I’ve written two blog postings about vaccination. One outlined two factors that may have contributed to recent outbreaks of whooping cough (pertussis) in the United States: increased rejection by parents of vaccination of their children against whooping cough, and replacement of a vaccine containing killed pertussis cells by an acellular preparation with fewer side effects. Members of the anti-vaccination movement argued that increased prevalence of whooping cough was due solely to the new vaccine being less effective than the original vaccine, and not at all to vaccine rejection by parents. I disputed this argument, on grounds that causation is complex and that both vaccine effectiveness and rejection likely contributed to whooping cough outbreaks
My second posting described the 1977 success of a worldwide vaccination program to eliminate smallpox and the current status of attempts to eliminate polio. In that posting, I reported that there were 359 cases of polio worldwide in 2014 compared to 350,000 twenty-six years earlier. In 2018, there were only about 30 identified cases of infection with wild polio virus, all in Pakistan and Afghanistan.
Vaccination is in the news again because of a rash of outbreaks of measles in the United States. The year 1963 was a turning point for measles in the U.S. Before 1963, almost all children got measles by age 15 and there were at least 400 to 500 deaths from measles annually (400 to 500 deaths were reported each year to the Center for Disease Control and Prevention [CDC]; the total would have been greater because not all deaths were reported). Scientists began developing a measles vaccine in the 1950s; this was first deployed in 1963 and replaced by an improved version in 1968. Cases of measles in the U.S. dropped rapidly, and the CDC declared in 2000 that measles was no longer continuously present in the U.S., although cases can be brought into the U.S. by travelers each year.
In most years since 2010, the CDC reported fewer than 200 cases, but there were 667 in 2014, 372 in 2018, and 387 already in the first three months of 2019 (465 by April 4). Many of these were not isolated cases, but occurred in outbreaks in which the disease was apparently introduced by a traveler from abroad and then spread through contacts between the traveler and unvaccinated people and between those who got the disease from the traveler and additional unvaccinated people. For example, 213 cases between January and March 2019 occurred in Brooklyn, New York, and 73 cases occurred in the Vancouver area in southern Washington. Most cases of measles in these outbreaks occurred in unvaccinated children.
Public health departments in the various states of the U.S. are responsible for setting guidelines for vaccination of residents. Vaccines for measles, mumps, and rubella are combined in the MMR vaccine, which the CDC recommends for children in two doses, at ages of 12-15 months and 4-6 years. All states require that children attending public schools be vaccinated, and many also require vaccination for children in daycare centers and private schools. All states also allow exemptions for medical reasons, and some allow exemptions for religious or philosophical reasons.
Medical exemptions to vaccination requirements are non-controversial, since some children may not be able to tolerate vaccination because they have compromised immune systems or allergic reactions to some constituent of a vaccine. Religious and philosophical exemptions are more problematic, and some states are eliminating one or both of these options. There is a vigorous anti-vaccination movement in the U.S., and anti-vaxxer parents who don’t wish to home-school their children may claim religious or philosophical exemptions to vaccination requirements so they can send their kids to school with other children. Indeed, legislators in 19 states have introduced bills to expand options for exemption from vaccination, although no state has passed such legislation since 2003.
Suppose you have a 3-year-old daughter who can’t be vaccinated against measles because she is allergic to one of the components of the MMR vaccine. You wish to send her to a preschool where some children have received their first MMR dose while others have not been vaccinated because their parents claim a religious or philosophical objection. How dangerous is this for your daughter? The answer depends on how infectious the measles virus is and on what percentage of people in the area where you live are able to transmit the disease because they haven’t been vaccinated. In fact, measles is extremely infectious – before the introduction of vaccination in 1963, a person infected with measles would transmit the disease to about 12 new individuals. This means that measles will spread in a population unless more than about 92% of the population is vaccinated. Conversely, if more than 92% of the population is vaccinated, measles likely won’t spread because the disease will have run its course in the initial infected person (making him no longer infectious) before he has a chance to encounter a vulnerable, unvaccinated person.
