By Peter C. Doherty
All prediction is probabilistic. Maybe that statement is unfamiliar. It’s central to the thinking of every scientist, though this is not to the way media commentators like Jenny McCarthy approach the world. Scientists make certain predictions, or recommend courses of action on the basis of the best available evidence, but we realize that there is always an element of risk. Particularly in regard to childhood vaccination, that idea of relative risk can dominate thinking.
What the public health authorities ask is that young parents take a perfectly normal child to their physician or nurse practitioner where they are then exposed to a mild form of virus, or injected with a non-living product derived from some human pathogen. Given the nature of any immune response, the kid may be grumpy for a couple of days due to various chemicals produced by the body, but that’s hopefully the end of it and the child will then be protected from some horrible disease. Where this becomes problematic is that, because most kids in western society are vaccinated and we have sufficient “herd immunity” to protect the others, younger adults have never seen (or experienced) diseases like measles, whooping cough, diphtheria, and so forth that can be truly horrible and even kill.
Then there are often well-publicized reports of vaccinations that went very wrong. For any scientist to give credence to such information they will want to see a proper, statistically valid analysis. But that is not, of course, the world most of us live in. As was pointed out several years back by Stephen Colbert when discussing comments on the human papilloma virus vaccine, many will believe an anecdote from The New England Journal of the Lady I Just Met rather than the statements based in evidence from responsible authorities like the Centers for Diseases Control and Prevention (CDC) or the American Academy of Pediatrics.
Still, things do occasionally go wrong with vaccines. A case in point would be giving the live Sabin polio vaccine to an infant with a hitherto undetected, profound immunodeficiency. Perhaps the child was protected until then by passively acquired maternal antibodies, but as those wane, the risk posed by life-threatening infections would soon become clear. Then there’s a rare genetic condition that can cause a child with fever to go into severe convulsions and become epileptic: that would have happened anyway, but vaccination may bring on the first episode. Convulsions associated with fever are relatively common in small children, usually without untoward sequelae, but there have been situations in which this has happened with childhood vaccines and, as a consequence, the product has been rapidly withdrawn. The massive preponderance of evidence with the standard, long available vaccines of childhood is, though, that any risk of adverse effects is infinitely smaller than the dangerous short- and long-term consequences of contracting the actual disease.
There are some situations where vaccination is mandatory. If, for instance, you want to travel to West Africa, then return without taking the risk of being quarantined, you must take the yellow fever vaccine. It’s a very good and long-established product, so most will be happy to do that when they understand what an awful disease this is. If we were to face a truly horrible pandemic like that imagined in the movie Contagion, I very much doubt that many would refuse an available vaccine, especially for their children.
Emergency situations are where the relative risk equation with vaccines can become a prominent consideration. When we need to get something out very quickly in the face of a major threat, there is just not time for the extensive safety testing that is normally required. Over the decades this has, in practice, been mostly an issue for influenza vaccines. The influenza viruses change constantly and, because they spread with such speed, any vaccine has to be rolled out fast if it is to be of value. Though the protocols used to make the product will be thoroughly tested and approved, modifying the virus proteins slightly (which is what nature actually does) increases the possibility of untoward consequences.
We’ve got better at making influenza vaccines safely over the decades, but there is still a minimal, finite risk, especially with very young children. Any danger associated with injecting large amounts of viral protein and adjuvants (substances that promote immunity) is eliminated when we turn to the alternative of low-dose, live attenuated vaccines that are just dropped in the nose. Such products are, though, not approved in the USA for the elderly, the population that is generally at greatest risk from influenza. I take the opportunity to be injected with the killed vaccine every year.
Vaccine research is a very dynamic area of science, with the focus being on developing better and safer products. There are also infections where no vaccine is currently available, particularly for HIV and Hepatitis C virus. The same is true for the noroviruses that can destroy your cruise ship vacation. In general, there is little problem of vaccine acceptance in developing countries where the common infectious diseases of childhood are still at a high prevalence. But we can’t be complacent, and we’re seeing far too high an incidence of, particularly, whooping cough and measles in western societies, leading to some deaths. Vaccinating children is a collective responsibility. The very young and the elderly are particularly at risk from, especially, the respiratory infections of childhood. Keeping herd immunity high protects all of us.
Relative risk defines the human experience. Most of us don’t think about it when strapping our kids into a car safety seat, but what we are in effect doing is acknowledging that we are trying to minimize a finite risk. There may also, for example, be the danger that restraining a small child could lead to their being trapped in a serious accident. But we acknowledge that, following an impact, the probability of being thrown out the window or against a hard surface is infinitely higher. We also allow children to take risks climbing on playground equipment, riding bicycles, or driving quad bikes because we understand that they have to grow in judgment so that they can live satisfactorily in a potentially dangerous world. Emphasizing personal freedom, some parents will not mandate that their kid wear a bike safety helmet, though the relative risk justification for that could not be more obvious.
With infectious diseases, childhood vaccination offers the infinitely lower risk alternative. And it’s not just for the short term. Imagine how you would feel if, for example, your unimmunized, pregnant daughter contracts rubella. Many diseases, like measles, are much worse in previously un-exposed, or unvaccinated, adults. Will your son who contracted mumps while travelling in a developing country thank you if he suffers testicular atrophy, and even sterility, as a consequence?
If you are a young adult who is not sure whether your parents had you vaccinated, check with your local physician or travel clinic before you head off to the more exotic parts of the world. Do that at least six to eight weeks ahead of time so that you can be given the standard childhood (and any other) vaccines, and there is time for them to take effect. Nothing in life is absolutely safe, but much of what we do as sensible human beings is concerned with minimizing avoidable risk.
Peter C. Doherty is Chairman of the Department of Immunology at St. Jude’s Children’s Research Hospital, and a Laureate Professor of Microbiology and Immunology at the University of Melbourne. He received the Nobel Prize in Medicine in 1996. In addition to Pandemics: What Everyone Needs to Know (September/OUP), he is the author of The Beginner’s Guide to Winning the Nobel Prize: Advice for Young Scientists, Their Fate is Our Fate: How Birds Foretell Threats to Our Health and Our World, published in Australia as Sentinel Chickens: What Birds Tell us About Our Health and the World; and A Light History of Hot Air. Follow him on Twitter at @ProfPCDoherty.