Quick thread on messaging about new studies showing reduction in transmission from vaccinations. All the studies have several sources of bias so I urge caution in sharing estimates in headlines and tweets. 1/8
Reporting a single number with a decimal point rather than a range conveys more certainty about the truth than we have right now. Reporting similar biased estimates from multiple studies just reinforces the problem - wisdom of crowds doesn't apply here. 2/8
My ballpark guess for what the true vaccine efficacy is, taking all the possible sources of bias into account? I'd say maybe 70-80%. Strong selection bias into getting vaccinated is going to be a major driver that is really hard to fully adjust for in the estimates. 3/8
Most people don't understand selection bias very well and tend underestimate how much this bias can shift estimates. Remember, selection bias is the main reason why RCTs are so valuable and why we put so many resources towards them. It really matters! 4/8
I know people are worried about underselling vaccines, but I think it's much easier to revise an uncertain estimate of 70-80% up to 90% than it is to revise a hard 90% down to 75%. Open to debate on this. But, note that my 70-80% ballpark is based on *evidence* not *caution*. 5/8
The key message is that there is strong evidence of substantial reductions in transmission from vaccination. And these reductions are definitely high enough to get us to herd immunity if enough people get vaccinated. This is very good news! 6/8
Another key message: the lower the vaccine efficacy, the higher take-up we need to get to herd immunity and end the pandemic.

If people understand this, they'll be more likely to get vaccinated even if efficacy proves to be lower than 80-90%. 7/8
Interpret new preprint data with caution. Most MDs and many epidemiologists don't get a lot of statistics training or hands-on experience to easily identify sources of biases in non-RCT studies. That's why this needs to be a team effort. 8/8
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More from @ZoeMcLaren

24 Feb
BREAKING: @NEJM Israel study by Dagan, @mlipsitch et al. finds large reduction in SARS-CoV-2 infections among vaccinated. Estimates are likely biased. But accounting for bias, reduction is still clearly substantial across all age groups 16-80+ and in presence of UK variant. 1/11 ImageImage
This is excellent news! The reduction in infections from the Pfizer vaccine is clearly high enough to relax many precautions, which will persuade many people at low risk of severe illness to get vaccinated to help slow transmission. Herd immunity just got a lot closer. 2/11
Now for the technical details: Two key sources of bias are differences between vaccinated & unvaccinated in
1. Frequency of COVID19 testing
2. Individual characteristics & risk exposure 3/11
Read 12 tweets
21 Feb
Headlines in @Bloomberg @business and @FT reporting an "89.4% reduction in transmission" from the Pfizer vaccine are *wrong* because the Israeli pre-print overstated its main results. Let me explain. 1/8
Haas et al. use data from the national surveillance system which includes test results from people who had PCR testing for COVID19. The issue is that vaccinated and unvaccinated people are not testing at the same rates. So cases in unvaccinated people are overestimated. 2/8
Unvaccinated people are required to be tested when returning from travel or after contact with a confirmed case, vaccinated people are not. Unvaccinated people are also likely to be more vigilant about contracting or spreading COVID19, so they'd test more often. 3/8
Read 12 tweets
25 Jan
Here's a helpful analogy: We are in a battle against the virus. Each vaccine dose can be used to give armor to the most vulnerable OR to lay a stone in the wall to keep the virus out. 1/4
If doses are limited, it makes sense to start by *only* giving armor to the vulnerable since it offers them a lot of protection from illness and death.

We could try to build the wall first, but the vulnerable wouldn't be protected from the virus until the wall was finished. 2/4
So, while doses & infrastructure are too limited to vaccinate enough less-vulnerable people to control the virus, we must focus on getting the most-vulnerable vaccinated. It's the most effective way to protect them.

Only then should we turn to vaccinating the less vulnerable.3/4
Read 4 tweets
25 Jan
Could throwing some COVID19 vaccine doses in the trash actually help save lives? Buckle up. This thread might break your brain. 1/9
Some argue that it's a net positive for non-prioritized people (e.g. young people) to get leftover COVID19 vaccine doses that would otherwise end up in the trash. Let me try to convince you that this is far less helpful than you think and actually likely costs lives. 2/9
According to the CDC, someone aged 65-74 is 90 times more likely to die from COVID19 than someone 18-29.👇 So vaccinating people 65-74 is *about 90 times more effective* at preventing death than vaccinating someone 18-29. 3/9
Read 9 tweets
23 Jan
I respect @lindy2350's work, but this article draws the misguided conclusion that "declining a Covid-19 shot because you think it should go to someone else won’t help anyone." Let me explain. 1/9
nytimes.com/2021/01/21/opi…
The pandemic has been long and hard on everyone. And I don't want to add to anyone's burden. But the idea that letting other more vulnerable people ahead of you in the queue "won't help anyone" is patently false. It may not fix the system, but it could save someone's life. 2/9
If you are at relatively low-risk *within your priority group* then you have many reasons to believe that your vaccination appointment would go to someone with higher risk. A 65 year old who works from home is at far lower risk than a 65 year old who works in retail. 3/9 Image
Read 9 tweets
23 Jan
@gregggonsalves you are a treasure and I want you to stay healthy. But I also know how much you care about health equity so please forgive me, but I feel compelled to respond. You can take the vaccination appointment, but you may want to consider postponing. Let me explain. 1/10
If you postpone your appointment, the dose will either (1) go to someone at higher risk, (2) go to someone at lower risk, or (3) in rare cases end up in the trash. 2/10
What is your COVID19 risk relative to the rest of your priority group? If you have low exposure risk (e.g. WFH) and have no underlying risk factors for hospitalization or death from COVID19 then you likely fall in the lower range of risk for your group. 3/10
Read 10 tweets

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