𝐔𝐧𝐝𝐞𝐫𝐬𝐭𝐚𝐧𝐝𝐢𝐧𝐠 𝐓𝐡𝐞 𝐃𝐚𝐭𝐚

AsᴛʀᴀZᴇɴᴇᴄᴀ (AZ)

Disclaimer: I assembled this thread to help me grasp covid data. I’m not a virologist so there are probably errors. Please feel free to correct me. Accurate information is important.

AZ was trialed under different conditions–
1) two Standard Doses, 28 days apart (SD/SD trial)
2) a half-dose (Low Dose), then a Standard Dose (LD/SD trial)
3) some participants had a single dose
4) some had dose #2 after more than 28 days


Overall, there were…
• Zero COVID-19 hospital admissions in the groups getting the AZ vaccine.
• The unvaccinated group had ten hospital admissions: two of which were severe.

Some acronyms…
• SD = standard dose
• LD = low dose
• AR = disease attack rate
• ARU = attack rate, unvaccinated group
• ARV = attack rate, vaccinated group
• RR = risk ratio

Risk Ratio is what determines Efficacy.

RR is calculated using the number of participants showing covid symptoms in the vaccinated and unvaccinated groups.



Efficacy is not the same as “Effectiveness”.

• Efficacy = 1 – RR
• Efficacy % = (1 – RR) x 100

In one SD/SD trial…
• 27 of 4,440 (0.6081%) developed covid symptoms: vaccinated group
• 71 of 4,455 (1.5934%) developed symptoms: unvaccinated group

Using the numbers in the previous tweet…

• RR = (0.6081) ÷ (1.5934) = 0.3816
• RR % = RR x 100 = 38.16%

• Efficacy = 1 – 0.3816 = 0.6184
• Efficacy % = (1 – 0.3816) x 100 = 61.84%

• this is where the much discussed 62% comes from.

So what does “Efficacy = 62%” mean?

A vaccine efficacy of 62% means there was a 62% reduction in the number of cases expected had the participants not been vaccinated.


The sample group with 62% efficacy had 8,895 total participants. In the vaccinated group, 27 developed covid symptoms while 71 of the unvaccinated developed symptoms.

71 ÷ 27 = 2.6

Thus there's 2.6 times more chance of developing covid symptoms when unvaccinated.

Things to note about Efficacy.

• It is determined using ideal conditions
• It is a short-term assessment and only applies to the time-length of the trial

On the trial's results…
• AZ reduced symptomatic covid cases by 62%
• AZ reduced covid's severity
• No one died from covid when vaccinated
• Unvaccinated people were 2.6 times more likely to develop covid symptoms than vaccinated people

Addendum 1: It's worth noting that in the "low dose/standard dose" (LD/SD) group of 2,741 participants, AZ had 90% efficacy. This is currently being researched further because the group was not large enough for substantial confidence in the results.
Addendum 2: While AstraZeneca isn't perfect, I see many positives about being vaccinated with it. I'll be using it. To me, it's another "Personal Responsibilities" slice of swiss cheese in the model @MackayIM put together to explain protecting oneself against covid.

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