The association between influenza vaccination, cardiovascular mortality and hospitalization: A living systematic review and prospective meta-analysis.

Background: The effects of seasonal influenza vaccination on cardiovascular disease (CVD) outcomes, including among individuals with established CVD, are uncertain.

Methods: To evaluate the efficacy and safety of influenza vaccines compared to no vaccines or placebo for preventing all-cause/CVD mortality or all-cause/CVD hospitalization in the general population and in populations with pre-existing CVD, we conducted a living systematic review (LSR) and prospective meta-analysis (PMA). Published randomized controlled trials (RCT) and observational studies between 1994 and 2023 were searched. PRISMA guidelines were followed in the extraction of study details, and risk of bias was assessed using the Cochrane tools. Analyses were stratified by study design and CVD history. Study quality was evaluated using GRADE system. Meta analyses based on random-effects models were performed between July and October 2022. Pooled risk ratios (RRs) for all-cause/CVD mortality and all-cause/CVD hospitalization were main outcomes.

Results: Six published RCTs comprising 12,662 participants (mean age, 62 years; 45 % women; 8,797 with pre-existing CVD) and 37 observational studies comprising 6,311,703 participants (mean age, 49 years; 50 % women; 1,189,955 with pre-existing CVD) were included. Only those RCTs judged to be low risk were included in the analyses, and observational studies at anything greater than moderate risk of bias were excluded. In RCTs, influenza vaccine was not significantly associated with lower all-cause mortality (RR, 0.85; 95 %CI, 0.61-1.17), cardiovascular death (RR, 0.80; 95 %CI, 0.60-1.07), or CVD hospitalization (RR, 0.69; 95 %CI, 0.47-1.02). A statistically significant reduction in all-cause hospitalization (RR, 0.86; 95 %CI, 0.76-0.97) was observed. The evidence level was assessed as moderate for all-cause hospitalization, and low for other outcomes. Overall, observational studies suggested a stronger protective association between influenza vaccination and outcomes, except for CVD hospitalization. Based on RCTs, there was no difference in the effects of influenza vaccination on all-cause mortality among the general population compared to those with pre-existing CVD, although the summary point estimate favored benefits only in those with pre-existing CVD.

Conclusions: While observational studies suggest that influenza vaccination may be associated with lower all-cause and CVD mortality and all-cause hospitalization, RCTs reported to date suggest a reduction in the risk of all-cause hospitalization but do not provide clear evidence to support preventive effects on mortality (all-cause or CVD) or CVD hospitalization.