Thread by @chrischirp

QUICK THREAD ON VACCINES & DELTA: Friday's @PHE_uk report said that viral load in infected vaccinated people was similar to that of uninfected people.

As has been pointed out by others, this does *not* mean all is lost and vaccines don't work.

10 tweet thread explaining why!

1. when you get a PCR test, they measure how many "cycles" you need before getting a positive signal. The more cycles you need, the weaker the original signal.

Lower numbers for "cycle threshold" (CT value) are thought to indicate higher viral load at the time test was taken.

2. In Friday's report, PHE report that CT values were similar between vaccinated and unvaccinated cases for both Alpha and Delta.

They conclude that once infected there "is limited difference in viral load" bewteen vax & unvax.

So how bad news is this?

3. Firstly, remember that NHS Test & Trace gets PCR cases from mostly *newly* symptomatic people. So this means that in the *early* days of symptomatic infection, a vaxxed person could be as infectious as an unvaxxed person.

BUT

4. Other studies have shown that vaccinated people get over infection more quickly - unsurprising since vaccination basically gives your body a massive head start in fighting Covid.

So vaxxed likely infectious for *less long* than unvaxxed.

medrxiv.org/content/10.110…

5. If this holds, then if you *randomly* sample people at any stage of their infection, you should see that *overall* CT values are lower for vaxxed people than unvaxxed (because green CT values are lower than red ones for most of the PCR detectable time).

6. This is exactly what the REACT study showed which does random testing and so isn't finding people mainly in the first few days of their symptomatic infection. (also includes asymptomatic people... and vaxxed more likely to be asymptomatic?)

https://twitter.com/kallmemeg/status/1423608928051605505?s=20

7. Then of course, you are also much less likely to get syptomatic infection in the first place if you are fully vaccinated - 80% *less* likely according to PHE.

(and much less likely to need hospital if you do get infected).

So what does this mean?

8. It definitely means that vaccinated people who test positive should be isolating during their first week of infection *at least*. Personally I think vaccinated contacts should still isolate for a week - but *at least* test themselves every day for a week.

9. But it also definitely means that vaccination is better than being unvaccinated at preventing infection and then clearing infection if you do still get infected.

So get vaccinated, don't despair but get tested if you get symptoms and isolate if you're positive. /END

PS H/T to @kallmemeg for her tweet thread on this which I've just adapted a bit!

https://twitter.com/kallmemeg/status/1423605470967775234?s=20

sorry - typo! that should say red CT values lower (and so viral load higher) than green CT values.

Note that the Y axis in figure above is opposite direction than the one in earlier PHE graph.