A magia sacrificial faz ter um efeito protetor imediato.

Lily sacrificando-se para salvar Harry teve um efeito imediato sem que alguém tivesse que fazer mágica adicional - isso protegeu Harry da Maldição da Morte do Lorde das Trevas.

“Voldemort laughed softly in his ear, then took the finger away, and continued addressing the Death Eaters. ‘I miscalculated, my friends, I admit it. My curse was deflected by the woman’s foolish sacrifice, and it rebounded upon me.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

Harry também foi capaz de "proteger" todos morrendo de bom grado por eles. O sacrifício dela também fez com que o Lorde das Trevas não tocasse Harry, o que também aconteceu "automaticamente" sem necessidade de mais magia para invocá-lo.

“You all know that on the night I lost my powers and my body, I tried to kill him. His mother died in the attempt to save him – and unwittingly provided him with a protection I admit I had not foreseen … I could not touch the boy.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

Não está claro se o sacrifício de Harry também fez com que o Lorde das Trevas não pudesse tocar todos os outros, mas protegeu todos eles dos ataques do Lorde das Trevas. É esse efeito imediato que Harry se refere.

“You won’t be killing anyone else tonight,’ said Harry as they circled, and stared into each other’s eyes, green into red. ‘You won’t be able to kill any of them, ever again. Don’t you get it? I was ready to die to stop you hurting these people –’

‘But you did not!’

‘– I meant to, and that’s what did it. I’ve done what my mother did. They’re protected from you. Haven’t you noticed how none of the spells you put on them are binding? You can’t torture them. You can’t touch them.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)}}

Nenhuma mágica adicional era necessária para permitir que um sacrifício intencional protegesse a pessoa pela qual ele foi feito.

O charme de Dumbledore era apenas proteger Harry nos Dursleys.

O que Dumbledore fez foi criar um encanto que garanta que Harry estivesse protegido enquanto estivesse nos Dursleys. O sacrifício de Lily entrou em vigor antes disso, e Dumbledore não precisou "ativar" toda a proteção do sacrifício - ele apenas fez a parte que protegia Harry dos Dursleys.

“For he has been better protected than I think even he knows, protected in ways devised by Dumbledore long ago, when it fell to him to arrange the boy’s future. Dumbledore invoked an ancient magic, to ensure the boy’s protection as long as he is in his relations’ care.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

O charme de Dumbledore simplesmente usou o poder do sacrifício de Lily, que já havia acontecido e deixou seu efeito em Harry, permitindo que Harry fosse protegido enquanto estava na casa dos Dursleys. Dumbledore explica isso a Harry - Lily deu a Harry uma proteção demorada morrendo para salvá-lo, e é por isso que Dumbledore escolheu usar o feitiço que o protegia enquanto estava na casa dos Dursleys.

“You would be protected by an ancient magic of which he knows, which he despises, and which he has always, therefore, underestimated – to his cost. I am speaking, of course, of the fact that your mother died to save you. She gave you a lingering protection he never expected, a protection that flows in your veins to this day. I put my trust, therefore, in your mother’s blood. I delivered you to her sister, her only remaining relative.’

‘She doesn’t love me,’ said Harry at once. ‘She doesn’t give a damn –’

‘But she took you,’ Dumbledore cut across him. ‘She may have taken you grudgingly, furiously, unwillingly, bitterly, yet still she took you, and in doing so, she sealed the charm I placed upon you. Your mother’s sacrifice made the bond of blood the strongest shield I could give you.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)}}

O efeito do sacrifício de Lily existia antes de Dumbledore usá-lo para criar seu charme - seu efeito é porque ele escolheu usar esse charme específico para proteger Harry.

Harry sobreviveu como um bebê imediatamente depois que sua mãe foi morta, então não pode ser por causa de algo que Dumbledore fez mais tarde. Supõe-se que seja uma conseqüência automática das circunstâncias da morte da mãe dele.

Em uma entrevista, Rowling explica os requisitos:

JKR: Don't you want to ask me why James's death didn't protect Lily and Harry? There's your answer - you've just answered your own question - because she could have lived - and chose to die. James was going to be killed anyway. Do you see what I mean? I'm not saying James wasn't ready to; he died trying to protect his family, but he was going to be murdered anyway. He had no - he wasn't given a choice, so he rushed into it in a kind of animal way. I think there are distinctions in courage. James was immensely brave. But the caliber of Lily's bravery was, I think in this instance, higher because she could have saved herself. Now any mother, any normal mother would have done what Lily did. So in that sense, her courage too was of an animal quality but she was given time to choose. James wasn't. It's like an intruder entering your house, isn't it? You would instinctively rush them. But if in cold blood you were told, "Get out of the way," you know, what would you do? I mean, I don't think any mother would stand aside from their child. But does that answer it? She did very consciously lay down her life. She had a clear choice.

