Hanging with the herd, for the immunity of it all

All opinions are my own and do not necessarily reflect those of Novo Nordisk.

When I hear about events such as the recent outbreak of measles among a small group in Texas, I am reminded of how complex, complicated and difficult public health efforts can be. In the US, for example, there are conflicting imperatives:  the rights of people to practice their beliefs versus the right of the community to be protected against preventable health threats.  This particular situation involved members of a church congregation, many of whom had not gotten vaccinated for measles due to worries about a link between autism and the Measles-Mumps-Rubella (MMR) vaccine.  While no scientific evidence has been found to support any such link, many had chosen not to be vaccinated “just in case.”

One day I hope to write about the link between the phenomenon of science denial and personal identity (one perspective can be seen here), but for now I just want to point out how this event and a recent publication by the Centers for Disease Control (CDC) on rotavirus vaccines demonstrate nicely the concept of herd immunity (article behind paywall, but writeup here).  There are different usage patterns for the term, so I’ll say up front I am using “herd immunity” to describe not just the proportion of individuals within a population who are immunized to a given pathogen but also the indirect effects for non-immunized individuals.  The term was first used in a publication in 1923, by Topley and Wilson, in the context of how to describe the host side of their studies in bacterial infection among mice.  The concept later gained mathematical underpinnings, including formulas describing how the different ratios of vaccinated to nonvaccinated individuals defines the degree of herd immunity depending upon how infectious a disease agent is.

In the Texas outbreak, the infection was caused by an individual who had been overseas and had been exposed to measles.  Upon return to Texas that individual visited the congregation and came into close contact with several members.  Eventually at least 25 people came down with measles.  Among the members of the congregation where the outbreak started, the infected individuals either had not been completely vaccinated or had no record of vaccination.  So in this case, insufficient numbers of people in the group had been vaccinated to prevent the spread of disease; herd immunity could not work.

The report from the CDC, on the other hand, is an example of successful herd immunity.  This epidemiological work examines the general effect that immunization of children for rotavirus has on their family members.  While the full text is behind a paywall, the first page preview contains the table with the meat of the study.  If one compares the annualized data from 2000-2006 with subsequent years, a huge drop in hospitalizations is seen for young children and infants.  And a substantial drop is seen in all age ranges above that, which include populations never vaccinated for rotavirus.  For this disease, given its normal pathway of infection via the very young, vaccination of the youngest children has a huge beneficial effect on the rest of the herd.

Given these stories, it seems a wonder that vaccination has faced such a battle on so many fronts.  The pushback against the human papilloma virus comes to mind.  But again, there’s often (always?) more to vaccinations than just public health.  As we learn more about the reasons why vaccines aren’t taken up by some segments of the population, maybe we can figure out different approaches to make vaccines more acceptable so we can welcome more people into the herd.


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