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A dosing error has put a spotlight on the AstraZeneca COVID-19 vaccine trial. Nathan Stirk/Getty Images
  • AstraZeneca announced that its vaccine is 70 percent effective, but a major dosing error during the trials may have affected the overall efficacy.
  • Some clinical trial participants were mistakenly given half a dose rather than a full dose in their first round of shots.
  • Researchers discovered that those who were given the weaker dosage produced a better immune response.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date.

AstraZeneca’s COVID-19 vaccine is thought to be one of the biggest game changers when it comes to getting a handle on the pandemic.

Unlike some other vaccines, AstraZeneca’s vaccine, which is given in two doses a month apart, doesn’t need to be stored in super-cold temperatures.

Each dose is significantly cheaper than the other shots and will be easier to distribute to rural areas and developing countries.

Recently, AstraZeneca announced its vaccine is about 70 percent effective at preventing COVID-19.

But a closer look at the clinical trials shows the efficacy isn’t as clear-cut as the pharmaceutical initially stated.

During the clinical trials, some participants were mistakenly given half a dose rather than a full dose in their first round of shots, according to BBC News. Still, the trial continued and the researchers discovered those given the weaker dosage produced a better immune response.

Here’s what experts make of AstraZeneca’s vaccine trials.

When regulators were informed of the mistake — that nearly 3,000 participants had received a lower dose — they permitted the trial to continue, as the dosing error had no impact on the vaccine’s safety.

In fact, the dosing mistake led to an interesting discovery.

In the participants who received two full doses, the vaccine efficacy was 62 percent. But in those who received a half dose and a full dose, efficacy was close to 90 percent.

“It’s a happy coincidence that they sort of stumbled onto a way that seems to be a better way for the human immune system to receive that particle vaccine, process it, and prepare for a longer-lasting immunity,” said Dr. Matthew Heinz, a hospitalist based in Tucson, Arizona, noting that the findings aren’t yet conclusive.

Averaged together, AstraZeneca announced via a press release its vaccine is 70 percent effective at preventing COVID-19.

The researchers are investigating why the weaker dose produced a more robust immune response.

Some experts have criticized the fact that AstraZeneca combined the efficacy results from what’s essentially two different trials and say the company will need to conduct another trial properly evaluating the effectiveness of the half-dose full-dose regimen.

“Small mistakes are common, but giving thousands of participants the wrong dose unintentionally is not a common mistake. Time will tell whether this particular mistake leads to a discovery, but at this point in time there is a lot of uncertainty around the findings,” said Dr. Philip Smith, an assistant professor in the department of kinesiology and health at Miami University in Ohio.

Smith, whose research focuses on public health and health policy, believes AstraZeneca will seek approval for the full dose, which is 62 percent effective.

The big question is whether regulators will trust the 62 percent, considering the sample size was smaller than intended, said Smith.

AstraZeneca plans to publish the full, final findings in a medical journal. Regulators who approve the vaccine will have access to this data and will make the final call if the evidence is sufficient.

Results from a phase 2 study previously published in The Lancet showed the full two-dose vaccine produced a strong immune response in participants of all ages.

“Publishing the full report will hopefully help clarify the situation so that a broad scientific community of experts can weigh in based on complete information,” Smith said.

There have been two adverse reactions publicly reported during AstraZeneca’s trials.

The first took place in September in the United Kingdom and caused trials to pause temporarily across multiple countries.

A few days later the trials resumed. No further details have been shared about the case, so much of what occurred remains unclear.

“I understand if the pause causes public concern, but in my opinion the pause means that regulators are doing their job,” said Smith.

In addition, in mid-October, a man involved in a trial held in India claimed he experienced neurological and psychological side effects after receiving the vaccine, according to news reports.

India’s medical research regulator, Indian Council of Medical Research, told Reuters the event was no cause for concern and that the trial wouldn’t be paused.

According to Heinz, it’s difficult to tell if there’s a causal relationship here — meaning if the vaccine caused the neuropsychiatric symptoms — or if it’s a coincidence.

There’s also a chance the participant received the placebo, but there’s no way for the public to know since it’s a double-blinded trial, said Heinz.

“With the thousands or tens of thousands that are enrolled in vaccine trials, it is probable that someone gets sick while they are participating, and the regulators need to do their due diligence to investigate whether the illness is related to the vaccine,” said Smith.

Heinz said with a vaccine like this, there’s no room for error.

“When you’re going to be giving something to potentially billions of people, you can’t even have a fraction of a percent of people having an adverse event of any kind. There is truly no room for a significant error at all,” Heinz said.

AstraZeneca will need to look into the case, see if anyone else experienced similar symptoms, and follow the participant’s health closely to determine if his neurological and psychological symptoms could be linked to the vaccine.

The regulators who oversee the vaccine’s trial will review the data and make a decision in the coming weeks.

Smith said he’s less concerned about the adverse reactions than he is about the small sample size, considering the vaccine is going to be administered to millions, potentially billions, of people if approved.

“I think we need to trust the regulators, but we will not fully know about the safety of the vaccine until there is widespread use by general populations,” said Smith.

AstraZeneca recently announced that its vaccine is 70 percent effective, but a major dosing error that occurred during the trials may have impacted the overall efficacy.

Some participants accidentally received a weakened dose, but ended up producing a more robust immune response.

Though the mistake led to an important discovery, some health experts would like to see AstraZeneca and regulators further investigate the dosing mishap to determine the vaccine’s true safety and efficacy.