Nope, we don’t.
After even 10 years with no virus exposure there is little immunity left, and now it’s been decades for us old-timers. Back in the old days re-vaccination of exposed people was routine during outbreaks.
Originally vaccination was thought to be life-long, but that was back in the days when the vaccinated were still exposed by the outbreaks that occurred in society continuing to “remind” their immune systems. As outbreaks ceased this reminder no longer occurred and immunity waned. This is also seen in other diseases, hence the need for boosters at various intervals.
Well… yes… but stockpiles of the vaccine are far too small to vaccinate all but a small minority of people immediately. It would be a very ugly race between producing more/sufficient vaccine and the virus spreading. It’s also a live vaccine, and also one that historically had one of the higher complication/mortality rates for a vaccine, which means the immunocompromised (cancer patients, people with HIV, organ transplant recipients, and others) can’t use it. It’s also contraindicated for anyone with a history of eczema or atopic dermatitis because it can lead to a horrifying death for them (although for some reason I got vaccinated as an infant despite my eczema - it was a different era, I think, and fortunately I survived the experience. Lethal side effects were not universal, but there were some cases). Unfortunately, people with those conditions aren’t going to surive a smallpox infection, either. There is a very limited supply of a new version of a smallpox vaccine that can be used by such people, but I expect it wouldn’t be sufficient for a world-wide outbreak. The newer versions of smallpox vaccine haven’t been tested in humans against actual smallpox, although they have been used against monkeypox in recent decades.
A lot would depend on which variety of smallpox gets loose. Some varieties have a relatively low fatality rate, about 10 times that of covid (hence my use of the term “relative” - it’s low only compared to its nastier cousins). The problem with the less evil version, sometimes called alastrim or Variola minor, is that being less nasty people were less likely to be completely knocked over by it and thus more likely to be and around and spreading it. All those yahoos wandering around with active covid ignoring that they don’t feel good (as opposed to genuinely asymptomatic cases) would be wandering around with this, spreading it far and wide.
Variola major strains had fatality rates between 30-90%, with survivors always left scarred. Blindness and joint damage can also occur and when it does it’s permanent, leaving a certain percentage of survivors permanently disabled. The “upside” - if you can call it that - is that V. major makes a person so sick that they tend to go to bed and not wander around as much as in the milder version, so it doesn’t spread quite so fast. Also, unlike covid, it’s not infectious before symptoms show. It still spreads very nicely, though, because it can survive outside the body on surfaces for significant periods of time.
In sum: If V. minor gets loose it will be something like covid with 10 times the death rate. So… instead of 750,000 dead Americans you’d see 7,500,000 dead. If V. major gets loose you’d see between 99 million dead and 297 million dead. Just in the US. Worldwide that would be between two and a quarter billion dead, and six and three-quarters billion dead. Think that’s unrealistic? Remember - the vast majority of the world is, at this point, unvaccinated (the jab I got 55+ years ago is now largely useless… I might have a tiny greater chance of surviving an infection, that’s the most it would give me at this point) and few have any sort of immunity. It’s hard to prove at this point due to time and lack of written records, but a LOT of people in the Americas were wiped out by disease shortly after the Europeans arrived, perhaps as many as 90% of the pre-contact population, and smallpox is a leading candidate for having done the deed (measles is also a possible contributor). There are historical instances of outbreaks leaving a high percentage of people in an area dead.
Humans of course will fight back. It will be a little harder to call a smallpox outbreak a “hoax” with “bring-out-your-dead” levels of mortality. But not all the deaths will be from infection because if you think covid disrupted supply lines and lead to shortages well… you ain’t seen nuthin’ yet!
A distant upside would be that the severe version with 90% of humans dead there will be a lot more resources for the survivors, greenhouse emissions and other forms of pollution will drop, and basically a lot of problems in the modern world related to too many people and not enough to go around, as well as waste disposal, will be greatly alleviated. The situation will be hellish for a generation, maybe two, but eventually those who remain may live in a better world. Unless/until we find some other way to fuck it up (such as nuking infected cities, which would be a Bad Thing).