Can vaccinated people 'shed' the vaccine onto others?
Here is what the checked sources say. Each title links to the original.
No Scientific Basis for Vaccine 'Shedding' Claims
FactCheck.org (May 2021) explains that shedding means a vaccinated person releasing a live vaccine virus that can reproduce. It happens with some live vaccines: oral polio vaccine (no longer used in the US), rotavirus vaccine (rarely), and commonly the nasal-spray flu vaccine. COVID-19 vaccines contain no live virus: mRNA vaccines instruct cells to make spike protein, and the Johnson & Johnson adenovirus cannot replicate, so FactCheck finds no biological path for shedding them. Vaccine expert Paul Offit is quoted saying spike protein and antibodies are not shed.
Transmission of Vaccine-Strain Varicella-Zoster Virus: A Systematic Review
Reviewed published reports, through 2018, of the live chickenpox vaccine virus spreading from vaccinated people. Across 20 articles, 13 people caught it from 11 healthy vaccine recipients, mostly household contacts. In every reported case the vaccinated person had a rash, and the resulting illnesses were mild. The authors conclude healthy vaccinated people have minimal risk of spreading the vaccine virus, and only if a rash is present; they note other cases likely went unpublished. One author disclosed a contract with Merck, which makes the vaccine.
Detection of Messenger RNA COVID-19 Vaccines in Human Breast Milk
Tested 131 breast milk samples from 11 mothers after mRNA COVID-19 vaccination. Measured: trace vaccine mRNA (about 1 to 12 picograms per mL of whole milk) in 7 samples from 5 mothers, 1 to 45 hours after the shot, and none after 48 hours. Not measured: whether the mRNA was active, or any effect on babies. The authors' interpretation, not tested here, is that breastfeeding after vaccination is safe, with caution for babies under 6 months in the first 48 hours. They note the small sample.