In 1980, smallpox was declared eradicated following an intensive global vaccination campaign.  The virus, Variola, has some close relatives that can infect humans, one of which is monkeypox.  Monkeypox isnt nearly the problem that smallpox was; it has a much lower mortality rate and outbreaks tend to fizzle out quickly due to poor human-to-human transmission.

However, a recent paper suggests that monkeypox infections are becoming an increasing problem.  So why is it emerging now?  Its a problem we’ve been anticipating, actually.  Turns out that when you get the smallpox vaccine (or smallpox itself), it also protects you from monkeypox.  So pre-eradication, most people were immune to monkeypox.  If you met up with an infected animal, chances are you were immune and wouldn’t get infected.  If you did somehow get infected, chances are most people around you were immune so you couldn’t transmit it to others.  An immune host is not fertile ground for viral replication, so whenever immune hosts are encountered, the chain of viral transmission ends.  In fact, a highly vaccinated population helps those few individuals that are not vaccinated by greatly limiting the potential of the virus reaching the unvaccinated (“naive”) individual.  Thats called herd immunity.

Turns out herd immunity to smallpox, and therefore monkeypox, is waning.  Vaccinations stopped in 1980 so anyone born after that is naive and therefore there is a major lapse in herd immunity.  Risk of infection with monkeypox virus is now as much as 20 times greater than 30 years ago.  Interestingly, all those old people born before 1980 who were vaccinated have a much lower risk of infection, telling us that immunity from vaccination lasts 30+ years.

So why should we worry about waning herd immunity to a rare and relatively mild disease that is hardly contagious?  Well, variola and monkeypoxviruses are about 96% identical.  We dont know how much  monkeypox needs to mutate to become sustainable in humans or more virulent.