The vaccine is not without some serious side effects. Because it is a live virus vaccine it cannot be given to those who have inadequate immune systems, (HIV infection, chemotherapy for cancer, certain arthritis drugs, long term steroid use, and pregnancy). It also is not recommended for people with atopic dermatitis or eczema. For some reason they are prone to a widespread skin involvement by the vaccinia virus. These same people are also at risk for complications if exposed to people who have been immunized. They would, however, also benefit from widespread public immunization in that they would be less likely to come into contact with a natural case of smallpox, which would certainly be fatal for them.
Although the rate of side effects with smallpox vaccine is higher than we have become accustomed to with our modern immunizations, the incidence of severe complications is still extremely low, especially when compared to the thirty percent death rate from natural smallpox infection. The two most serious adverse reactions are encephalitis and progressive vaccinia, both of which can result in death. Encephalitis occurs when the cowpox virus infects the brain. It happens in roughly 12 out of one million people who are vaccinated for the first time, and in two out of one million who are vaccinated for a second time. Of those who are unfortunate enough to get this, 15% to 25% die, and another 25% have neurological deficits. Progressive vaccinia occurs when the vaccinia virus causes a serious local reaction at the site of the innoculation. The skin and muscle die, and the reaction can spread, and be potentially fatal. It occurs at a rate of 1 to 2 per one million first time recipients of the vaccine, and 6 to 7 per one million repeat vaccine recipients. It seems to be limited to people with defective cellular immunity, a condition that is not always recognized and is entirely different from the other usual immune deficiencies.
Overall, the CDC estimates that there would be about one death per one million first-time vaccine recipients, and one death per four million people receiving the vaccine for the second time. That means that if we assume the highest fatality rate, we could expect about 290 deaths nationwide if every man, woman, and child in the United States were vaccinated. The actual number would probably be lower since those with contraindications would be excluded from getting the vaccine, and a significant number of us would have the lower mortality rate of second time recipients. Compare that to the fatality rate of having smallpox. Thirty out of every one hundred infected people die. From a general population standpoint, if a city is struck with a smallpox epidemic, and its citizens aren't vaccinated, we can expect to lose five percent of the population, (not everyone exposed would become infected, but a good many of them would.) That would be 5,000 people in a city of 100,000. Expand that to several large cities and the loss would be devastating. The infection rate could well be even higher since those estimates are based on past experience, before the disease was eradicated and when there was more community wide immunity to it. Today, we are more vulnerable. Everyone under the age of 35 has no immunity to it at all.