A low dose of the Pfizer-BioNTech COVID-19 vaccine didn’t produce a significant immune response among children between the ages of 2 and 5, the two firms said on Friday. Amid clinical trials that are ongoing, the two drugmakers tested 3 micrograms of the vaccine in kids aged 6 months to less than 5 years of age. Children between 6 months and 2 years produced an immune response comparable to individuals aged 16 to 25 years after two doses of the mRNA vaccine, the companies said. However, the vaccine didn’t produce a significant immune response after the same dose regimen among children between the ages of 2 and 4, according to the companies. Pfizer and BioNTech said they will change their plans and will test three doses of their COVID-19 vaccine on children in that age group rather than the usual two doses. Should the three-dose study prove successful, the companies said …