Thread by @BoomerangChris

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.

[cont’d…]

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

[cont’d…]

thelancet.com/journals/lance…

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.

[cont’d…]

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

[cont’d…]

Risk Ratio is what determines Efficacy.

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

RR = ARV ÷ ARU

scroll.in/article/979627…

[cont’d…]

Efficacy is not the same as “Effectiveness”.

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

Example
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

[cont’d…]

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.

[cont’d…]

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.

miamiherald.com/news/coronavir…

[cont’d…]

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.

[cont’d…]

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

[cont’d…]

𝐒𝐔𝐌𝐌𝐀𝐑𝐘
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

[end]

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.