As one-time events that are more fiscal than monetary in nature, however, I do not consider them particularly important from a monetary policy point of view. A similar effect was seen in the increase in the July CPI number, which was partly attributable to a rise in tobacco excise taxes imposed by a number of states.
Under the QQE, yields of liquid short-term assets have already approached zero across the board, and therefore even a small premium added to T-Bills could easily push their rates below zero. Foreign investors have also played an important role in driving yen rates into negative territory.