LONDON : Health chiefs in the UK on Thursday began what they have described as a world-first clinical trial of patients who will receive different COVID-19 vaccines for their first or second dose, to detect its impact on immunity against coronavirus.The study, backed by 7 million pound of government funding, will determine the effects of using different vaccines for the first and second dose – for example, using Oxford University/AstraZeneca’s vaccine for the first dose, followed by Pfizer/BioNTech’s vaccine for the second.It is being run by the National Immunisation Schedule Evaluation Consortium (NISEC) across eight National Institute for Health Research (NIHR) supported sites and also gather immunological evidence on different intervals.