Rights Management
The vaccine had been an obvious clone of a patented one - that much was obvious. But the cloning and manufacture had been a grassroots biotech effort, so the countries' governments shrugged and said they had had nothing to do with it and did not feel obligated to pay.
"That's fine," said the company owning the patent, if not in those words. They came from the software industry.
They did not release a new virus. That'd have been an atrocity. They came from the software industry anyway. They created a self-replicating RNA patch designed to identify and disable any unlicensed application of their intellectual property.
Once the deaths resumed they sat with the governments to discuss a purchase plan.