Underlying health issues. That pretty much means he would die from even the flu. Most people who die from covid are elderly, have pre existing conditions, poor health, morbidly obese, etc. Just like the people who die from the flu every year.
And being unvaccinated doesn't mean anything as proven here.
The first person in the U.S. diagnosed with the omicron Covid variant was fully vaccinated, has mild symptoms and has not been hospitalized, officials said.
www.cnbc.com
So just because you're fully vaccinated doesn't mean you can't get covid and doesn't mean you can't spread it.