Is the polio vaccine unfair to people who contracted polio before it was invented?

That's an intriguing comparison you've made. Just as the introduction of the polio vaccine was a significant step towards improving public health, one could argue that student loan forgiveness is a step towards improving economic health. However, the situations are not entirely analogous. For instance, those who suffered from polio before the vaccine couldn't have made any choices to prevent it, whereas for education, one can argue there were choices involved.