There's a paragraph of Methods on how they chose to deal with those: Confounding factors Age in years and smoking in three categories (never, past and current smokers) were assessed. Total cholesterol, triglyceride and high-density lipoprotein cholesterol (HDL) were measured by the automated enzymatic technique. We assessed dyslipidemia by total/HDL cholesterol ratio which was proven the best predictor of future atherosclerosis.30 Diabetes was ascertained by medical record review. Subjects were considered to have diabetes if documented diagnoses were in the medical records or if they were being treated for diabetes. To avoid confounding by affluence and high socioeconomic status, we adjusted educational levels, income and private insurance status.

For Covid, infection-generated immunity wanes over time just as vaccine-induced immunity does. Regardless of what taught a person's immune system to recognize and fight SARS-CoV-2, it doesn't last. This has not only been documented from many data sources from many countries (ex nejm.org/doi/full/10.1056/NEJMoa2118946 ) it's also apparent in anyone's personal life. I think at this point we all know someone who's gotten covid multiple times.

It's also clear that avoiding infection is important not just to avoid death, but to avoid the negative health effects of multiple bouts of covid. nature.com/articles/s41591-022-02051-3

Fun fact, the biggest contributing factors to increased life expectancy have actually been public health initiatives like sewage management, water treatment, hand washing, and vaccines.