Rubella???

i got 15 months old son, he have completed his MMR shots. 5 days ago he experienced cough, colds and intermittent fever...2 days ago i'v seen a maculo papular rash at his neck spreadind to his back and chest and resembles a viral exantem rash.... is it possible for him to catch up german measles?

it couldn't be an allergic reaction though... he got the last shot a couple of months ago.. couldn't be a delay reaction

Here're the signs and symptoms of rubella:

Often the signs and symptoms of rubella are so mild that they're difficult to notice, especially in children. If signs and symptoms do occur, they generally appear between two and three weeks after exposure to the virus. They typically last about two to three days and may include:

Mild fever
Headache
Stuffy or runny nose
Inflamed, red eyes
Enlarged, tender lymph nodes at the base of the skull, the back of the neck and behind the ears
A fine, pink rash that begins on the face and quickly spreads to the trunk and then the arms and legs, before disappearing in the same sequence
Aching joints, especially in young women

Click on the link below for more on rubella

If he just had his MMR, which covers Rubella (German measles) then it is probably an allergic reaction to the serum used in the vaccine. Take him in for an allergy test to test him against the components of the vaccine. He could be allergic to it and needs a substitute for the illnesses.

It's unlikely that he would get rubella or measles unless he got the vaccine less than 2 weeks ago. Roseola could be, it's hard to differentiate the rash from rubella but when the rash appears usually the fever goes away.
There are many other viral diseases that appear with rash. Enterovirus as echo or coxsackie virus could do it. If the fever pesists or if there are other symptoms like cyanosis(bluish discoloration), shortness of breath, persistent cough, etc. you should take him to the pediatrician ASAP. Most likely is an non-specific viral illness.
Hope he gets better real soon.