Separating out two common questions:

1) Do vaccines reduce transmission?

Yes. A vaccinated person is less likely to transmit because they are less likely to ever be infected. At a population-level, this translates to reduced transmission.
cdc.gov/mmwr/volumes/7…
2) But if a vaccinated person gets infected, are they less infectious?

This is hazier. Maybe they have lower viral load, shorter duration of infection. Maybe virus is contained to the nose only. But less infectious does not equal non-infectious.
A few replies mentioning the UK household study estimating 50% reduction in infectiousness. Studies of close contacts can provide valuable real-world evidence, and estimates from these studies will accrue over time.
khub.net/documents/1359…
Addendum: I used the word "hazier" above, but maybe I should clarify that the most likely outcome is that vaccines DO reduce infectiousness of breakthrough cases. But it's not a perfect (100%) reduction, and we will need more studies to estimate it reliably.
Appending this new paper to the thread. It addresses the same topic in far greater detail.

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More from @nataliexdean

13 May
“Everyone believes in coordination, but no one wants to be coordinated.”

In today’s @WHO forum, Sir Michael Jacobs (@RoyalFreeNHS) with a call to action to improve collaboration for therapeutics research. 1/4
He provides a successful example of three large-scale platform trials collaborating to harmonize protocols for antithrombotics. Data are more valuable when they can be combined and compared. 2/4
He provides another example of countries having committees to prioritize new drugs for trials. Within the last few months, the committees have realized the advantage of sharing briefing documents and resources. Reduces duplication of effort and minimizes risk of omission. 3/4
Read 4 tweets
4 May
The imminent FDA authorization of a vaccine for 12-15 year olds is great news, and adolescents should be able to access vaccine. But in the short term, we must also grapple with the ethics of vaccinating adolescents ahead of high-risk adults in other countries.
Read 4 tweets
26 Apr
I had the opportunity to write a comment about findings from a large vaccination cohort study in Scotland. I use the comment to discuss some of the challenges of observational vaccine studies and the potential for lingering bias.
THREAD 1/11
Confounding is a key challenge in observational studies. One way to gain confidence in findings is to check for bias in results where we know the answer (usually, where we know there is no relationship). 2/11
.@_MiguelHernan describes using this approach in an Israeli cohort. They verified that they didn't see a protective effect of vaccines earlier than observed in randomized trials. At first they did see this, so they adjusted for more covariates. 3/11
Read 11 tweets
15 Apr
The % vaccine breakthroughs in a population depends on:
- Vaccine efficacy
- Amt of virus circulating
- Length of time since vaccination
When you see 0.008% breakthroughs in fully vaccinated people, remember that many of these people haven't been exposed.
wsj.com/articles/cdc-i…
I love to see small numbers as much as anyone, but know that numbers like this cannot be directly interpreted as a measure of vaccine efficacy (although I have a feeling they will be). We can only interpret them against a background rate in unvaccinated people.
Similarly, "most breakthroughs have been in elderly adults" should not be read as the vaccine is less effective in elderly adults. The majority of vaccinations (and the longest amount of follow-up time) have been in elderly adults. Again, we need more info to interpret.
Read 4 tweets
13 Apr
Regarding vaccine safety, regulators are in a tough position. Transparency is a good principle. But one thing to do better is to limit the dead time between an announcement and the details of its rationale (the media briefing). Here be dragons. 1/3
The media briefing provided important context, that this is intended to be a short pause. That the aim is to increase awareness to (1) strengthen reporting and (2) make sure doctors use the correct drugs to treat this rare condition. They scheduled a rapid ACIP meeting. 2/3
In the dead time, the media scrambles for insights but everyone is short on details. Even states trying to set policy and provide vaccines don’t have all the information. So it’s a hard job, but there’s more to be done to get that important first version of the message right. 3/3
Read 4 tweets
31 Mar
The news about Pfizer's adolescent trial is excellent. As some debate whether we have enough data to reliably estimate vaccine efficacy in this subgroup, some important context is that efficacy was not even the primary outcome of the trial. 1/4 statnews.com/2021/03/31/pfi…
The 12-15 subgroup was comparatively small (an extension of the existing trial), and the focus was to measure safety and immunogenicity, although efficacy data was also collected. Though we don't have full details, the trial was unlikely powered for efficacy. 2/4
When we think about bridging a known efficacious vaccine to a new (here, younger) population, the bar for evidence is lower. Clearly we need high quality safety data. For immune response data, we see even higher antibody responses in adolescents than adults. 3/4
Read 6 tweets

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