Thread by @erlichya

It might be that we are back to square one (March 2020) in Israel in terms of prevention of transmission despite the impressive vaccine operation.

A thread 🧵
[translated from my Hebrew thread due to the popular demand]
👇👇👇

Increasing lines of evidence from Israel suggest a massive drop in the effectiveness of the Pfizer vaccine to prevent *transmission* of Delta. Recent numbers suggest ~60% compared to ~90% against previous VoC.

https://twitter.com/Nadav_Eyal/status/1417396568353824786

What is the blended protecting for the Israeli population?
Let's consider three groups: no vaccine (0% effectiveness), vaccinated (60%), recovered (100%, which is a bit of an overestimation).

The size of each group is from here (MoH data):

Let's translate the bottom line:
5.7M people who are vaccinated
540K who recovered
3M people who are not vaccinated (many of which <20y/o)

👉So the population average of the effectiveness against transmission = 42% (compared to March 2020)

Now there are CAVEATS in my analysis: it assumes that all age groups transmit and susceptible at the same level (beyond vaccination status) etc. But it gives us a ballpark number about the protection dynamics.

So isn't Israel in a better shape than March 2020 with respect to transmission?

No! The virus also evolved since March 20. Delta is around 1.6x-2x more transmissible than Wuhan. For the sake of the analysis let's assume it is 1.8x.

sciencenews.org/article/corona…

So extra transmissibility of Delta x (1-population blended effectiveness) =
1.8 * (1 - 0.42) =
1.02
😮!

The bottom line is that according this crude analysis that the gains by the vaccines for transmission reduction were eroded by viral evolution.
Hello March 2020.

Of course, the biggest difference from March 2020 is that the vaccine protects quite well against severe disease. But note that effectiveness is just the extent of risk reduction. With 90% effectiveness, 5000 cases/day are simply like 500 cases/day without a vaccine.

So the important lesson is that if you want to control a pandemic, you need to control the transmission.

[that does not mean lockdowns per-se, but prioritizing vaccines, updates, and prophylaxes in general that help control transmission]