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Herd immunity — defining the herd?
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If one is a cattle farmer, it is relatively easy.
Not so with human populations. Our mantra in immunization practice has been that for each disease there is a certain proportion of the population (herd) that must be immune to protect those susceptible members against infection. This threshold can only be estimated and varies among different diseases, the more contagious ones having a more stringent requirement. We seem to have achieved herd immunity for many diseases that now appear no more or only rarely.
The “canary in the mine” for many years has been measles. This disease is very contagious and even a slight fault in our system of immunization may be manifested by measles reappearance. In a study many years ago, we found that in two schools with only 4.1% of the students seronegative, an introduction of measles led to a sustained epidemic (N Engl J Med. 1987;316:771-774). Although the national immunization rates may be very high, packs within this larger herd might not be protected and an introduction to one of these can lead to outbreaks. This was well demonstrated during the, large measles epidemic in the late 80s when nationally we seemed to be doing fine, but in the inner cities, where we were not doing as well, those with low immunization rates were devastated by measles. We have had repeated examples of outbreaks that have occurred following introductions into populations that rejected immunization for one reason or another.
Some parents have declared openly that as long as everyone else’s child is immunized, their child does not have to be. This assumes that the rest of the herd will be immunized and thus protect their child from infections. One of the more interesting outbreaks of pertussis occurred in Boulder, Colo. Some parents did not immunize their children in the belief that they were protected by others who had chosen to protect their children. They had miscalculated.
The definition of the herd may change. Most notably, children grow up and move to new, larger herds, eg, school, out-of-home care, college and into a job. We have had examples of epidemics of meningococcus, measles, rubella and mumps and other diseases in these settings. What is more, when these childhood infections occur in adults, these generally are much more severe.
We also must take into account that we now are truly a “global village.” We cannot ignore the fact that childhood diseases that have become uncommon in the United States because of our immunization program are common in many parts of the world, including some “well-developed” countries. This year, there have been importations of measles from Italy, Switzerland, Belgium, India, Israel, China, Germany, Pakistan, the Philippines and Russia (MMWR. 2008;57:893-896). People from abroad come to this country and we travel to theirs. We try to assure that visitors in both directions are protected by immunization but we have innumerable examples that this has been imperfect.
One of the altruistic reasons for the need to be immunized is to protect others, some of whom cannot be immunized with live vaccines or are too young to be vaccinated and thus are vulnerable.
In the current epidemic of measles in London, a death was reported in a 17-year-old with a congenital immune defect, which contraindicated his immunization (Health Protection Report. http://www.hpa.org.uk/hpr.archives/2008/news2508.htm. Accessed Sept. 24, 2008).
In the recent measles outbreaks in the United States, many of the affected infants were too young to immunize (MMWR. 2008;57:893-896). The availability of acellular pertussis for adults has enabled some families to protect the newborns in their “pack” by immunizing or reimmunizing their members.
“For women who have not received TDaP previously (including women who are breast-feeding), TDaP is recommended as soon as feasible in the immediate postpartum period to protect the women from pertussis and reduce the risk for exposing their infants to pertussis” (MMWR. 2006;55:1-47). The child too young to be immunized might be born into one’s own household. Unfortunately, one of the major reasons that some choose not to immunize do so on the basis of their religious beliefs. One can only wonder what religions relinquish their responsibility to their fellow humans?
The concern about the threat of the unprotected in the herd has manifested itself in at least two other ways. Some parents have excluded children who are not protected by immunization from contact with their own children. Although on the surface of it this sounds harsh, is it any different from asking these parents not to bring their children around when they have an acute infection, eg, diarrhea? We exclude children from school and out-of-home care for either evidence of acute infection or lack of immunization.
Some physicians have asked parents who reject immunization of their children to leave their practice. Some feel, if they do not trust me with the decision to immunize, how can they trust me with other decisions concerning their child’s health? Yet another reason is that they do not want unprotected people sitting in their waiting room who might be incubating a vaccine-preventable disease from exposing their patients who are unable to be immunized and therefore unprotected. There are many examples of transmission of vaccine-preventable diseases in an office setting.
Measles
This brings us to measles.
As you probably have heard, during the first half of this year, there have been 131 cases including two sizable outbreaks with cases in 15 states and Washington, D.C. The vast majority of these cases could be linked to cases occurring outside of the United States. These have mainly been in individuals who were unimmunized in most cases because of philosophical or religious objections. Most were home schooled (MMWR. 2008;57:893-896).
Fortunately, measles has been virtually eliminated.
Unfortunately, because it is not visible we no longer are aware of what a devastating disease it can be, particularly in those too young to be immunized.
We can cite the frightening statistics of about 4 million cases, 450 deaths and 4,000 cases of encephalitis annually prior to vaccine. The true impact of this disease can only be appreciated by those of us who made house calls in those days and the parents and grandparents of those children, most of whom who were fortunate enough not to become a statistic. Now, parents are more concerned about the apparent adverse effects of vaccines. It is essential that those of us responsible for the care of children be conversant with both the dangers of these preventable diseases and the real risks of the vaccines to prevent them. The database refuting most of the claims of alleged adverse effects continues to grow (nytimes.com/2008/09/09/opinion/09tue3.html?_r=1&ref=opinion&oref=slogin. Accessed Sept. 24, 2008).
Monitoring adverse effects of vaccines is an ongoing process that is performed with diligence. The information gleaned is used to make changes in our recommendations when these data point to their need. It is important to remember that “the plural of anecdote is not proof” (MMWR. 2008; 57(Early Release);1-47).