evidence: A: An unvaccinated person who is infected with COVID-19 poses a much greater risk to others who are also unvaccinated. But vaccines are not 100% effective, so there is a chance that an unvaccinated person could infect a vaccinated person — particularly the vulnerable, such as elderly and immunocompromised individuals. [...] It’s not the first time that Carlson has cast doubt on the effectiveness of the vaccines. But experts told FactCheck.org that unvaccinated people with COVID-19 are a potential threat to people who have been fully immunized against the disease. [...] In particular, immunocompromised people, who already have moderately to severely weakened immune systems, "are especially vulnerable to COVID-19, and may not build the same level of immunity" from the vaccines compared with people who are not immunocompromised, the CDC says. That’s why the Food and Drug Administration authorized a third dose of the mRNA vaccines for certain immunocompromised people.