First, what is fertility? To most people, fertility is the ability to have children; the opposite of infertility. To demographers and epidemiologists, fertility is the number of children a person has. According the U.S. Census Bureau, fertility is the number of children ever born to a person (referring to the number of live births). "Typically it is asked of women age 15 to 50, or women of all ages but some surveys ask men how many children they have fathered." A fertility rate is the number of children born in a population over a period of time, usually per 1,000 people per year. To demographers and epidemiologists, the ability to have children in known as fecundity rather than fertility.
In his book, The End of Poverty, Jeffery Sachs wrote, "I have been asked dozens of times if help for Africa would ultimately backfire in an even greater population explosion. Would greater child survival rates not translate into more adult hunger and suffering?"
I've been asked similar questions about my interest in tropical medicine and my volunteer work in Africa. Sachs goes on to discuss the demographic transition, a phenomenon that has occurred in every industrialized country and is occurring in developing countries; that is, as standards of living improve, people have fewer children.
There are a several theories about why the demographic transition occurs. The primary theory links decreasing fertility to decreases in child mortality; parents have fewer children when more of their children survive to adulthood.
We can see this occurring today. In the graph below, I've used World Bank data from 2012 to plot child mortality rates (the number of deaths of children under 5 years of age per 1,000 population per year) against fertility rates. Each dot represents a country. In countries in which child mortality is low, women have fewer children. In countries with high death rates for children under 5 five years of age, fertility rates are higher.
Childhood vaccines are associated with improved child survival. Measles immunization has been shown to decrease all cause mortality. In the graph below I've plotted mortality of children less than 5 years of age against the percent of children 12 to 23 months of age who have received measles vaccines. Under 5 mortality is higher in countries with lower measles immunization coverage than in countries with high measles vaccine coverage.
Of course, there are a number of other factors associated with both decreasing child mortality and decreasing fertility including higher costs of raising a child (e.g., education), transitioning from agricultural to manufacturing markets and urbanization (people who live in cities tend to have fewer children than those living in rural areas), more women working outside of the home, changes in social norms, and, of course, access to contraception.
Although there have been improvements in the prevention and treatment of malaria, most malaria deaths occur in children under 5 years in sub-Saharan Africa. This is one of the reasons fertility remains high in Africa.
In 2010, Bill Gates made a passing reference to the demographic transition during a TED Talk on energy and climate:
First, we've got population. The world today has 6.8 billion people. That's headed up to about nine billion. Now, if we do a really great job on new vaccines, health care, reproductive health services, we could lower that by, perhaps, 10 or 15 percent, but there we see an increase of about 1.3.
Gates had discussed the demographic transition in the Bill and Melinda Gates Foundation 2009 annual letter:
Two things caused this huge reduction in the death rate. First, incomes went up, and with that increase, nutrition, medical care, and living conditions improved. The second factor is that even where incomes did not go up, the availability of life-saving vaccines reduced the number of deaths. For example, measles accounted for 4 million children’s deaths in 1990, but fewer than 250,000 in 2006.
A surprising but critical fact we learned was that reducing the number of deaths actually reduces population growth. Chart 3 shows the strong connection between infant mortality rates and fertility rates. Contrary to the Malthusian view that population will grow to the limit of however many kids can be fed, in fact parents choose to have enough kids to give them a high chance that several will survive to support them as they grow old. As the number of kids who survive to adulthood goes up, parents can achieve this goal without having as many children.
|Bill and Melinda Gates Foundation, 2009|
Unfortunately, Gates' TED Talk comments were misinterpreted by several people in the blogosphere to mean that vaccines cause infertility.
To answer the question in the title of this post, "do vaccines reduce fertility?" I will violate Betteridge's law of headlines and say, yes, vaccines reduce fertility, but not in the way some people would like you to believe.
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van den Ent, M. M. V. X., Brown, D. W., Hoelstra, E. J., Christie, A., & Cochi, S. L. (2011). Measles mortality reduction contributes substantially to reduction of all cause mortality among children less than five years of age, 1990-2008. Journal of Infectious Diseases, 204(Supple. 1), S18-S23. http://jid.oxfordjournals.org/content/204/suppl_1/S18.long.
World Bank. (2014). Data. http://data.worldbank.org.