Thread by @NGKabra

If, like me, you're confused about the Oxford/AstraZeneca (Pune-walla!) vaccine, which is 70% effective, but also 90% effective, then here is an explanation. 🧵

The Oxford vaccine requires two shots one month apart. The original design called for two equal shots. However, someone made a mistake and a small fraction of the volunteers got only half a dose in the first shot, and the full dose in the second shot.
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Later, analysis of the data showed—to everybody's surprise—that the ½+1 dose regimen was 90% effective and the standard 1+1 dose regimen was only 66% effective.

The average of these two regimens across all volunteers is 70%, and that is what has to be reported 🧵

Why don't they ignore the standard 1+1 dose data, focus only on the ½+1 data, and report 90% effectiveness.

For complex reasons out of scope for this thread, that is considered cheating, so they *have* to report the overall figure of 70% 🧵

But does it make sense that a lower dose is more effective than a higher dose?

Nobody knows for sure. It could be a statistical problem. Or there can be a perfectly good biological explanation.
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This vaccine consists of a chimp adenovirus modified to carry pieces of SARS-CoV-2. It is possible that our immune system reacts to the adenovirus and kicks it out before the pieces of SARS-CoV-2 are exposed to the immune system. So no anti-SARS-CoV-2 antibodies are produced.
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And so, it's possible that a lower first dose gives our immune system a chance to get to know the pieces of SARS-CoV-2 that are inside the adenovirus, and produce strong anti-SARS-CoV-2 antibodies.

But that's just a theory. We can't know for sure without additional testing.
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But, if the Pfizer and Moderna vaccines are 90% and 94% effective, why are we getting excited about a 70% effective vaccine that might, at best, be 90%?

Because of cost and temperature.
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The Pfizer vaccine requires storage at -70°C. Moderna can stay for a month in the fridge (-2 to -8 °C) but needs -20°C for longer periods

The Oxford vaccine can be stored for 6 months at fridge temperatures. Much more practical, especially for third-world countries.

Remember, all this is preliminary data. These vaccines have only been shown to provide protection for a few weeks after the second dose.

Whether they protect us for 1 or 3 or 6 months, 1 year, or a lifetime isn't known. The answers to those questions will decide our future.

And, as @iamsarnaik points out, the Pfizer/Moderna vaccines use mRNA, which has never been used in an actual vaccine.

Oxford's adenovirus vaccine technique is tested on many more humans and used widely in a non-human rabies vaccine

https://twitter.com/iamsarnaik/status/1331094395597123584

Also, remember that all 3 vaccines have so far only published press releases. We have to wait for the actual data and papers to be published and studied by outside experts for a better understanding.

Research by press release is fraught with problems *cough*Biocon*cough*.

And, it appears that the Oxford AstraZeneca vaccine results have more than usual statistical gymnastics, so seeing the full data and expert opinions is even more important (especially since that's the vaccine poorer countries are most likely to get):

https://twitter.com/zeynep/status/1331658933082198019