US-based Pfizer and German biotech firm BioNTech have announced the start of a late stage clinical trial of a Covid-19 vaccine jointly developed by the two companies.The Phase 2/3 study will involve up to 30,000 participants between 18 and 85 years of age, the two companies said on Monday.The vaccine candidate, BNT162b2, recently received US Food and Drug Administration (FDA) Fast Track designation, encodes an optimised SARS-CoV-2 full length spike glycoprotein (S), which is the target of virus neutralising antibodies."The Phase 2/3 study protocol follows all the US Food and Drug Administration (FDA) guidance on clinical trial design for COVID-19 vaccine studies," Kathrin Jansen, Senior Vice President and Head of Vaccine Research and Development, Pfizer, said in a statement."Our selection of the BNT162b2 vaccine candidate and its advancement into a Phase 2/3 study are the culmination of an extensive, collaborative and unprecedented R&D program involving Pfizer, BioNTech, clinical investigators, and study participants with a singular focus of developing a safe and effective COVID-19 RNA vaccine."Pfizer and BioNTech last week announced that the US government had agreed to pay $1.95 billion for 100 million doses of a potential Covid-19 vaccine jointly developed by them."The initiation of the Phase 2/3 trial is a major step forward in our progress toward providing a potential vaccine to help fight the ongoing COVID-19 pandemic, and we look forward to generating additional data as the program progresses," said Ugur Sahin, CEO and Co- Founder of BioNTech.During preclinical and clinical studies of four BNT162 RNA vaccine candidates, BNT162b1 and BNT162b2 emerged as strong candidates based on assessments of safety and immune response.Pfizer and BioNTech selected BNT162b2 as the candidate to progress to a Phase 2/3 study based on the totality of available data from its preclinical and clinical studies, including select immune response and tolerability parameters.Monday marked the beginning of another late stage Covid-19 vaccine trial as pharmaceutical company Moderna announced the start of a phase-3 clinical trial to evaluate an investigational vaccine known as mRNA-1273.This trial will also include 30,000 participants.