049 - Immunization Experiments with Swine Influenza Virus

Now for something different: influenza. By 1936, people knew that the disease was viral, not bacterial, though bacterial infections often complicated flu, causing even more sickness. So searching for a way to prevent the viral disease in the first place was a good idea.

Richard E. Shope took advantage of earlier studies of influenza in animals, and tested the possibility of immunizing animals against the flu by injecting them with the virus. No killing or inactivating at all, just injecting the live virus into muscle or skin or other areas of the body, apparently is enough to safely generate an immune response.

This was already known, but what Shope intended to test with this study was whether flu virus that had been cultured in one kind of animal (pig, ferret, or mouse) could provide immunity to another type. As secondary goals, he was looking at dosage and route of inoculation (either subcutaneous, under the skin, or intraperitoneal, into the body in the spaces between organs).

The flu Shope used had been isolated from swine originally, but in the lab he had passed some of it through mice and some through ferrets, so those strains were adapted to those animals. He used infected lung tissue from each animal, ground up and dissolved in saline, as vaccines and infecting doses.

First he tested seven pigs, inoculating them with virus from pigs, ferrets, or mice either subcutaneously or intramuscularly. Then, along with two non-immune controls, he infected them with virus through the nose. After 4 days, Shope killed the pigs and examined their lungs for flu lesions and their blood for anti-flu antibodies.

And all seven turned out to be immune, while the controls got sick. One of the seven pigs had live virus in its nose, but none had it in their lungs, where the infection is worst. And they all had antibodies against the virus.

Next he tried ferrets, inoculating them subcutaneously or intraperitoneally. The 8 controls all had severe illness with virus easily found, but those immunized with ferret-derived virus were immune, all except one out of nine. Of those inoculated subcutaneously, all the ferrets immunized with swine or mouse virus got sick, but intraperitoneal inoculations from those animals seemed better able to protect (66% from swine, 100% from mice).

Finally was the test in mice, which apparently are a useful model because the virus kills them easily but doesn't really spread between them, so strict isolation is not necessary. The setup was pretty much the same as with the ferrets. In this case, 79 out of the 83 control mice died within 7 days after infection, as expected.

Of those immunized with mouse virus, 77 out of 99 immunized with two large doses survived, about equal proportions subcutaneous or intraperitoneal (it didn't matter), though only 29 out of 68 given one large dose survived, and 42 out of 83 given two smaller doses.

Of mice given swine or ferret virus subcutaneously, only 8% survived from swine virus and 17% from ferret. Intraperitoneal injection was better: 70% from swine, especially with a higher immunizing dose, and 67% from ferret.

Since it is a live virus vaccine, Shope monitored the inoculated mice for illness, and a few did die, but mostly from intestinal infections. A very few did die from pneumonia, but he wasn't able to find any flu virus present in these, so it could've been something else.

These results are promising, but Shope also presents some data that are more troubling. Some pig farm in Iowa had done an experiment with this sort of vaccine, immunizing more than 1500 pigs. Mostly the vaccinated were kept separate from the others, but in a few droves that was not possible. In one drove of 223 animals, 23 were vaccinated, and soon after, a flu infection broke out in the drove, infecting all but 30 (including 20 of the vaccinated). There were no other infections going on in Iowa at the time, and it was too early in the season for flu to be going around, so Shope suspected that it came from the inoculation somehow. A similar thing happened in another drove.

So it seems that live virus in body tissues can generate a protective immune response, especially if the virus originates from the same kind of animal (except in pigs, who can gain immunity from any virus apparently). And it's possible that with this type of vaccine, other susceptible individuals in the environment may be at risk of infection from the vaccinated. So it's promising, but not quite ideal.

Citation: Shope, R. E. Immunization Experiments with Swine Influenza Virus. J Exp Med 64, 47–61 (1936).