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
If testing rates differ between vaccinated and unvaccinated people then we shouldn't interpret the *observed* difference in cases as the *actual* difference in cases. Many asymptomatic cases among the unvaccinated will go undetected because of lower testing rates. 4/8
This means that the estimate of 89.4% (95% CI 88.8 – 89.9%) is higher than the true (currently unknown) value. How much higher? We need more evidence to know for sure. We expect there to be a reduction in transmission, we just need to know by how much. 5/8
So why did @Bloomberg & @FT report the wrong number? Because the study overstated its (non-peer-reviewed) results in the abstract and news outlets reported that uncritically. News outlets should have asked one or more experts to read the study and weigh in first. 6/8
In fact, the pre-print included this caveat: "Israel’s testing policy exempts fully-vaccinated persons from required testing when returning from travel abroad or if they are in contact with a confirmed case. Thus, this could lead to overestimating VE against infection." 7/8
So, is there a way to back out the unbiased estimate of the reduction in transmission from Pfizer's vaccine from this data? You bet. I developed a method to do exactly this with tuberculosis testing data in South Africa. Thread on my paper below: 8/8
Addendum: In case there was any question, I've read the preprint carefully and frequently peer review papers like this for top journals. Happy to answer questions.
Addendum: @Bloomberg has updated their story (but not their headline) to reflect my comments in this thread. Thanks @jwgale!
bloomberg.com/news/articles/…
Obtaining an unbiased estimate of the impact of the vaccine on infections means addressing bias due to differences between vaccinated and unvaccinated in
1. testing frequency and
2. baseline characteristics (age, exposure risk, etc.)
Think critically and read past headlines.

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More from @ZoeMcLaren

23 Feb
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
Read 10 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
22 Jan
Is it naive to hope that the pandemic will get better now that we have a new President? The truth is that there is a lot of low hanging fruit that the Biden administration is already working on. I predict our trajectory will start to look better soon. Let me explain 1/14
Health information: Better information about relative COVID19 risks of different activities. @CDCDirector Walensky plans to review all guidance for scientific merit and update it as necessary. This will help reduce transmission right away. 2/14
Funding: HHS had been inexplicably holding up funds appropriated by Congress last summer. Under Biden's administration more funds will be allocated to states and health departments to be spent on COVID19 priorities. 3/14
Read 14 tweets

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