A man walking past an AstraZeneca sign in Macclesfield, England. Phil Noble/Reuters The UK pharmaceutical company AstraZeneca is likely to run a second global trial to assess its COVID-19 vaccine's efficacy, its CEO told Bloomberg News on Thursday. AstraZeneca and the University of Oxford announced Monday that preliminary results indicated their two-dose vaccine could be up to 90% effective at preventing COVID-19. But the team later said an error in the trial left some participants with half-doses instead of full doses. Experts said that error cast doubt on the validity of the efficacy rate and warranted further study. Visit Business Insider's homepage for more stories. Experts' trust in the validity of AstraZeneca's reported COVID-19 vaccine data has quickly eroded.AstraZeneca and the University of Oxford said on Monday that their coronavirus vaccine was found to be up to 90% effective based on preliminary results from studies of 23,000 volunteers in Brazil and the UK.But reports of an error during its initial trial prompted a barrage of questions.On Thursday, the pharmaceutical company's CEO, Pascal Soriot, acknowledged the concerns and said it would likely conduct a second trial of the two-dose vaccine, Bloomberg News first reported Thursday."Now that we've found what looks like a better efficacy we have to validate this, so we need to do an additional study," Soriot told Bloomberg News.The error meant about 2,700 participants got 1 1/2 vaccine doses instead of the intended two.