 12 months.  However, the effect lasted only if the vaccine was given early in the flu season (September to mid-November); giving the vaccine late in the flu season (mid-November to February) was not associated with significantly reduced risk.

The research benefits from using data coded within the General Practice Research Database for England and Wales for almost 50,000 people with stroke or TIA, matched by age and gender to the same number of controls who had attended the GP at the same time. There is still potential for missing or miscoded information in the database, but overall the data is thought to be fairly reliable.

They also adjusted their analyses for a large number of potential confounders. The researchers say there is still the potential for what they call “healthy vaccine” bias, with healthier people more likely to be vaccinated, and are perhaps less likely to have a stroke.

The findings support previous studies, which the researchers say have suggested an association between recent respiratory illness and risk of a stroke; they have also studied findings that influenza vaccination may offer protection against another heart attacks. However, the biological mechanisms by which respiratory infections or influenza might precipitate cardiovascular events is unknown. It is also unknown if the findings could apply to younger people at risk.

In summary, it is plausible that there could be a link between the protection the flu vaccine gives against flu strains and a risk of suffering a stroke in the same season.

The purpose of the seasonal influenza vaccine is to protect against respiratory illness, not to offer possible protection against a stroke. However, the researchers “reinforce current recommendations for annual influenza vaccination” and that there is “the potential added benefit of stroke prevention”.

Even if the link between the flu jab and reduced stroke risk is unproven, it is always a good idea to get the jab if you are in one of the groups recommended to receive it. This is if you are:

  • 65 years of age or over 
  • pregnant
  • have a long-term (chronic) medical condition such as asthma or diabetes
  • living in a long-stay residential care home or other long-stay care facility
  • receive a carer’s allowance, or you are the main carer for an elderly or disabled person whose welfare may be at risk if you fall ill
  • a healthcare worker with direct patient contact or a social care worker

Read more on who should get the flu jab.

Analysis by Bazian. Edited by NHS Choices.

 

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