What is Going on with the Astra-Zeneca Vaccine’s Strange Efficacy Results?

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Photo by CDC on Unsplash

Okay, so, we have preliminary efficacy results on the Astra-Zeneca vaccine…and scientists are baffled.

You have probably seen both 70% and 90% figures circulating, and there’s a reason for that.

The trials tried two different regimens. One was two full doses of the vaccine, the other was a half dose followed by a full dose.

So, guess which one is 90% effective? Yeah, I led you with this one.

The Half Dose Works Better

In completely counterintuitive results, a much higher efficacy was obtained when the first dose was half the amount of the second one.

This virus is not a mRNA vaccine, but a viral vector vaccine. It’s made by modifying a chimp adenovirus…essentially a chimpanzee cold. (The Chinese have been using a human adenovirus as a vector, but this is generally considered more risky as prior immunity to the adenovirus could diminish the efficiency of the vaccine).

The actual results were 62% efficacy (about at the higher end of the flu shot) for the full first dose and 90% for the half first dose.

Now scientists are scrambling to work out why. Is it an artifact of the data? (if so, we’ll know and probably end up with the 70% average, which is less effective than the mRNA vaccines, but also cheaper).

Or are we seeing something real that might impact how we do vaccination series in the future?

Image for post
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Photo by CDC on Unsplash

There Are Two Theories

If the efficacy data turns out to be real, then there are two theories as to why this might be happening:

  1. Our immune systems just like it better. The first dose kind of wakes our immune system up and then the second goes “Hey, look at this.” This might produce better T-cell immunity.
  2. It has to do with the viral vector. In this theory, the full dose is blunting the reaction to the spike protein in favor of one to the viral vector. This could potentially be tested by looking at antibody responses. And there’s some evidence that it happens in mice.

If the data is real and #1 proves to be the hypothesis, though, this might be worth looking at with other multi-dose vaccines. Does cutting the dose for the first shot actually help?

We’ll have to see what happens when full data is revealed, but it’s quite fascinating.

Immunology is weird.

Written by

Freelance writer, freelance editor, novelist and short story writer. Jack of many trades. https://www.jenniferrpovey.com/

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