I wasn't aware that the under 5 trial had failed, we are currently deciding if we will give it to our 6 year old. From a biologial/science perspective can someo

I wasn't aware that the under 5 trial had failed, we are currently deciding if we will give it to our 6 year old. From a biologial/science perspective can someone here explain why might the vaccine work in a 5 year old and not in a 3/4 year old?