Q: If an unvaccinated person gets blood from a vaccinated person, will the vaccine transfer?
A: No. If an unvaccinated person gets a blood transfusion from a vaccinated donor, the unvaccinated person does not become vaccinated. That’s true for COVID-19 vaccines and all other vaccines, too. Not even if the blood donation happens right after the vaccine is given. Receiving a blood transfusion from someone who has had COVID-19 is also not at all like vaccination.
A vaccine contains extremely small amounts of both active & inactive ingredients which never enter our bloodstream in the same way as other medications like pain relievers, antibiotics, or antidepressants.
All the action comes from your own immune system. 
Your body makes antibodies in response to the very tiny amount of active ingredient at the site of the vaccine. Within a few days, the vaccine ingredients themselves break down at the site of the injection.
Your new antibodies do circulate in your blood–but in whole blood, they’re at such a low concentration that it likely has no impact on the person who receives your donation.
For convalescent plasma therapy, plasma is collected from someone who is recovering from COVID-19. This concentrates the antibodies (which are in plasma) and could help someone who is fighting COVID-19 themselves (if it’s given early and the donor has a lot of antibodies; see JHU link below).
Our immune system cannot make more antibodies in response to the introduction of antibodies from another source, whether it’s donated blood or antibody therapy. (Vaccines contain anti*gens* which stimulate the immune system, not anti*bodies* which is what the immune system makes; similar words but not the same).
So when a blood transfusion patient gets a few of your antibodies, the recipient’s immune cells will not remember or replicate them. The antibodies will just disintegrate after a while and get cleaned up by the body’s waste mechanisms, like all the rest of the cells in that transfusion–and the vaccine ingredients themselves.
In fact, you can donate blood immediately after you’ve received a COVID-19 vaccination, as long as you meet two criteria:
You’re feeling good.
The vaccine you got was from any one of the common manufacturers (Pfizer, Moderna, J&J, Novavax, and AstraZeneca are all 
in the U.S., and many others are in other places too).
Some other vaccines do have a cooldown of a couple of weeks before you can donate blood. Generally, those ones contain a live but weakened virus, which can cause a mild infection. The infection could (theoretically) be passed on to the transfusion recipient. In an immunocompromised patient receiving the donation, we don’t want to introduce infections–even mild ones. The live attenuated vaccine you’re most likely to encounter if you’re old enough to donate blood is the 1-dose shingles vaccine called Zostavax from Merck. You have to wait 4 weeks to donate blood after you get it.
There are currently no live attenuated COVID-19 vaccines, though one called COVI-VAC is in Phase 1 trials in the UK right now.
The American Red Cross declared the first-ever national blood crisis in January, and desperately needs donations. Whole blood donations from people who are O+ or O- and platelet donations are especially needed.
Thanks for the question, Cindy from New Lothrop, MI!
Here’s our recent post about the blood crisis.
And some links for further reading:
