A 2006 study, published in the Journal of the American Medical Association, found that
“Green tea consumption is associated with reduced mortality due to all and due to cardiovascular disease but not with reduced mortality due to cancer.” source
Another in Arteriosclerosis, Thrombosis, and Vascular Biology (2010) claimed :
“High tea consumption is associated with a reduced risk of CHD mortality. Our results suggest a slight risk reduction for CHD mortality with moderate coffee consumption and strengthen the evidence on the lower risk of CHD with coffee and tea consumption.” Source
Neither study, or any of the numerous others since then, were able to pinpoint the active ingredients that would lead to such effects. That's still the case as of 2017.
Research from Charité Universitäts, Berlin, examined the hypothesis that tea might help alleviate cardiovascular problems by acting as a vaso-dilator. But no firm results were able to pinpoint which chemical components were leading to tea's apparent benefits.
“Consumption of tea is associated with reduced progression of atherosclerosis and lower cardiovascular mortality, However, the substance(s) mediating the favorable cardiovascular effects of tea in vivo have not yet been identified.”
Note: the article gives links to numerous other studies claiming a wide range of medical benefits for tea drinking.