How did they arrive to this estimate? Please show your calculations

The authors stated that “By the end of the trial, 999 major cardiovascular events (253 per 100 000 person-years) had occurred: 482 in the vitamin E group and 517 in the placebo group. This corresponded to a nonsignificant 7% risk reduction with vitamin.” What is the term commonly used for this estimate the authors used? How did they arrive to this estimate? Please show your calculations