evidence: Some Vaccine Shedding Is Expected [...] Shedding is common after receiving a live attenuated influenza vaccine, according to the CDC, especially among younger people. Shedding is not the same as transmission, since transmission requires a larger amount of virus. Transmission of shed influenza vaccine viruses from vaccinated to unvaccinated people has been documented, according to the agency, "but has not been reported to be associated with serious illness," the CDC explains. Close contacts of immunocompromised people can receive the live attenuated influenza vaccine, unless the person with immunocompetence is in a protective environment, according to the agency. [...] But sometimes the vaccine virus can change back into a more dangerous virus that can cause paralysis. This happens when the virus accumulates mutations after circulating for a long time in populations with low rates of vaccination or in immunocompromised people. This strain of vaccine-derived poliovirus can shed and infect others, putting unvaccinated people at risk of getting polio.