|Sopona, the Yoruba god of smallpox
A while ago I listed a number of reasons why smallpox was eradicated, whereas other diseases haven’t been (yet). One of the reasons was that the vaccine against smallpox is so effective. Vaccinia immunization induces immunity for an extraordinarily long time — memory immune responses have been shown for up to 60 years after vaccination.
So why is vaccinia such an effective vaccine? Part of it is that vaccinia is a live virus: It replicates after you’re inoculated (so there’s lots of antigen there), and it stimulates the innate immune response (which is geared toward detection of live viruses, among other things). (The yellow fever vaccine is another live virus vaccine that’s also famous for inducing long-term immunity.) Vaccinia is also a large virus that has a lot of antigens available, so that there are lots of different modes of immunity triggered. That is, both B cell (antibody-based) immunity, and broad T cell-based immunity, are likely to be present and to have lots of different targets.
A recent paper suggests that the route of vaccination is also important. Unlike most vaccines, which are given by intramuscular (e.g. influenza vaccine) or subcutaneous (e.g. yellow fever) injection, or orally (the live polio vaccine), vaccinia is delivered by scarification — scraping the most superficial layers of the skin. I don’t think this was the result of deliberate comparisons — scarification was the traditional method, and it was easy and convenient. Before 1967:
A scratch about 5 mm long was made in the skin with a needle, a lancet or a small knife and the vaccine suspension was rubbed into the site. A single cut or cross cuts were made, in 1 , 2 or 4 different sites. This was essentially the same method as had been used for variolation in Europe during the latter part of the 18th century. ((Smallpox and its Eradication (Chapter 7). F. Fenner, D. A. Henderson, I. Arita, Z. JeZek, I. D. Ladnyi. World Health Organization, Geneva, 1988))
Later, a bifurcated needle was used:
Experiments soon showed that the multiple puncture method, in which the bifurcated needle was held at right angles to the skin, which was then punctured several times with the prongs, was very efficient and very easy even for an illiterate vaccinator to learn. It became the standard method of vaccination throughout the world.
Scarification was a simple and convenient way to deliver the vaccine. It turns out that scarification isn’t just a convenience, it’s the most effective way to get immunity:
VACV immunization via s.s. [skin scarification], but not conventional injection routes, is essential for the generation of superior T cell–mediated immune responses that provide complete protection against subsequent challenges.
|Langerhans cells in the skin
This includes protection against respiratory-spread disease, not just skin infection. My first thought was that this is probably simply because the vaccinia virus replicates better in the skin than by intramuscular injection, but the improved immunogenicity is also seen with a non-replicating version of vaccinia, “MVA”.
My next thought is that Langerhans cells are probably part of the reason. Langerhans cells (see the figure to the right) are a subset of dendritic cells, probably extremely good at triggering immunity, that form a dense network under the skin, and probably act very efficiently at filtering skin-delivered antigen and delivering it to the immune system.
Also, the fact that the skin is damaged in the process evokes Polly Matzinger’s “danger” concept of immune stimulation.
At any rate, something, even if we don’t know exactly what, about scarification leads to better immunity, at least for vaccinia virus. That’s useful to know. Having said that, I’m not quite sure why this paper appeared in Nature Medicine, a very high-impact journal — the mechanism wasn’t shown at all clearly, and this isn’t the first time that the general observation has been made:
This study strongly indicated that, although less reactogenic, vaccinia vaccine administered im [intramuscularly] at a dose of 105 pfu fails to induce an immune response comparable to that elicited by standard scarification.
Even more broadly, the skin inoculation concept has been shown to lead to high immunogenicity in other systems; for example, it was shown a couple of years ago that yellow fever vaccine is more immunogenic when delivered intradermally than when given by its conventional subcutaneous route:
Intradermal administration of one fifth of the amount of yellow fever vaccine administered subcutaneously results in protective seroimmunity in all volunteers.
|Bifurcated needle used for smallpox vaccination
(I do have to add that apparently scarification — which is much easier than intradermal injection — does not work for yellow fever, based on some experiments in the 1950s. I haven’t read those papers myself, though. I’d be interested to see if the bifurcated needles used in the late 1960s and on for vaccinia might be more effective for the yellow fever vaccine.)
Anyway, seeing this in at least two instances makes it seem possible that it’s a general effect. If skin administration enhances immunogenicity, perhaps this is a way of extending limited vaccine stocks in an emergency.