She's talking about the strength of an individual's immune system. She doesn't seem to be talking about herd immunity or how the pandemic has potentially change

She's talking about the strength of an individual's immune system. She doesn't seem to be talking about herd immunity or how the pandemic has potentially changed population dynamics. For example, "debt" could be referring to the fact that many people might've already gotten [insert infection name here] over the last couple of years, but they did not, and now there's an increased number of people who don't have immunity to it compared to a world where there not been a pandemic. The fact that this was ignored almost seems misleading given that she claims to be an immunologist.