Hi Dumboiu, always nice to see you here.

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Great questions as usual, but I'm DEFINITELY not a microbiologist. The data which is available suggests that immunization provides complete coverage for 3-5 years, after which immunity "gradually" decreases. Since smallpox was eradicated, it's difficult to really assess how much conferred immunity previous vaccination recipients actually have; obviously, we're not about to attempt research which infects healthy volunteers who were previously immunized!!!!!
As you know,we're talking about a Live vaccine which contains the vaccinia virus. Up to 1 out of every 100 people vaccinated can experience mild symptoms. The CDC estimates that the incidence of death with vaccinations is probably 1 or 2 out of 1,000,000.
Groups who should NOT be vaccinated include:
Pregnant women
Breastfeeding women
Those with impaired immunity (HIV or cancer treatment)
Those with a history of eczema or atopic dermatitis
children under 1 year of age.
That's about all that I know
