The Health Ministry is studying whether Sinovac vaccine recipients who had been jabbed with a Pfizer booster would need a fourth dose to increase their protection against the highly infectious Covid-19 Omicron variant.br br Health Minister Khairy Jamaluddin said the ministry had taken note of a recent study which found that Sinovac's two-dose Covid-19 vaccine followed by a booster Pfizer shot was less effective in curbing the spread of the Omicron variant.