Original paper
The protective effect of influenza vaccination on the clinical course of coronary disease in patients with acute coronary syndromes treated by primary PCI – a report from FLUCAD study

Background Inflammation and infection play an important role in the progression of atherosclerosis and destabilization of angina [1-5]. Several studies have indicated that influenza vaccination reduces cardiovascular mortality, the incidence of myocardial infarction (MI) and stroke [6-9]. In our previous report of the randomized, double-blind, placebo controlled FLUCAD study, we found that influenza vaccination reduced the incidence of coronary ischemic events in patients with confirmed coronary artery disease (CAD) [10].
As our study group was not clinically homogenous, we decided to investigate the potential difference in the protective effect of influenza vaccination between patients with stable angina and those with acute coronary syndrome (ACS) vaccinated after primary PCI.

Material and methods
Patients and protocol
FLUCAD study is a prospective, randomized, double blind, single center, placebo controlled study performed to evaluate the effect of influenza vaccination on the incidence of coronary events in patients with CAD. The details of the study protocol were published previously [10]. In brief, 658 patients (477 men) aged 59.9 ± 10.3 years randomly received intramuscular inactivated sub-unit influenza vaccine (Influvac, SolvayPharma) for 2004/2005 season, or placebo containing all vaccine compounds except viral antigens. The randomization was 1 : 1 (influenza or placebo vaccine).
Patients were eligible if they were aged 30-80 years, and had CAD confirmed by angiography with at least 50% stenosis of at least one large epicardial coronary artery. We excluded patients with congestive heart failure NYHA III/IV, planned cardiovascular surgery within 6 months, evolving renal failure, contraindication to vaccination, or any factor impeding follow-up. The ACS group consisted of patients with: ST elevation ACS (STEMI), non-ST elevation ACS (NSTEMI) and troponin negative ACS. All ACS patients were treated by primary PCI, and were vaccinated within a week after coronary intervention, before discharge from the hospital. Outpatients were vaccinated during office visits to cardiologists.
Patients were recruited and vaccinated from October 2004 to February 2005. All cardiovascular events and study endpoints were confirmed by source documents. The follow-up was censored for all participants on October 31st, 2005.
The primary endpoint was 12-month cardiovascular death. There were two secondary endpoints: major adverse cardiac event (MACE), which was the composite of: cardiovascular death, acute myocardial infarction (MI), or coronary revascularization and coronary ischemic event, defined as a combination of MACE or hospitalization for myocardial ischemia, both assessed at 12-month.
STEMI and NSTEMI were diagnosed on the criteria described previously, basing on AHA guidelines [10].

Statistical analysis
All analyses were done on a intention-to-treat basis. Results are presented as median, (25th and 75th percentile), for continuous variables, and as percentages for categorical data. Groups were compared (all tests two-sided) by unpaired t-test or one way analysis of variance for normally distributed variables or Mann-Whitney-Wilcoxon test, where data were not normally distributed. Frequencies were compared using chi-square or Fisher’s exact test, where appropriate. Survival curves were assessed by Kaplan-Meier method and compared using a log-rank test. Hazard ratios (HR) and 95% confidence intervals were calculated by the Cox proportional hazards model. Estimated 12-month cumulative rate of the study endpoints in vaccinated versus placebo patients was presented as percentages based on Kaplan-Meier method. In measuring the time to an event for cases in which a patient had multiple end-points, the first event was taken into account.
Multivariable Cox regression model included major causal risk factors for CAD: age, sex, smoking, arterial hypertension, elevated LDL-cholesterol ( 3.12 mmol/l), low HDL-cholesterol ( 0.9 mmol/l), and diabetes along with influenza vaccination, and recent ACS.
Data were analyzed using the SAS package, ver. 8.2. The p values < 0.05 were considered significant.
The authors had full access to the data and take responsibility for its integrity. All authors have read and agreed to the manuscript as written.

Results
Study population
We included and vaccinated 658 Caucasian patients (477 men), mean age 59.9 ± 10.3 years. Three hundred twenty five patients received influenza vaccine and 333 patients received placebo. Median follow-up time was 298 (263-317) days, range 4-370 (measured to the first event or to the end of the observation period). No patient was lost to the follow-up. Detailed characteristics of study groups are listed in table 1.
Among 157 ACS patients, 83 received active vaccine and 74 placebo. In ACS group there were 81 STEMI, 43 NSTEMI and 33 Troponin negative ACS patients. Of the 501 stable angina patients, 242 received active vaccine and 259 placebo. Stable group consisted of 370 outpatients with angiographically confirmed CAD and 131 patients hospitalized for elective PCI.

