COVID-19 vaccine developed by China's Sinovac could undermine confidence in the shot, experts warned, as several countries prepare for mass inoculations with the vaccine.Turkish researchers said on Thursday Sinovac's CoronaVac was 91.25% effective based on an interim analysis only to see a confusing readout the same day from Brazil which said the vaccine's efficacy was between 50% and 90%.Brazil has also delayed release of its efficacy data on the vaccine three times, as Sinovac seeks to consolidate data from global trials that include Indonesia, Turkey and Chile.The efficacy data is closely watched as many developing countries have signed deals to use the Sinovac vaccine which is easier to store and transport than rival shots developed by.