Why could vaccinated people with mild smallpox still spread the disease?

Vaccinated individuals could still spread smallpox because the smallpox vaccine could sometimes result in only a very mild attack of the disease in those who were vaccinated. Because the symptoms were so mild, these individuals might not realize they were infected, yet they remained capable of unwittingly transmitting the fatal form of the disease to others.1

This phenomenon presented a significant public health challenge, as the primary danger was the silent spread of the virus by people who appeared largely healthy. Consequently, during outbreaks, it was essential for health officials to conduct thorough inquiries into all possible contacts of patients, regardless of the severity of the symptoms observed, to isolate those who might be carrying the infection.1