  We prove a character formula for the irreducible modules from the category
$\mathcal{O}$ over the simple affine vertex algebra of type $A_n$ and $C_n$ $(n
\geq 2)$ of level $k=-1$. We also give a conjectured character formula for
types $D_4$, $E_6$, $E_7$, $E_8$ and levels $k=-1, \cdots, -b$, where
$b=2,3,4,6$ respectively.
