Possible Vaccine Reactions About 20 percent of those who receive their first dose of MMR vaccine may get a rash, slight fever or swelling of the neck glands within one or two weeks of vaccination. This may last for a few days. Occasionally there is mild swelling of the salivary glands. About 5 percent of children will have some aching or swelling of the joints lasting two or three days. This may occur from two to ten weeks after the shot.
Reactions are much less common after the second shot.
Uncommon side effects such as pain, numbness or tingling in the hands and feet may also occur and then disappear. However, the risk of encephalitis from the vaccine is 1,000 times less than the possible risk of encephalitis from the actual measles disease. Women in their child bearing years, particularly those over 15 years of age, may have joint pains, and sometimes arthritis, but these problems occur four times less often with the vaccine than with the actual rubella disease. Occasionally a complication called thrombocytopenia occurs about 1 week after the shot. Blood cells that normally help the blood to clot can be reduced.