Flu Vaccine May Prevent Cardiovascular Events

A meta-analysis of previous studies suggests that getting vaccinated against the flu reduces one’s risk of adverse cardiovascular events, with the greatest benefit in those with a recent acute coronary syndrome.

Patients at high risk for cardiovascular events who get vaccinated against the flu may receive protection against adverse cardiovascular outcomes as well, according to the findings of a recent meta-analysis.

Previous research has suggested a link between respiratory tract infections and increased risk for major cardiovascular events, and smaller studies and meta-analyses of observational studies have shown that those who are vaccinated against the flu reduce that risk. To account for possible bias and to further assess the relationship between the flu vaccine and risk of fatal and nonfatal cardiovascular events, the current study, published in the October 23/30, 2013, issue of the Journal of the American Medical Association, reviewed randomized clinical trials that studied influenza vaccination and cardiovascular events.

The researchers performed a systematic search for randomized clinical trials that compared influenza vaccination with placebo or standard care, included at least 50 patients, and had a follow-up of less than 1 year. Studies that described an appropriate method of randomization, blinding, and completeness of follow-up were considered to be high-quality. Baseline characteristics, outcomes, and safety events were extracted from studies included in the meta-analysis and were analyzed for a composite of major adverse cardiovascular events such as death or hospitalization caused by heart attack, angina, stroke, heart failure, or urgent coronary revascularization.

A total of 1 unpublished and 5 published clinical trials were included in the analysis. Overall, 6735 patients were enrolled in the 6 studies, and 36.2% of those enrolled had a cardiac history. Among the 3238 patients in the studies who received the influenza vaccine, 2.9% experienced a major adverse cardiovascular event compared with 4.7% of those who received a placebo or no vaccine. The researchers estimated that 1.7 major cardiovascular events could be prevented for every 100 patients who receive the flu shot.

A sub-analysis of 3 clinical trials of patients with coronary artery disease also found a significant association between the influenza vaccine and cardiovascular risk, especially among patients with recent acute coronary syndrome. Among patients who experienced recent acute coronary syndrome, 10.25% of those who received the flu vaccine experienced cardiovascular events, compared with 23.1% of those who received a placebo or no vaccine. The researchers estimated that 12.9 cardiovascular events could be prevented for every 100 patients in this population vaccinated against the flu. The association was significant but not as strong among patients with stable coronary artery disease: 6.9% of these patients who got the flu vaccine had a cardiovascular event compared with 7.4% of placebo patients.

In an accompanying editorial, however, Kathleen M. Neuzil, MD, MPH, suggests that the results may not be as strong as the researchers indicate. She writes that only 2 of the studies were of high quality and that the prevention rate of 12.9 cardiovascular events per 100 patients with recent acute coronary syndrome calculated in the sub-analysis is most likely inflated, as it would require a high attack rate, high vaccine efficacy, and that a majority of influenza cases would lead to major cardiovascular events. Dr. Neuzil does note, however, that the prevention rates estimated in the primary analysis are plausible and suggest additional benefits accompanying flu vaccination.

“Regardless of whether influenza vaccine reduces cardiovascular disease, the known morbidity of influenza in older adults with and without high-risk conditions and the known efficacy of the vaccine warrant its use,” Dr. Neuzil writes.