Here is another question from one of the students in my recent vaccine lecture at Columbia University:
“I wanted to ask you a quick question about the Pox Virus. In class, you explained how the live vaccine can lead to the spread of the virus through the urine/feces of vaccinated individuals.
When this happens, are these vaccinated people spreading the virus (non-attenuated) to others, and would they also be spreading the Ab specific for that virus? I was confused by your explanation because I wasn’t sure how Pox doesn’t spread to non-vaccinated people more often.
I have heard of the concept of ‘herd immunity’ that allows for individuals with out the vaccine to still be immune to a disease due to the overwhelming immunity of the surrounding community, which is immune to the given disease. Is what you were explaining in class about the Pox virus, a mechanism by which herd immunity acts?”
My response:
Not every individual in a population need be immunized to stop viral spread, but a sufficient number must become immune to impede virus transmission. Infection stops when the viral load drops below the threshold required to sustain the infection in the population. This effect is called herd immunity. Is varies between 80-95% depending on the virus and the population.
OPV can spread to non-immunized individuals because it is shed in the feces (not urine nor antibody, as you wrote). When it infects a non-immune person, it replicates in them and effectively immunizes them. Herd immunity does not depend on the spread of vaccine from one person to another, only the number of immune persons. Inactivated vaccines, which do not spread from person to person, induce herd immunity as well as live vaccines.
OPV spreading contributes to herd immunity because it increases the percentage of the population that is immune. However, spread of vaccine is not the concept embodied by herd immunity.
The vaccine against smallpox, called vaccinia virus, is a live vaccine and also spreads from person to person, effectively increasing the number of vaccinated individuals. However, I wasn’t referring to this virus during my explanation of herd immunity.
The point is that herd immunity is conferred when immunity of the population reaches a certain level. Whether that level is achieved by immunization programs, or by the spread of vaccine virus from a vaccinated to a non-vaccinated individual does not matter.