My 11.5 month old had suspected german measles. is it important to confirm this diagnosis via blood test or of no relevance now its over?

Define : In the US it is considered a reportable illness, with documentation and follow up studies required as part of the regional/national efforts at infection control. You find and eliminate outbreaks when you keep track of the cases & investigate where they started. It is a rather minor illness in most, but deadly to the fetus when it infects a pregnant woman.