First RIP Russ, and yes he did die suddenly, from either a mechanical failure or pilot error, it was not from the vax.
“After being waived by the 49ers during the 1987 season, Francis signed with his old team, the Patriots, before the season’s final game.[13] His second tenure in New England was less successful than his first, however, and he played just one more season. Francis spent 1989 injured before being waived[14] and then retired.”
Wikipedia reference. As I remember Francis was coming off the disabled list and was informed he’d be 2nd string behind John Frank. Instead of being 2nd string on a Super Bowl contender he choose to fly across the country and play for a lousy New England team. Missed out on 2 playoff appearances. He was not into winning.