The process outlined in the last paragraph is called herd immunity – unvaccinated individuals may be effectively immune if a large enough percentage of the population is vaccinated that the disease can’t spread. The percentage that must be vaccinated depends on the infectiousness of the disease; smallpox is much less infectious than measles and whooping cough, which is why public health workers were able to eliminate smallpox by a worldwide vaccination campaign, but we still have outbreaks of measles and whooping cough. In Tools for Critical Thinking in Biology (TCTB), I outlined a simple algebraic model of herd immunity. News reports of the current measles outbreaks in the U.S. often mention herd immunity as a protection for children and others who can’t be vaccinated, sometimes without explaining how it works. If you don’t have access to TCTB, Pam Belluck and Adeel Hassan provided an excellent overview of measles, including the process of herd immunity, in a recent column in the New York Times.
Returning to your hypothetical 3-year-old about to enter preschool, thinking about herd immunity will help you assess how vulnerable she is to contracting measles if you live in a place where a measles outbreak is underway, like Brooklyn or Vancouver Washington in spring 2019. Such an outbreak likely means that fewer than about 92% of the children in the epicenter of the outbreak have not been vaccinated and that measles is spreading among these unvaccinated children. This may include unvaccinated children in your chosen preschool, who won’t be able to transmit measles to their vaccinated playmates in the preschool, but will be able to transmit the disease to your daughter who is unable to be vaccinated.
In TCTB, I explained a moral question that arises from a basic mathematical model of herd immunity. As a parent, you have a moral obligation to protect your children. Anti-vaxxers argue that this justifies refusing vaccination for their children, but this argument ignores an overwhelming amount of scientific evidence for the benefits of vaccination. As members of society, we have an additional moral obligation to avoid harming others. This includes children who can’t be vaccinated for medical reasons, which these days is only a problem if such children live in a community where an outbreak occurs and many parents have refused to vaccinate their children on religious or philosophical grounds.
Anti-vaxxers resist having their children vaccinated for various reasons, some with a modicum of rational justification, others that ignore or deny abundant scientific evidence. One common reason for refusing vaccination against measles is the claim that this can cause autism in children. This claim is based on fraudulent research reported by Andrew Wakefield and his colleagues in 1998. Even ignoring the fraud involved in Wakefield’s research, the hypothesis of a causal association between childhood vaccination and autism has been repeatedly refuted in subsequent research, most recently in a study of 657,461 children born in Denmark between 1999 and 2010. Some anti-vaxxers argue that children don’t need to be vaccinated against measles because they can be treated with antibiotics if they get the disease, ignoring the fact that measles is a virus and that antibiotics don’t affect viruses. Other anti-vaxxers argue that natural immunity from getting a disease is somehow better than “artificial” immunity from vaccination. The best example to refute this claim comes from considering chicken pox. The virus that causes chicken pox, often in childhood, persists throughout life in nerve cells in the brain, and can cause a very painful skin disease called shingles in adults. Older adults are especially vulnerable to shingles, which can cause blindness or hearing loss or contribute to cardiovascular disease, in addition to producing a painful rash.
Frank Bruni published an opinion piece in the New York Times on March 9, 2019, in response to news of measles outbreaks in preceding weeks. This is how he summarized his main point: “The parents who are worried or sure about grave risks from vaccines reflect a broader horror that has flickered or flared in everything from the birther movement to “Pizzagate,” that nonsense about children as Democratic sex slaves in the imagined basement of a Washington pizza joint. Their recklessness and the attendant re-emergence of measles aren’t just a public health crisis. They’re a public sanity one, emblematic of too many people’s willful disregard of evidence, proud suspicion of expertise and estrangement from reason.” Just last week (April 2, 2019), the tweeter-in-chief added another example of estrangement from reason in his claim that noise made by windmills causes cancer.