Thread by @DearPandemic

1/ Q: Do we know more about whether the vaccines reduce transmission?

A: YES! Evidence continues to mount that the vaccines DO in fact reduce transmission.

#vaccineswork
dearpandemic.org/do-vaccines-re…

2/ ➡️ While the Nerdy Girls are still dreaming of transmission being tested in the clinical trials (call us☎️), the data pouring in from around the world strongly suggests that the vaccines are very good (but not perfect) at reducing ALL infections & by extension transmission.

3/ Recall the clinical trials (largely) tested only participants who developed symptoms. One possibility is that vaccines reduce the severity of COVID-19 to the point of no symptoms, but still allow the virus to replicate in the nose and throat & potentially be spread to others.

4/ This is the top scenario in this (awesome) infographic from Dr. @nataliexdean.

In the 2nd scenario, ALL types of infections are reduced by the vaccine, including asymptomatic. This means much less virus hanging around in anyone’s nose or throat ➡️ much less transmission.

5/ Scientists have always expected this based on other vaccines, but quantifying the effect is harder.

To answer this, we’re most interested in studies that also tested for ASYMPTOMATIC ILLNESS, which we now have from several cohorts of Health Care Workers (HCW) in the UK & US.

6/ ➡️SIREN study: 23K+ HCWs in UK tested w/ PCR tests every 2 weeks & additional lateral flow tests from Dec 8 '20- Feb 5 '21, a period of surging case rates in the UK.

dx.doi.org/10.2139/ssrn.3…

7/ The study found:

💥70% reduction in asymptomatic and symptomatic infection >21 days after 1st shot.

💥85% reduction in asymptomatic and symptomatic infection >7 days after 2nd shot.

8/ We Nerdy Girls like this study design because it started at the very beginning of vaccine roll-out in the UK, & almost all HCWs got the vaccine during the study (~90%).

9/ This reduces (but doesn't eliminate) our concern that people who chose to get the vaccine might be different from those who don’t in ways that impact their infection risk, the problem that randomized trials are designed to avoid.

@ZoeMcLaren #causalinference #epitwitter

10/ ➡️ A similar study of 4K US HCWs from @CDCgov @CDCMMWR, participants also tested weekly to pick up BOTH asymptomatic & symptomatic infections.

💥 80% ⬇️ in infections >14 days after 1st shot.
💥 90% ⬇️ in infections >14 days after 2nd shot.

cdc.gov/mmwr/volumes/7…

11/ This study is smaller & raises concerns about the comparability of vaccinated & unvaccinated w/ lower take-up among HCW in the US. Nonetheless, the low levels of asymptomatic infection among vaccinated workers is striking & very encouraging.

https://twitter.com/ZoeMcLaren/status/1377254751801315328?s=20

12/ 40K were tested coming in for other procedures through Mayo Clinic. Again, the advantage here is the ability to pick up *asymptomatic* infections that the person is not aware of.
💥79% reduction in asymptomatic infection >10 days after 1st shot.
academic.oup.com/cid/advance-ar…

13/ ⚠️But there are still very likely selection issues to consider here in who was vaccinated prior to procedures. #epitwitter

💡What about viral load–if I’m vaccinated but test positive, am I less infectious?

14/ 🎱Signs point to YES! Recent work showed average viral load of positive tests 12+ days after 1st vaccination was significantly lower than those < 12 days or unvaccinated.

Lower viral load *likely* means people are less infectious.

nature.com/articles/s4159…

15/ FINALLY-good news from Israel! Cases dropping dramatically even in young w/o high vaccine coverage, despite re-opening. This is not a *complete* slam dunk but + sign that high rates (> 55% fully vaccinated) reduce transmission for all, 1 of ultimate goals of vaccination!

16/ BOTTOM LINE…THE NEWS IS GOOD!

💥 Vaccines reduce both asymptomatic & symptomatic infection
💥 People without detectable virus unlikely to pass infection to others

➡️ VACCINES GREATLY REDUCE TRANSMISSION

#somegoodnews #VaccinesSaveLives #ThoseNerdyGirls

17/ Important caveat:

⬇️ This protection is high but not 100%, so caution with #StaySMART principles is still warranted in the presence of unvaccinated people, especially those at higher risk of severe disease.

#MaskUp

18/ See some of our recent mosts on navigating this challenging in between world w/ mismatch between your own individual vaccination status & family or population coverage:

dearpandemic.org/vaccinated-adu…

@CDCgov #vaccinated #pandemiclife

19/ What's my @CDCgov recommendation?

dearpandemic.org/cdc-guidance-o…

20/ Remember that in public you don’t always know who is vulnerable or has lower immune response to vaccines due to immunosuppression, so be a good citizen and #MaskUp until we push case levels down much further!

Great new piece by @apoorva_nyc on this:

https://twitter.com/apoorva_nyc/status/1382771323093532674?s=20