But however novel and strange the opinion may appear, it must be admitted that while Smallpox was in full force, it had the power of modifying and rendering Measles mild; and now that Smallpox is in great measure expelled, Measles is gradually coming to occupy the same ground. I am sorry to make this statement, but the facts, at least with regard to Glasgow, are too strong to admit of doubt....
That Measles should have been modified by Smallpox is rendered highly probable by the manner in which the Vaccine Disease prevents Smallpox or renders it so mild as to be without the smallest danger. May not Smallpox have a similar effect in relation to Measles?
When Smallpox was in full force, few children escaped, and most of those who had Smallpox and Measles had Smallpox first. This, I believe, will have been the case with more than nine-tenths of the community. Still, however, as Measles came round, it occasionally had precedence of Smallpox, and it was perhaps chiefly among such patients that it proved fatal. In looking over the registers of former years, I find the deaths by Measles were generally among very young children.
He was even disposed to believe that Smallpox, on the whole, exercised a beneficial influence in the eradication of latent disease—
An opinion has prevailed with some, that Vaccination does positive harm by infusing peccant or vicious humour into the constitution. I do not see the smallest ground for this hypothesis; but that Smallpox does good to those who survive the disease by rendering the system insusceptible of other infections, or by rendering them milder when incurred, must, I think, be admitted.... I do not presume that the constitution is improved by Smallpox, but perhaps by eradicating certain unobserved deviations from health, which, if not early removed by the accession of some acute disease, would have proved the seeds of early mortality by gaining a deeper hold of the constitution before Measles and other epidemics of later appearance came round.
In this point of view, we are not to consider Smallpox as peculiarly fatal, but fatal merely as having the start of some other diseases. Measles, Chincough, Croup, and Scarlet Fever would have proved equally fatal had any of them occurred first.... It is only on this principle that we can explain how it happened that thirty years ago not one in a hundred died of Measles, whereas now one in ten dies. Thirty years ago as few escaped Measles as now, but before they were affected they had generally passed through Smallpox, by which the secondary disease was so modified as to be almost completely divested of danger.
Watt, it will be observed, treats smallpox throughout as a malady of childhood; thus confirming Monro’s observation in 1765, that “the inhabitants of Scotland generally have smallpox in their infancy or childhood, very few adults being seen in the disease.”
From the preceding excerpts, it is not difficult to comprehend Watt’s position. He was persuaded of the prophylaxy of vaccination; he was satisfied that it had reduced smallpox in Glasgow; but, to his astonishment, he discovered that it had not reduced the general death-rate; and that in so far as smallpox had been displaced, other ailments, and specially measles, had maintained the tale of fatality.
The discovery that the fall in smallpox was compensated for by a rise in deaths from other diseases was a remarkable discovery, the importance of which is as yet far from appreciated. Watt was however at fault in attributing the decline of smallpox in Glasgow to vaccination; and in failing to inquire whether the phenomenon had any relation to vaccination whatever. He is the best scientific demonstrater who most completely exhausts the possibilities to the contrary of what he seeks to establish. Supposing vaccination to be as powerful against smallpox as its promoters averred, the causes in Glasgow was not commensurate with the effect. Nowhere was vaccination more practised. 15,000 were vaccinated at the Faculty Hall, says Watt, “and perhaps twice or thrice that number in private practice”—a loose and questionable statement. The 15,000 operated on at the Faculty Hall in the course of ten years were the poor, the vast majority in Glasgow and elsewhere, and the chief sufferers from smallpox. The assumed “twice or thrice that number” were those who employed their own medical men—a luxury less common then than now. The population of Glasgow approached 100,000, and it is obvious that the larger part must have lived outside the fortification of the Jennerian rite.
But admitting that all, or nearly all, in Glasgow were vaccinated who had not had smallpox, still that would afford no proof for what was claimed. Watt was cautious, and held closely by his Glasgow evidence, content to have it taken for what it was worth; but had he ranged wider, he would have discovered that the fall in smallpox extended over Europe, and was as well marked in Vienna as in Glasgow, in Stockholm as in London, in Italy as in Denmark. As in Glasgow the credit for the fall was claimed for vaccination, but the fall had set in before vaccination was heard of, and extended over populations to which vaccination had no application. Indeed Watt allows that the fatality of smallpox had begun to decline and that of measles to increase prior to the conveyance of the Jennerian salvation to Glasgow; but he failed to discern the significance of the fact. So too in other cities where smallpox fell off: the death-rate did not fall off; but, as in Glasgow, was kept up by cognate varieties of fever.