Mortgage investors who think the 40-year lows in mortgage rates reached this fall provide some immunity to future prepayment risk are whistling past the graveyard, according to some securitization analysts. As the current refinance wave ends, a significant supply of the mortgage universe will be in the 5.5 percent coupon and the probability of another refinance “event” is reduced dramatically, says Bear Stearns in a new prepayment analysis. The Wall Street firm’s report is