Thread by @x2IndSpeculator

A Belgium study of VE vs. infection & onwards transmission of #COVID19, from Jan-Jun 2021; mostly #Alpha, no #Delta.
"VE vs. infection for a fully vaccinated high-risk contact (HRC) & an unvaccinated index was estimated at 74% for BNT162b2 (Pfizer)..."
sciencedirect.com/science/articl…

Other vaccines had fewer events.
"...NO SIGNIFICANT DIFFERENCE between protection by full-dose vaccination & previous infection."
"The VE against onwards transmission was estimated at 62% for BNT162b2... Vaccination with mRNA-vaccines had a SIMILAR EFFECT as previous infection."

It's striking VE vs. onward transmission is lower than vs. infection. Common preconceptions would expect higher, since VE vs. symptomatic #COVID scores better than vs. infection.
This confirms some vaccinated people tested negative but still transmitted further (even pre-#Delta)!

As @nzm8qs immediately noticed that's why some countries are doing repeated rounds of PCR testing & isolating HRCs regardless of test results.
Most countries don't. Instead they insist PCR thresholds are too high & should be lowered to count as a positive.

https://twitter.com/nzm8qs/status/1429060151953166337?s=20

On the contrary.
"...Drancourt & colleagues made an interesting but underestimated observation... they were able to isolate infectious #SARSCoV2 virus from a clinical sample with a low concentration of viral RNA, which is reflected by a PCR Ct-value of 33.
mdpi.com/1999-4915/13/8…

This finding is of foremost importance, because many current hygiene concepts rely on the statistical assumption that the likelihood of infectivity decreases reciprocally to the increase of the Ct-value."
I.e., patients with high Ct-values no longer transmit infectious particles.

All this reminded me of an old @kakape 2015 article.
"The infected chickens were dead in no time, leaving them no chance to spread the disease to their healthy cagemates. But when vaccinated birds were infected with the highly virulent strain,...
sciencemag.org/news/2015/07/c…

they lived longer & all the healthy birds housed with them became infected and died. Thus, vaccination enabled the onward transmission of viruses otherwise too lethal to transmit, putting unvaccinated individuals at great risk of severe disease and death..."

If a rampant #SARS2 spread continues, we the vaccinated might select more virulent variants (because we endure less severe disease & less often die), and then unknowingly, asymptomatically, even while testing negative transmit it to the non-vaccinated.
I guess, we'll know soon.

To summarize. Even if only 1/3 of the vaccinated or naturally infected HRCs are capable of onward transmission (more with #Delta!), it's a huge number that MUST be isolated. Without it, we'll have more people than before participating in the selection & transmission of variants.