The weakened viruses only cause infection in the cooler temperatures found in the nose.

The vaccine that helped eradicate smallpox used a weakened virus.

The nasal flu vaccine, FluMist, is made with weakened live virus.

Many human vaccines are derived from weakened live viruses.

In these people, the weakened virus can cause the original disease.

(injection of some dead or weakened virus, or bacteria that causes the disease).

LAIV is made from weakened virus and does not cause influenza.

The weakened live virus reproduces inside the person's nose and produces viruses that the immune system learns to attack.

"They say they're worried about the autism link, or that it's just not natural to inject weakened viruses into their kids."

Also, both are oral vaccines using weakened live virus, like the polio vaccine used in poor countries.