So, if anyone who has been exposed to smallpox is vaccinated any time within a week after exposure, the vaccine will take hold first, and the patient will have either simple vaccinia, with its trifling headache and fever, or else a very mild form of smallpox.
"A Handbook of Health"
Woods Hutchinson
The reason being that vaccinia, the disease resulting from successful vaccination, being far milder than smallpox, runs its course more quickly,-taking only two days to develop,-while smallpox requires anywhere from seven to twenty days to develop after the patient has been infected, or exposed.
"A Handbook of Health"
Woods Hutchinson
In other words, after the true vaccinia has run its course, secondary affections of the skin of the cows usually take place, and if dairy workers became infected from these lesions, then no protection against smallpox is afforded them.
"Makers of Modern Medicine"
James J. Walsh