Tuesday September 21 2010
Flu can kill but flu jabs offer important protection
“The winter flu jab may prevent heart attacks,” reported The Daily Telegraph. The newspaper said that having a seasonal flu jab may reduce the risk of a heart attack by one-fifth.
The story is based on research that analysed data on 79,000 UK patients. The study compared rates of heart attacks in people given the flu vaccine with those in people who were not vaccinated. Researchers say that the flu vaccination was associated with a 19% lower rate of heart attacks, which may be due to the jab preventing chest infections that lead to heart attacks.
While this study was well conducted, it cannot prove that vaccination cuts heart risk. It is possible that other health and lifestyle factors may have affected the results. For example, those people who were vaccinated may have been more likely to see their doctor and, therefore, adopt behaviour to prevent heart problems. However, the researchers adjusted their findings for a number of important factors, which increases the likelihood that the results are valid.
Where did the story come from?
The study was carried out by researchers from the University of Lincoln, NHS Lincolnshire and the University of Nottingham. It was funded by the UK National Institute for Health Research and was published in the peer-reviewed Canadian Medical Association Journal.
It was reported fairly in the newspapers, and The Daily Telegraph included the opinions of independent experts.
What kind of research was this?
This was a matched case-control study, which looked at the possible association between vaccinations against flu and pneumonia and the risk of heart attack. This type of observational study is often used to identify factors that may contribute to a medical condition. It compares people who have that condition (the case subjects) with patients who do not have the condition but are otherwise similar (the control subjects).
The researchers point out that heart attacks peak in winter and that this may be related to peak incidence of flu and pneumonia. Previous studies have shown a link between heart attacks and a preceding respiratory infection, although there is contradictory evidence as to whether protection against chest infections offered by flu and pneumococcal vaccines can help prevent heart attacks. The researchers set out to investigate whether there is any association between the two vaccines and the risk of heart attack.
What did the research involve?
The researchers looked at people who had had a first heart attack between 2001 and 2007 (the cases) and matched each of them to four people (the controls) who had not had a heart attack by the same date (called the index date). They extracted all the data from a validated UK database that contains reliable, anonymous information about 5% of the population of England and Wales.
The cases had to be at least 40 years of age at the time of their first heart attack and were identified using standard diagnostic codes. The controls were also matched according to age, sex and the GP practice they attended. The four controls were selected at random from all those eligible.
From this data, the researchers extracted information on flu vaccinations in the year before the index date and in the same flu season as the index date, as well as information on whether the vaccination was given early (between September 1 and November 15) or late (between November 16 and February 28 or 29). They also included information on any pneumococcal vaccination given at any time before the index date, and on combined vaccines.
They extracted data on factors that might influence the risk of heart attack (confounders), such as other cardiovascular risk factors, treatment with medications, how often someone had seen a GP in the previous five years and smoking status. They also noted whether they belonged to the high-risk “target groups” who are encouraged to have both vaccines. The researchers used validated statistical methods to analyse the association between heart attack and vaccines, adjusting their results for these confounders.
What were the basic results?
The study included 16,012 people who had had a heart attack and 62,964 matched people who had not. Just over half those in each group had had a flu vaccination in the year before the index date and just over one-third in each group had had a pneumococcal vaccination before the index date. The researchers noted that people with risk factors for heart attack were also more likely to have been vaccinated, which would be expected since some of the risk factors for heart disease are also indicators that a person has a greater need for vaccination.
After adjusting their results for possible confounders (including being in a target group for vaccination), the researchers found that:
- Overall, the flu vaccine was associated with a 19% reduction in the risk of heart attack (adjusted odds ratio [AOR] 0.81, 95% confidence interval [CI] 0.77 to 0.85).
- Early vaccination was associated with a 21% risk reduction (AOR 0.79, 95% CI 0.75 to 0.83).
- Pneumococcal vaccine was not associated with reduction in heart attack risk (AOR 0.96, 95% CI 0.91 to 1.02).
How did the researchers interpret the results?
The researchers concluded that their study lends support to previous research suggesting that the flu vaccine may have a protective role against heart attack, even in people without cardiovascular disease. They say this may possibly be because it helps prevent flu, which may be a risk factor for heart attack. They added that further research is needed to confirm the finding that vaccination earlier in the season confers greater benefit.
The researchers said that pneumococcal vaccination or combined vaccination had no additional benefit in preventing heart attacks compared with flu vaccination alone.
This large, well-conducted study suggests that the annual flu vaccine may protect against heart attacks, even in people without known risk factors for heart disease. The researchers minimised the possibility of bias by including all cases of heart attack within a given time period, carefully matching them to controls and only including patients who had at least five years’ up-to-date information on the database. They also adjusted their findings for important confounders. Additionally, the size of this study means that its results are more likely to be reliable.
However, this type of study cannot prove that a particular intervention (in this case, flu vaccination) causes an effect (prevention of heart attack). This is because it is possible that confounding factors (both measured and unmeasured) may have had an influence on the results. As the authors note, they did not account for some confounders such as stressful life events, and data was missing on some confounders such as smoking status, blood pressure, BMI and cholesterol levels (although they carried out additional calculations to take into account the missing data).
While we cannot tell whether there is a reduced risk of heart attacks from receiving a flu vaccination, we do know that immunisation is a powerful tool for protecting vulnerable groups from some of the harms of flu and pneumonia. Just like heart attacks, flu and pneumonia kill thousands of people every year. The winter flu season is approaching and anyone who knows or thinks they would benefit from a vaccination should read our guide to getting a flu jab.