It’s way too close for comfort.
Comfort in politics seems to be a rarity.
I wouldn't worry to much, the margin will widen as Sanders becomes more exposed. Early polls are not very reliable. In January 1984, a Harris poll of registered voters said Ronald Reagan should NOT seek re-election by a 55-40 margin. Even in early June, Reagan only led Mondale in the Harris poll 50-45.
I think in a Trump-Sanders matchup, what will be remembered after the election is the debate night where President Trump triggers Bernie into a public temper tantrum that craters his support among all but the extreme hardcore left. Sanders is just a seething volcano waiting to erupt and President Trump will find the right needle. Voters will realize that temperamentally Bernie is unfit to serve as President.