That the vampire must be invited into a building is a staple of vampire fiction. But the rule is often interpreted differently by various writers. Sometimes the vampire need only be invited at one entrance to gain access through any entrance as in “Dracula” (1931). I didn’t watch “Buffy, the Vampire Slayer”, but I understand the vampires on this show only needed invites into private residences.
In “Let the Right One In”, the rule seems to be that Eli needs an invite through any entrance. We hear an off-screen voice give her permission to enter the hospital but when she gets to Hakan’s hospital room window she has to ask to come in there. He can’t speak, so…
Nit-pickers will notice at the beginning of the film we see Eli enter the apartment building for the first time, apparently uninvited. And we can only guess at the circumstances of Virginia’s entrance into the hospital (we only see her being wheeled down a hall on a gurney) after the cat attack.
“Let the Right One In” seems to be the first time in a film where the consequences of the vampire breaking the invitation rule have been depicted. And wonderfully so, in my opinion.