ES: And James didn't.

JKR: Did he clearly die to try and protect Harry specifically given a clear choice? No. It's a subtle distinction and there's slightly more to it than that but that's most of the answer.

MA: Did she know anything about the possible effect of standing in front of Harry?

JKR: No - because as I've tried to make clear in the series, it never happened before. No one ever survived before. And no one, therefore, knew that could happen.

MA: So no one - Voldemort or anyone using Avada Kedavra - ever gave someone a choice and then they took that option [to die] -

JKR: They may have been given a choice, but not in that particular way.

_{The Leaky Cauldron, 16 July 2005}

O livro foi lançado em 21 de julho de 2007, então a entrevista é anterior a ele.

De acordo com essa afirmação, é necessário que a vítima tenha vivido e escolhido morrer. Agora Voldemort nunca teria dado a Harry a escolha de viver, então claramente os requisitos para uma proteção não são cumpridos aqui.

Na verdade, não há evidências no livro de que alguém seria protegido pela morte de Harry, exceto que Harry diz que sim. A sequência é esta:

Voldemort was in the centre of the battle, and he was striking and smiting all within reach. Harry could not get a clear shot, but fought his way nearer, still invisible, and the Great Hall became more and more crowded, as everyone who could walk forced their way inside.

...

Voldemort was now duelling McGonagall, Slughorn and Kingsley all at once, and there was cold hatred in his face as they wove and ducked around him, unable to finish him –

(Molly kills Bellatrix)

Harry felt as though he turned in slow motion; he saw McGonagall, Kingsley and Slughorn blasted backwards, flailing and writhing through the air, as Voldemort’s fury at the fall of his last, best lieutenant exploded with the force of a bomb. Voldemort raised his wand and directed it at Molly Weasley.

‘Protego!' roared Harry, and the Shield Charm expanded in the middle of the hall, and Voldemort stared around for the source as Harry pulled off the Invisibility Cloak at last.

The yell of shock, the cheers, the screams on every side of ‘Harry!' ‘HE’S ALIVE!' were stifled at once. The crowd was afraid, and silence fell abruptly and completely as Voldemort and Harry looked at each other, and began, at the same moment, to circle each other.

(Harry and Voldemort talk)

‘Potter doesn’t mean that,' he said, his red eyes wide. ‘That isn’t how he works, is it? Who are you going to use as a shield today, Potter?'

(more talk)

‘You won’t be killing anyone else tonight,' said Harry as they circled, and stared into each other’s eyes, green into red. ‘You won’t be able to kill any of them, ever again. Don’t you get it? I was ready to die to stop you hurting these people –'

‘But you did not!'

‘– I meant to, and that’s what did it. I’ve done what my mother did. They’re protected from you. Haven’t you noticed how none of the spells you put on them are binding? You can’t torture them. You can’t touch them.‘

(DH)

Então, depois que Voldemort tentou matar Harry, ele ainda estava "batendo e golpeando". A palavra "ferir" significa, entre outras coisas, "atacar com efeito mortal ou desastroso". McGonagall, Kingsley e Slughorn estão se debatendo e contorcendo-se no ar, pelo menos o pouso os deixaria desorientados e fáceis de matar para os Comensais da Morte. Harry tenta proteger Molly, mas o Protego não pode parar a maldição mortal que Voldemort provavelmente usaria. Em vez disso, o fato de Harry estar vivo distrai Voldemort, ele não joga contra Molly. Então Harry e Voldemort conversam, e Harry afirma que Voldemort não pode tocá-los, embora seu ataque a McGonagall, Kingsley e Slughorn tenha sido eficaz.

Se apenas alguém precisa estar disposto a morrer para proteger todos os outros, certamente haveria alguém disposto a fazê-lo, e os Lordes das Trevas teriam muita dificuldade.

Então não havia proteção, era apenas o blefe de Harry.

Editar

Apenas para o contexto adicionado, quando Voldemort disse: "Quem você vai usar como escudo hoje, Potter?" ele realmente não queria que alguém fosse um escudo para Harry, porque ele se lembra de como isso terminou da última vez:

"I miscalculated, my friends, I admit it. My curse was deflected by the woman’s foolish sacrifice, and it rebounded upon me." (GoF)