Washington: US biotech company Moderna announced on Wednesday that it has begun clinical trials of a booster dose of vaccine designed specifically to combat the Omicron variant of Covid-19. The trials will involve a total of 600 adults — half of whom have already received two doses of Moderna’s vaccine at least six months ago, and half of whom have received two doses plus the previously authorized booster dose. The booster specifically targeting Omicron will therefore be evaluated as both a third and a fourth dose.