024 - Studies on Experimental Pneumonia V. Active Immunity Against Experimental Pneumococcus Pneumonia in Monkeys Following Vaccination with Living Cultures of Pneumococcus

This is very similar to the last post, by the same authors and in the same journal around the same time, but instead of vaccinating monkeys with dead bacteria, this time they vaccinated with living ones, subcutaneously (right under the skin).

The numbers of subjects in each experiment were still quite small, only a few each time. They tried two different strains of pneumococcus, one very virulent and the other very non-virulent (henceforth known as "attenuated"). Not surprisingly, vaccinating with the virulent strain often caused significant illness in the monkeys, sometimes fatal, though sometimes not very serious.

The attenuated strain seemed to be pretty mild in all cases though, only causing mild local reactions (in the two monkeys that received it).

Both live vaccines appeared to confer much more protection against pneumonia (transmitted the natural way, not injected) than the dead vaccines had. The monkeys even seemed to have some cross-protection against some other types of pneumococcus.

So, if one accepts the results of such tiny experiments, it seems that infection with live organisms, even minimal infection in the case of attenuated, induces better immunity than simple exposure to dead pathogen, at least in monkeys. And the researchers observed that immunity to pneumococcus didn't seem to correlate with the presence or levels of antibodies in the monkeys' blood. We'll see how these findings play out in the future.

Citation: Cecil, R. L. & Blake, F. G. Studies on Experimental Pneumonia V. Active Immunity Against Experimental Pneumococcus Pneumonia in Monkeys Following Vaccination with Living Cultures of Pneumococcus. J Exp Med 31, 657–683 (1920).