[Question] Why don’t we vaccinate people against smallpox any more?

The eradication of smallpox in 1979 represented one of the greatest achievements of modern civilization. However, since then most countries have elected to stop vaccinating their populations against the disease. This seems like a very concerning vulnerability to me, with waning herd immunity due to more and more of the world’s population being replaced by unvaccinated young people.

What if there was an unintentional release from one of the labs around the world that still hold on to samples of the virus? What about an intentional release by terrorists/​rogue nations? What gives scientists the confidence that there are no undiscovered animal reservoirs or uncontacted tribes in remote places where smallpox is still circulating? Smallpox is highly contagious and hundreds of times more deadly the SARS-Cov-2.

How would the world respond to such a release? Is there enough capacity to rapidly produce and deploy billions of doses of smallpox vaccines? (Right now we’re at an all-time high in terms of pandemic preparedness; I’m thinking decades down the road when the lessons from Covid-19 has been all but forgotten)