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Miguel Hernán Profile picture
@_MiguelHernan
Aug 16, 2017 • 7 tweets • 5 min read • Read on X
Draw your assumptions before your conclusions. Registration is open for our "Causal Diagrams" course @HarvardOnline bit.ly/2uQssso
@HarvardOnline This piece on our #CausalDiagrams course includes my truest quote ever: "If they get bored, they will stop watching" bit.ly/2w748W6 Image

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More from @_MiguelHernan

Miguel Hernán Profile picture
Jun 19, 2023
1/
 One day everyone will recognize #selectionbias due to a #collider and the world will be a better place.

This time observational studies found a higher risk of omicron reinfection after a 3rd dose of #COVID19 vaccine. As usual, alarms went off.

Can you see the obvious bias? Image
2/
 Those who receive a booster and get infected are, on average, more susceptible to infection than those who don't receive a booster and get infected.

So no surprise than those who receive a booster and get infected are more likely to get reinfected.

Led by @susanamcorella...
3/
 ... we described the bias @bmj_latest with simulations + real data.

We show that a vaccine booster will be associated with higher reinfection risk even if the booster has no harmful effect.

Now the good news: Preventing this #selectionbias is easy...
bmj.com/content/381/bm…
Read 6 tweets
Miguel Hernán Profile picture
Apr 13, 2022
1/
 Our findings on a fourth dose (2nd booster) of the Pfizer-BioNTech #COVID19 vaccine are now published.

Compared with 3 doses only, a fourth dose had 68% effectiveness against COVID-19 hospitalization during the Omicron era in persons over 60 years of age.

Interestingly...
2/
 ... this is yet another example of the need for good #observational studies that emulate a #TargetTrial.

Would it be better to have a real randomized trial? Yes

Do we have a randomized trial? No

Will we have a randomized trial? Perhaps, but too late for a timely decision.
3/
 Last year, observational evidence was also used to recommend a first vaccine booster.

Our and others' studies provided evidence on the booster's protection against hospitalization after infection with Delta:


Policy makers listened. Lives were saved...
Read 8 tweets
Miguel Hernán Profile picture
Dec 3, 2021
1/
 We emulated a target trial of two #COVID19 mRNA vaccines in the largest healthcare system in the US.

Both vaccines were similarly effective, with Moderna slightly better than Pfizer-BioNTech.

But that isn't the most important conclusion of our study.

2/
 Spring of 2020: #COVID19 vaccines are developed.

October 2020: Results from randomized trial are announced.
businesswire.com/news/home/2021…

~6 months from development to evaluation of effectiveness.

Utterly impressive. Unprecedented.

Kudos to the pharmaceutical industry.

Now...
3/
 December 2020: Vaccines become available.

December 2021: Where are the big randomized trials for COMPARATIVE effectiveness?

1 year, still *crickets*

Billions of taxpayer dollars and we don’t get to know which vaccine is better and safer?

Not so impressive, pharma industry.
Read 6 tweets
Miguel Hernán Profile picture
Aug 25, 2021
1/
 Vaccine safety: We compared excess adverse events after #COVID19 vaccination (Pfizer-BioNTech) and after documented #SARSCoV2 infection.

nejm.org/doi/full/10.10…

Take-home message: Low excess risk of adverse events after vaccination, higher after infection.

Some thoughts👇
2/
 Preferring #SARSCoV2 infection over vaccination has become even harder. (Remember: infection also increases the risk of severe disease/death)

This is a good illustration of how #randomized trials and #observational studies complement each other for better #causalinference...
3/
 The original #randomized trial estimated vaccine effectiveness to prevent symptomatic infection, but was too small to quantify vaccine safety.

That's what #observational studies do.

Now a different sort of question: Why could we do this study in the first place?

2 reasons.
Read 8 tweets
Miguel Hernán Profile picture
Mar 18, 2021
1/

Many countries are vaccinating their elderly. Can we relax control measures now?

No.

Even with 50% of elderly vaccinated, uncontrolled #SARSCOV2 transmission may overrun the healthcare system.

We explain @AMJPublicHealth today, led by @_gmales
ajph.aphapublications.org/doi/10.2105/AJ…
2/
Data from Madrid, Spring 2020:

Critical care requirements peaked at 5 times the usual capacity.

Hospitals managed to increase ICU capacity by 3-fold.

Heroic but, sadly, insufficient.

The healthcare system collapsed. Not everybody who needed critical care received it.
3/
If 50% of the elderly had been vaccinated, critical care requirements still would have peaked at almost 4 times the usual capacity.

Greater than the ICU capacity of any country in the world.

Only a prolonged lockdown could return ICU requirements to normal.

One last thing:
Read 4 tweets
Miguel Hernán Profile picture
Mar 12, 2021
@ProfMattFox 1/
The odds ratio from a case-control study is an unbiased estimator of the

a. odds ratio in the underlying cohort when we sample controls among non-cases

b. rate ratio in the underlying cohort when we use with incidence density sampling

No rare outcome assumption required.
@ProfMattFox 2/
Because the odds ratio is approximately equal to the risk ratio when the outcome is rare, the odds ratio from a case-control study approximates the risk ratio in the underlying cohort when we sample controls among non-cases and the outcome is rare.

But...
@ProfMattFox 3/
... for an unbiased estimator of the risk ratio (regardless of the outcome being rare), we need a case-base design, not a classical case-control design.

Of course, all of the above only applies to time-fixed treatments or exposures.

As for the causal interpretation...
Read 5 tweets

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