077 - Reinfection (Second Attack) in Experimental Poliomyelitis

A common thing I hear from those that don't like vaccines is that coming down with the "natural" version of the disease gives a much stronger, even life-long immunity to it, while vaccine-derived immunity only lasts a few years. The truth of this depends on the vaccine, the disease, and the person in question, of course, but it's worth asking if it's true that getting a disease makes one immune thereafter.

This study, by Dr. Simon Flexner, investigated whether monkeys that had recovered from a "natural" (though experimental) infection of polio could be reinfected with the same or related virus—a second attack. Others had previously observed second attacks in children and monkeys infected before with polio, but not in such a formal setting.

Polio tends to be more severe in monkeys, often paralyzing and killing them, but they're not as susceptible to it, so it needs to be introduced to them experimentally, in the lab, with larger doses of virus than people encounter. Because of this, it's a bit less "natural" than human infections, but easier to work with. Those that survive have high levels of anti-polio antibodies.

So Flexner took monkeys that had recovered from polio and tried to infect them again, using the same strain of virus as the first time, or a different strain. He found that it wasn't too hard to reinfect these monkeys, even with the same strain of virus. The disease sometimes was just as severe as the first time, even in those that had severe disease the first time. It was even possible in monkeys that had been hyperimmunized through a kind of vaccination.

The final question was whether monkeys could even be reinfected a third time. There had been a case report of a third attack in a child, but none in monkeys yet. Flexner took the monkeys he had left after the second attack and tried reinfecting them, but none of them got sick that time.

So "natural" immunity is not some magical shield, at least not in lab monkeys with polio.

Citation: Flexner, S. Reinfection (Second Attack) in Experimental Poliomyelitis. J Exp Med 65, 497–513 (1937).