All vaccine used for routine immunization are very effective in preventing disease, although no vaccine attains 100% effectiveness. More than one dose of a vaccine is generally given to increase the chance of developing immunity.
Vaccine are very safe, and side effects are minor- especially when compared with the disease they are designed to prevent. Serious complications occur rarely.
For example, severe allergic reactions result at a rate of one for every 100,000 doses of measles vaccine. Two to four cases of vaccine-associated paralytic polio have been reported for every one million children receiving oral polio vaccine.