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When will there be enough evidence that vaccines reduce asymptomatic carriage (e.g. risk of transmission) to be able to change guidelines for vaccinated people not being a risk to unvaccinated? Also, how should vaccinated people be tested (if at all?) given PCR sensitivity issue?
As we discussed before, this Singapore study is the most compelling I've seen that asymptomatic infection less likely to transmit (4 fold) than symptomatic infection: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)32651-9/fulltext
And this study from Catalonia, Spain also told us that viral load of index cases is the most predictive factor of spreading the virus to others https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30985-3/fulltext
Now combine this with the fact that 2 studies on Feb 8 demonstrated that nasal viral load massively reduced after vaccination-here is 1st from Pfizer in Israel where >75% of those >60 yrs were >14 days post-first dose compared to 25% of those 40-60 yrs https://www.medrxiv.org/content/10.1101/2021.02.08.21251329v1
CT (cycle threshold) of 16,297 positive qPCR tests reported from Dec 1st to Jan 31st in these 2 age groups (lab didn't know vaccination status). Hypothesis- if vaccines reduce
viral load, difference in CT values between the two age group should be seen in late Jan but not before.
viral load, difference in CT values between the two age group should be seen in late Jan but not before.
Consistent with hypothesis no difference in CT values in two age groups before Jan 15th - after that, stark difference, much higher CT values (lower viral loads) in >60 yr old age group. 2nd study from Feb 8 is here and tweeted before https://www.medrxiv.org/content/10.1101/2021.02.06.21251283v1
Following inoculation with Pfizer mRNA vaccine, viral load is reduced 4-fold for infections occurring 12-28 days after the first dose of vaccine (and that is just first dose!). So, important question becomes- if you want to test someone after vaccine, should you use
oversensitive PCR test & not incorporate CT? I would argue (along with @michaelmina_lab) that - if you must test after vaccination, use a rapid antigen test to assess infectiousness if your question is - can this person transmit (seems less & less likely) https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1890/6052342
And with that I am taking a day off Twitter to work. Good night all!
