What does it even mean for a vaccine to be open-source?

As per the founder in the thread:

"Once the Phase I trial of PVX-001 concludes, we will open-source the design and manufacturing information needed for others to produce and build on it, and we will not enforce our intellectual property rights against anyone who uses it to make vaccines against any betacoronavirus [...]"

The headline seems slightly inaccurate. A pledge of later going open-source isn't the same as open-source. Sure, "marketing".

Hey, it worked for OpenAI!

They kind of did the opposite. Started off open source until they had gotten what they needed from the goodwill of the open source community, then made it closed source to make sure they wouldn't have to return any of that goodwill

Guess my sarcasm wasn't heavy enough...

I assume it's not patented or copyrighted?

Open source software is still copyrighted, its just that the rights holders grant you a license that allows you to do certain things as long as you stick to certain conditions

But that's not what the word "open-source" means