Ireland as example; question: COVID virus killed mostly elderly & not younger; yet COVID virus is mild (BA.5, XBB1.5) for at least one year now while deaths are elevated in young, not elderly: why?

If the virus no longer kills & has not been killing at the rates during the peak of the pandemic & if those now dying are young & not elderly & virus is not killing elderly, what kills younger people?

This virus does not kill younger persons as the at risk group was always elderly. Now that deaths are down from the virus across all age groups, what is killing young persons? Could it be the COVID vaccine?