AstraZeneca admits the best results in its COVID-19 vaccine study came from a dosing error, and experts are raising new questions about the shot

  • AstraZeneca and the University of Oxford are facing criticism over their COVID-19 vaccine trial results after admitting a mistake in the vaccine dosage.
  • During the trial, some patients got two full doses of the vaccine, whilst others had a half-dose in their first shot, followed by a full dose.
  • An AstraZeneca executive on Wednesday described the “mistake” as “serendipity”: The vaccine was 90% effective in the group with the initial half-dosage, compared to 62% effective in the full-dosage group.
  • The head of US Operation Warp Speed said in a call with reporters Tuesday that patients that received the half-dose were under 55 years old, per the New York Times. 
  • Some health experts have expressed concern about the mistake. “Astrazeneca/Oxford get a poor grade for transparency and rigor,” one biostatistician at the University of Florida said.
  • Visit Business Insider’s homepage for more stories.

AstraZeneca and the University of Oxford on Wednesday acknowledged a dosing error in their COVID-19 vaccine trial — and some health experts have said it has eroded their confidence in the shot.

The COVID-19 vaccine is on average 70% effective, according to the latest large-scale trial results announced Monday. The vaccine was found to be 62% effective in those who had two full doses, but rose to 90% effective in those who had a half-dosage in their first shot. 

On Wednesday, AstraZeneca admitted that the half-dosage was initially a “mistake,” Reuters first reported. The plan was for patients in the British trial to receive two full doses of the vaccine one month apart, but some received a half-dose for their first shot instead.

“We went back and checked … and we found out that they had underpredicted the dose of the vaccine by half,” Mene Pangalos, an AstraZeneca executive responsible for the company’s research and development, told Reuters.

The half-dosage appeared to increase the vaccine’s effectiveness, and Pangalos described the error as “serendipity.”

“That, in essence, is how we stumbled upon doing half dose-full dose (group),” Pangalos said. “Yes, it was a mistake.”

The trials included more than 20,000 volunteers in the UK, Brazil, and South Africa.

In a press release on Monday, AstraZeneca said less than 2,800 patients were given the smaller dosing regimen, compared with almost 8,900 patients who received two full doses.

Some experts have raised concerns about the mistake.

On Tuesday, Moncef Slaoui, the head of the US vaccine program, Operation Warp Speed, said in a call with reporters that participants receiving the half-strength initial dose were under 55 years old, per the New York Times

If the half-dosage wasn’t tested in older patients, this raises questions about the efficacy of AstraZeneca’s vaccine, as younger people generally have a stronger immune response than older people to vaccine shots.

In addition, efficacy results were pooled from clinical trials in Britain and Brazil that were designed differently, per the Times.

John LaMattina, a former president of Pfizer’s global research and development unit, said in a tweet it was “hard to believe” the Food and Drug Administration would issue an emergency approval for a vaccine whose most effective dose has only been given to 2,300 people.

John Moore, a professor of microbiology and immunology at Weill Cornell Medical College told the Times: “The press release raised more questions than it answered.”

Natalie Dean, a biostatistician specializing in vaccine study design at the University of Florida, posted on Twitter: “Astrazeneca/Oxford get a poor grade for transparency and rigor when it comes to the vaccine trial results they have reported.”

She questioned how AstraZeneca had monitored the trials. 

“I really don’t know what to make of the results,” Dean wrote in a follow-up tweet. “I’m glad this is not the first vaccine to read out, because it is awfully confusing for experts and non-experts alike.”

Read more: Wall Street analysts lay out how the unexpected pause in AstraZeneca’s coronavirus vaccine trial could slow the race to halt the pandemic

AstraZeneca didn’t immediately respond to a request for comment from Business Insider but a spokeswoman told the Times that the trials “were conducted to the highest standards.”

This isn’t the first hurdle that AstraZeneca and the University of Oxford have faced. Their vaccine trials were paused after an “unexplained illness” in one UK trial participant in September. But they were soon given the green light to restart

In November, drugmakers Pfizer and Moderna announced that trial results suggested their respective vaccines were 95% effective in preventing COVID-19.

Although AstraZeneca’s vaccine came in at 70% effective, manufacturers say it’s considerably cheaper and easier to distribute.

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