Impact of vaccination on clinical outcomes in ACS patients
In ACS group (tab. 2, fig. 1 A, C, E) the primary endpoint – the estimated 12-month cumulative cardiovascular death rate was 1.2% in vaccinated and 0% in placebo patients (p = 0.623).
The MACE tended to occur less frequently in the vaccine group in comparison to placebo, with a statistical trend 4.66 vs. 9.63% respectively, HR 0.36 (95% CI 0.09-1.39, p = 0.139).
For coronary ischemic event the estimated 12-month cumulative event rate was significantly lower in the vaccine group (11.63%) than in the placebo group (19.29%), with a HR 0.37 (95% CI 0.14-0.99, p = 0.047) (tab. 2).

Impact of vaccination on clinical outcomes in stable angina patients
In the stable angina group (tab. 3, fig. 1 B, D, F) we found no statistically significant effect of the influenza vaccination either on cardiovascular death, or any of the secondary endpoints (MACE and coronary ischemic event).

Discussion
Our results show in a prospective, randomized, double blind, placebo-controlled study the difference of the protective effect of influenza vaccination on the clinical course of CAD between patients with ACS treated by primary PCI and those with stable angina. The beneficial effect of vaccination is evident in patients vaccinated in the clinically unstable period of angina, with recent ACS (tab. 2, fig. 1 E). Despite a much bigger cohort of patients, we did not prove such an effect with statistical significance in patients with stable angina (tab. 3, fig. 1 B, D, F).
Another important finding is that the protective effect of influenza vaccination was present through the whole follow-up and it extended beyond the period of viral circulation (fig. 1 E). The study also confirms previous sparse reports that influenza vaccination, early after emergency PCI, is safe, and carries no increased risk of cardiovascular events [9, 11]. Despite the recommendation of influenza vaccination in CAD by the European Society of Cardiology some aspects concerning the nature of the effect still remain unclear [12].
The effect of influenza vaccination might be different in relation to the clinical profile and treatment of CAD patients, and it may affect the results of sparse studies. Gurfinkel et al. found in FLUVACS study a significant, 75% reduction in mortality in a group of 301 CAD patients due to influenza vaccination, but his group was treated very conservatively, developed a large number of cardiac events (5% mortality rate in 6 months), and the impact of influenza vaccination on: mortality, reinfarction and composite end point was significant [9]. Madjid et al. postulated that influenza vaccination may have a plaque stabilizing effect and may be more beneficial for the subjects who lack access to efficient medical care. Our results confirm this hypothesis. Our ACS patients were vaccinated during the first days after primary coronary interventions, in the phase of unstable CAD and aggravated inflammation. Their hs-CRP level was significantly higher than in the stable angina group (median 0.60 vs. 0.21 mg/dl, p = 0.0001). In contrast, the stable group represented optimally treated stable patients, with over 90% use of beta-blockers, antiplatelets, statins and ACE-inhibitors. In our opinion, it might be very difficult to prove the beneficial effect of influenza vaccination on the already stable angina patients receiving optimal treatment.
The mechanism of the protective effect of influenza vaccination in CAD is not clear. Some authors think that as influenza and all severe acute infections trigger a plaque destabilization [4, 6, 11].
Other investigators believe in a special, specific relation between influenza and atherosclerosis. They suggest that autoimmune cross-reactivity between influenza antigens and plaque antigens may be responsible for the progression of atherosclerosis and influenza cardiovascular complications [13]. Our observations that the protective effect of influenza vaccination was not restricted to the period of viral circulation suggest that influenza vaccination may influence an immuno-inflammatory mechanism of atherosclerosis.
In conclusion, influenza vaccination reduces the rate of coronary ischemic events and may be safely performed in patients early after ACS and primary PCI. In contrast, in optimally treated, stable angina patients, further stabilization of the CAD course by influenza vaccination, is not evident. The beneficial effect of influenza vaccination in ACS patients was present through the whole follow-up, and was not restricted to the period of viral circulation. A further study is still needed to evaluate life saving and MI-protecting properties of influenza vaccination, and to clarify the mechanism of its protective effect on the clinical course of coronary disease.


Study limitations
Our group represented only selected patients who received optimal medical treatment and uncomplicated invasive procedures. The number of cardiac events was lower than expected, and the sample size may be considered too small to evaluate the effect of influenza vaccination on mortality and MACE. Another limitation was that the effect of flu vaccination on restenosis is unknown. Restenosis might be the cause of cardiac events in post PCI patients, but routine angiography was not performed in this study.

Disclosures and aknowlegments
The study was financed by the Grant of Polish Ministry of Education and Science No. 2 P05B 01627. Solvay Pharmaceuticals B.V. provided influenza vaccine and placebo vaccine free of charge.

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