https://www.ghsa.org/sites/default/files/2016-11/RearBelts_FINAL.pdf.pdf
https://www.nhtsa.gov/risky-driving/seat-belts
Here's some research into seatbelt use. Pages 12 and 13 of the first show that back-seat occupants were 3 times less likely to die or be injured if using a seatbelt.
Also, wearing a seatbelt reduces your likelihood of serious or fatal injury by 45-60% (depending on the type of vehicle).
The death rate of Covid 19 is roughly 1.2%, with 80% of deaths being people over 65.
People have a death rate of about 2% at 65-74, 5% at 75-84, and 15% 85+. Covid-19 killed approximately 800k of 55 million people over 65. That's a rate of about 1.45%. That doesn't even consider effects of a weak immune system, obesity, other health problems, and the fact that many "Covid deaths" were actually just people who died of other causes while they had Covid.
The death rate under 65 was about 0.07% total, or 0.36% of people who had Covid (roughly 20000 deaths out of 275,000,000 or 56,000,000 people, respectively).
In short, there is a substantial chance that wearing a seatbelt will save your life or decrease injury. The vaccine doesn't provide nearly that much defense. It's also a rushed experiment that hasn't been researched and tested as long as most vaccines. I'm not against people getting it, but I don't need it.
Also, wearing a sling of leather over you doesn't change your body. The vaccine does. I'd like to keep my genes the way they are, thanks.