So what about the chupacabra?

This is strangely comforting:

Let us assume that a vampire need feed only once a month. This is certainly a highly conservative assumption given any Hollywood vampire film. Now two things happen when a vampire feeds. The human population decreases by one and the vampire population increases by one. Let us suppose that the first vampire appeared in 1600 AD. It doesn’t really matter what date we choose for the first vampire to appear; it has little bearing on our argument. We list a government website in the references [US Census] which provides an estimate of the world population for given any date. For January 1, 1600 we will accept that the global population was 536,870,911.3 In our argument, we had at the same time 1 vampire.

We will ignore the human mortality and birth rate for the time being and only concentrate on the effects of vampire feeding.

The results are persuasive, except possibly to those who denounced The Lancet study based on their earnest conviction that math doesn’t work. In short, the authors conclude, it would take surprisingly little time for vampires to rule the earth:

Relying on the anthropic principle — which declares that any condition essential for human existence must indeed be true — the authors conclude that because the non-existence of vampires is necessary for human survival, vampires cannot in fact be real.

Leaving that aside for the moment, let’s explore the problem further. Obviously a vampire conquest would pose different public health and policy challenges than, say, a zombie infestation. The only real viable alternatives, it seems to me, would be an aggressive pro-natalism (with the intent perhaps of reaching a state of coexistence with the vampires) or a genocidal campaign against the undead. As for the first option, the authors note that even when an ordinary human birthrate is factored into these tables, the population would still diminish geometrically; indeed, they point out, for humans even to survive a wave of vampire feedings at all, they would need to double their numbers (at least) each month. (Worse, such a birth rate would only stabilize the human population while catastrophically over-taxing the planet’s food resources, since pregnancy requires the consumption many more calories than ordinary). That said, even the most accelerated, biologically plausible birth rate would delay the vampirization of the human species by a mere month or so.

Of course, the authors’ grim, worst-case calculations assume that if vampires did exist, humans would either not notice the slow geometric growth of vampires, whose numbers would only swell beyond reason after about 20 months, or — having noticed the sudden appearance of vampire cells — lack the fortitude to do what’s necessary to exterminate the enemy. Assuming the threat were detected early enough, it seems perfectly plausible that humanity — acting through appropriate multilateral institutions and abiding by all the proper checks on abuses of state power — would easily have nearly two years to develop an effective genocidal program before some kind of “tipping point” were reached and the human species was forever transformed into nocturnal leeches.