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how do they know for certain there won't be any long-term debilitating effects?
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Because while you're right that side effects from vaccines have happened before, they know from those past times that side effects tend to show up within six months in adults. Which is why the vaccines, even though early signs are they will be effective, aren't being handed out already. They're waiting for the six months to be up to make sure there won't be side effects.
Bear in mind that they're not inventing new ways to create vaccines here. This is all established technology being used on a new virus.