From New York Times, I'm Michael Obaro. This is Adalee.

Today, the decision-making process that led the U.S. government to suspend the use of Johnson & Johnson's COVID-19 vaccine.

How long it may last, and the impact it could have.

I spoke with my colleague, Science reporter Carl Zimmer.

It's Thursday, April 15th.

Carl, walk us through how this very big decision was reached to pause vaccinations with Johnson & Johnson's vaccine in the U.S.

Where does that story start?

The story starts in Europe with another vaccine, one that's made by AstraZeneca.

AstraZeneca is going into widespread use in Europe. It's not yet authorized in the United States.

And after a while, the European regulators noticed a few rare cases of blood clots which start to get them concerned.

So they begin to investigate whether that has anything to do with the vaccine.

With the AstraZeneca vaccine, got it?

So blood clots are a surprisingly common thing. Thousands of people get blood clots every day.

So just because people who get vaccinated get a blood clot, that doesn't really tell you anything in particular about whether the vaccine is dangerous.

You have to take a closer look.

And when European regulators took a closer look at people getting vaccinated, they noticed that there's a small amount of people who are all getting the same unusual kind of vaccine.

They were taking place in the brain. And they were associated with very low levels of something called platelets.

And what are platelets?

Platelets are a component of the clotting process in our bloodstream.

So when we get little wounds in our blood vessels, platelets jump into the wound and basically create a little patch.

And that allows us to heal.

So that's good. What's bad is when the platelets start to form a big clot, which can then break loose and go around in our bloodstream and end up in dangerous places like in our brains.

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