Under intense criticism for his statement that convalescent plasma donated by former coronavirus patients could reduce COVID-19 deaths by 35%, FDA commissioner Stephen Hahn apologized on Twitter late Monday.

The agency announced on Sunday that it had granted an emergency use authorization (EUA) for plasma donated by coronavirus survivors as a treatment for new patients. President Trump, Hahn and U.S. Health and Human Services secretary Alex Azar all touted the 35% figure based on a small subgroup of hospitalized COVID-19 participants in a Mayo Clinic study — patients who were younger than 80 years old, not on ventilators and who received plasma that contained high levels of antibodies within 3 days of diagnosis, according to a report by the New York Times.

A World Health Organization (WHO) official on Monday cautioned that the use of convalescent plasma to treat coronavirus patients is still experimental, according to the Associated Press.

“The results are not conclusive,” WHO’s chief scientist Dr. Soumya Swaminathan said during a press briefing, according to the news agency. “At the moment, it’s still very low-quality evidence.”

Other scientists, including one who worked on the Mayo Clinic study, wondered about the origin of the statistic, the New York Times reported, as it was not mentioned in the EUA, an FDA memo or a Mayo Clinic analysis. The FDA’s request for the EUA does not contain the 35% figure, but rather a statement about the adjusted 30-day mortality level observed in patients who the high-antibody-level transfusion within 3 days of diagnosis.

“You need to correct the 35 lives saved per 100 sick with covid-19 so people understand that was absolutely wrong, Steve,” tweeted Dr. Eric Topol, a noted cardiologist, author and founder of the Scripps Research Translational Institute. “That there is no evidence to support that. That there is no evidence at this juncture to support *any* survival benefit. That we need RCTs (randomized controlled trials).”

On Monday night, Hahn wrote a series of tweets apologizing for the error and answering critics who accused the FDA of bowing to political pressure from the Trump administration.

“I have been criticized for remarks I made Sunday night about the benefits of convalescent plasma,” Hahn tweeted. “The criticism is entirely justified. What I should have said better is that the data show a relative risk reduction not an absolute risk reduction.”

“We at FDA do not permit politics to enter into our scientific decisions,” he added. “This happens to be a political season but FDA will remain data driven. On behalf of FDA‘s 18,000 career employees, I want to reassure the American public about this commitment.”

Hahn has been under increasing pressure from Trump, including the president’s insistence that the FDA hurry up and approve a vaccine for COVID-19. Earlier in August, Trump complimented the FDA commissioner for “really speeding up the process of therapeutics and vaccines.”

“It’s very important, Stephen,” Trump said. “Can you move it faster, please, OK? Thank you, great job.”