Pfizer's trial looked at antibody levels in the blood of primary school-aged children to estimate their immune response, unlike larger trials of older participants which compared Covid cases in the vaccine and control groups. A spokesperson said the company may later disclose vaccine efficacy from the trial but there had not been enough infections yet among the volunteers. Pfizer is also trialling its vaccines on babies as young as six months and expects the results of that study to be available by winter, in a move likely to cause international controversy. While the US is expected to open the vaccination programme to younger groups, at this stage the jab is unlikely to see the same sweeping approval in the UK, which only today started immunising healthy children aged 12 and above. Read More: Is a Covid jab for FIVE-year-olds next? Pfizer says its vaccine is safe for young children and plans to get it approved 'as soon as possible'