Evidence from the first study of long covid in children suggests that more than half of children aged between 6 and 16 years old who contract the virus have at least one symptom lasting more than 120 days, with 42.6 per cent impaired by these symptoms during daily activities. These interim results are based on periodic assessments of 129 children in Italy who were diagnosed with covid-19 between March and November 2020 at the Gemelli University Hospital in Rome.. [link to www.ncbi.nlm.nih.gov (secure)]

I believe the first Covid vaccine was given around the middle of December 2020.