Thread by @3DiMMUNE

When the vaccine drops to ~50% efficacy, what does it mean in practice? This is how I interpret it.

when a vaccine is ~50% effective it does not mean "breakthrough" infection will be 50% less severe.

No, it means that out of 100 vaccinated, 50 will be protected & 50 will not

The problem with such efficacy is that one cannot know in which "50" group one is: protected group or unprotected group.

In practice, it means that when one is vaccinated but the vaccine drops to ~50% efficacy, a given individual can be 100% protected or 0% protected.

Now, let's analyze what we know about current vaccines. Multiple studies reported that by ~ 6 months post 2-dose vaccination (and starting at 3 months) vaccine efficacy drops and reaches below 50%

nejm.org/doi/full/10.10…

So, at ~6 months, an individual with 2-dose vaccination may be 100% protected or 0% protected but no way to tell in which group one ends up. This applies to both documented infections and hospitalizations.

Now comes 3rd dose. What is its efficacy? It is also less than 50%.

What does it mean? It means that ~ 6 months, half of the 2-dose vaccinated people will be completely unprotected and after 3rd dose, again, half of them will be or stay unprotected.

medrxiv.org/content/10.110…

The most difficult part is that no way to tell if one is in a protected or unprotected group after 6 months or after 3rd dose. It's the toss of the dice if a given individual is protected or not when the vaccine shows 50% or less efficacy (after 3rd dose).

So, what can be done? Right now, the vaccines we have provide only a short window of protection (~3 months) and it appears 3rd dose effect is random (50/50). Unless something changes, think of #Mask and #SocialDistancing as something absolutely essential.