Nor was Pearson content simply to inquire of others: he experimented himself, and put Cowpox to the test a week or so before the appearance of Jenner’s Inquiry. He wrote—
Happening, on the 14th of June, to be with Mr. Lucas, apothecary, on professional business at Mr. Willan’s farm, adjoining the New Road, Marylebone, where from 800 to 1000 milch cows are kept, I availed myself of the opportunity to make inquiry concerning the Cowpox. I was told it was a pretty frequent disease among the cows of that farm, especially in winter, and that it was supposed to arise from sudden change from poor to rich food. It was also well known to the servants, some of whom had been affected with the malady from milking the diseased cows. On investigation, I found that three of the men-servants, namely, Thomas Edinburgh, Thomas Grimshaw, and John Clarke had been affected with the Cowpox, but not with the Smallpox. I induced them to be inoculated for the Smallpox, and, with the view of ascertaining the efficacy of the variolous infection employed, William Kent and Thomas East, neither of whom had either the Cowpox or the Smallpox, were also inoculated. (P. 14.)
The result conformed to expectation: Edinburgh, Grimshaw, and Clarke did not take Smallpox, even though inoculation was repeated, whilst Kent and East did. Pearson set forth his experiments much more philosophically than Jenner, but his bias was pronounced, and it blinded him to some obvious considerations; and it is marvellous how easily we may accumulate details for which we have a fancy. Summing up the testimonies he had collected, he held that—
The body of evidence is numerous and respectable, declaring that a person who has laboured under the Cowpox fever and local eruption, is not susceptible of the Smallpox. It does not appear that a single well authenticated contravening instance has fallen under observation. But I do not apprehend that accurate and able reasoners will consider the fact as completely established, though, I doubt not, they will allow that the testimonies now produced greatly confirm the probability, and that the cautious appropriation of it in practice is warrantable. (P. 64.)
In this summary we perceive the limit and imperfection of Pearson’s Inquiry. Smallpox did follow Cowpox: it was well known that it did: and Dr. Ingenhousz ascertained the fact as soon as he looked for it. Moreover Pearson showed himself ignorant of Jenner’s position, who, recognising the fallacy of the rural superstition, was compelled to discriminate Cowpox as genuine and spurious—the genuine being the variety derived from Horsegrease.
Upon Jenner’s assertion that Cowpox was unaltered by transmission from arm to arm, Pearson remarked, “The fact remained to be proved.” That Cowpox produced a harmless ailment was not, he thought, to be hastily assumed. Dr. William Heberden had recently inoculated 800 poor persons at Hungerford without a mishap, and 1700 had passed through Dr. Woodville’s hands in the current year (1798) with only two deaths; yet how erroneous would be to argue that variolous inoculation was harmless from such special experience!
Such instances of success can only be attributed to a certain favourable epidemic state of the human constitution itself, existing at particular times, for the proportion of deaths from inoculation is usually much greater, owing, probably, to certain unfavourable epidemic states. (P. 67.)
If Cowpox remained unchanged in transmission from arm to arm, it would be no harmless ailment; for the evidence was distinct that it was frequently a severe one. For example, Edinburgh told Pearson that when suffering from Cowpox he had to give up work and go into an hospital; and Grimshaw that the disease was uncommonly painful, with swellings in his armpits, sore to the touch; and the servant at Rhodes’s farm in the Hampstead Road, who had seen much Cowpox in Wiltshire and Gloucestershire, said the milkers were sometimes so ill that they had to keep their beds for several days, though none ever died of the Cowpox fever. If, however, by transmission from arm to arm, Cowpox became milder, it was not improbable that at the same time it would lose more or less of its protective efficacy.
Pearson might have seen and added, that resistance to inoculated Smallpox, when the constitution was in no humour for Smallpox, was no proof that the same constitution would resist Smallpox when epidemic, or in condition for the evolution of the disease.
Pearson likewise took objection to Jenner’s evidence (such as it was) that it was possible to take Cowpox after Cowpox, but not Smallpox after Cowpox; saying—