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You can’t hide in the herd

Can you hide in the herd?  Well, I suppose the title has given away the punch line.

Herd immunity is a fascinating effect, and one of the mainstays of a public vaccination effort.  The idea is that if enough people in the community are immune to a particular disease, then those who are susceptible will rarely come into contact with a person who is contagious, and the disease will be unable (or find it difficult) to spread.  This results in a greatly reduced risk of infection for the entire population regardless of their individual immunity.

This has lead to the belief that because of the protection of the herd’s immunity, individuals now have the option to avoid even the minimal cost and risk of vaccination while having the same reduced risk of infection as if they had vaccinated.

Let’s set aside the fact that that there are people who have no choice but to rely upon herd immunity as their sole line of protection against these infections.  Forget that there is a threshold below which herd immunity collapses, and that our current vaccination rates tend to be right on the cusp of that threshold.  Pay no attention to the fact that the personal decision to not vaccinate deprives others of their sole protection from these infections.  Finally, ignore the ethics and self-defeating nature of benefiting from the sacrifice of others while simultaneously eroding the efficacy of the herd immunity being exploited.  On a small enough scale, doesn’t the tactic of hiding in the herd provide the same protection as getting vaccinated without incurring the minimal risk of vaccination?

Not so much.

Countless reports of outbreaks around the world consistently describe a disproportionate number of infections during vaccine-preventable outbreaks occurring in the unvaccinated.  For instance, in Indiana in 2005 an outbreak of measles infecting 34 people was traced to a 17-year-old unvaccinated girl who had contracted measles in Romania.  In that outbreak 94% of the infected were unvaccinated.

This scenario was repeated during a measles outbreak in California in 2008 where 12 children were infected.  The index case, an unvaccinated boy who had traveled to Europe, infected both of his siblings, five schoolmates, and four children from his pediatrician’s office.  None of the children were vaccinated, though three of the four infected in the office were too young to have been vaccinated.  Zero vaccinated children contracted measles.

Reports such as these highlight how quickly these diseases can spread, how easily the unvaccinated are infected, and the limited effectiveness of voluntary isolation.  They also demonstrate the effectiveness of herd immunity in containing the infection, and rather strongly suggest that, even with intact herd immunity, the vaccinated and unvaccinated are not at equal risk of infection.

How much greater is the risk, though?  10%?  50%?  Perhaps fully twice as likely to be infected?  The magnitude matters to parents (and physicians) who are weighing the risks of vaccines and their corresponding diseases.

A group from Kaiser Permanente of Colorado has attempted to help put a number on that increased risk.  Within the last year they have provided two matched case-control studies that quantify the magnitude of the risk children incur because of vaccine refusal.

Their first study found that the act of refusing to vaccinate against pertussis (whooping cough) placed children at a 23 times greater risk of contracting pertussis.  That’s a 23 fold-increased risk of a disease that, in children under 12 months of age from 2000-2004 in the US caused 62.8% to require hospitalization, 55.8% to have apnea, pneumonia in 12.7%, and death in 0.8%.

Their second study, published just this month and following the same format as the first, focused instead on the risk of varicella (chickenpox) infection after vaccine refusal.  Here they identified an 8.6-fold increased risk of infection with a disease that as recently as 1995 (when the vaccine was released), tallied 3,000,000 infections, 10,000 hospitalizations, 4,000 cases of pneumonia, 600 cases of encephalitis and 100 deaths per year.

These findings further reinforce the fact that even in a community with intact herd immunity, the choice to remain unvaccinated places children at a markedly higher risk than their vaccinated counterparts.  The delusion that hiding children within the herd provides them with protection even remotely equal to vaccination must be abandoned.

It bears to be stated again, frankly and clearly.  The choice to refuse a vaccine, to “hide in the herd,” is an active decision to accept a markedly higher risk of infection, its complications, the associated medical costs and lost wages, the responsibility of spreading the disease to others should an infection occur, and to choose to undermine the very herd immunity on which we all depend.

Parents want to be fully informed about the medical decisions they make for their children, and rightfully so.  To that end, we do everyone a disservice by allowing the public discussion to be dominated by the risks of vaccines to the exclusion of other equally important topics, including the risks of not vaccinating.  Studies such as these are a needed and welcome addition to the literature, and should provide a valuable insight for people wanting to make a properly informed decision.

Posted in: Science and Medicine, Vaccines

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