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.
References:
Bloom, D. E., Canning, D., & Weston, M. (2005). The value
of vaccination. World Economics, 6(3),
15-39.
Brauner-Otto, S., Axinn, W., & Ghimire, D. (2007). The
spread of health services and fertility transition. Population Studies Center Research Report 07-619. http://www.psc.isr.umich.edu/pubs/pdf/rr07-619.pdf.
Conley, D., McCord, G. C., & Sachs, J. D. (2007).
Africa's lagging demographic transition: evidence from exogenous impacts of
malaria ecology and agricultural technology. National Bureau of Economic Research Working Paper Series No. 12892.
http://www.nber.org/papers/w12892.
Gates, B. (2009). Bill
and Melinda Gates Foundation annual letter 2009. http://www.gatesfoundation.org/who-we-are/resources-and-media/annual-letters-list/annual-letter-2009.
Gates, B. (2010). Innovating
to zero. TED Talks. http://www.ted.com/talks/bill_gates/transcript,
http://www.ted.com/talks/bill_gates.
Greenwood, J. & Sesharid, A. (2001). The U.S.
demographic transition. AEA Papers and Proceedings,
92(2), 153-159. http://www.econ.wisc.edu/~aseshadr/publication_pdf/usdt.pdf.
Newson, L., Postmes, T., Lea, S. E. G., & Webley, P.
(2005). Why are modern families small? Toward an evolutionary and cultural
explanation for the demographic transition. Personality
and Social Psychology Review, 9(4), 360-373.
Omran, A R. (1971). The epidemiologic transition. Milbank Memorial Fund Quarterly, 49(4),
509-538.
Sachs, J. D. (2005). The
end of poverty: economic possibilities for our time. New York: The Penguin
Press.
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.
