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1.3 What is herd immunity?


This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax lynng@alsirat.com with numerous contributions by others.

1.3 What is herd immunity?

If a large enough percentage of a population is immune to a disease,
their immunity protects the rest of the "herd."

Some discussion of this concept from misc.kids follows:

From: pburch@cmb.bcm.tmc.edu (Paula Burch)

>Paula Burch (pburch@cmb.bcm.tmc.edu) wrote:
>: If one child remains unvaccinated, but all other children are
>: vaccinated, the one child who does not get vaccinated is pretty safe
>: from getting the disease. If many children remain unvaccinated,
>: epidemics occur, and children die needlessly.

dolson@ucsd.edu (Mark Dolson) writes:
>This is exactly what occured with measles in the 80's, BTW. Fewer
>vaccinated, and the incidence skyrocketed, with resulting complications
>of eye problems, etc, and even some deaths. I agree that people who
>let their children remain unvaccinated are riding on the backs of
>everyone that does vaccinate, and I resent it.

mblum@world.std.com (Cerebus) writes:

To be fair, most of the major outbreaks (as well as most of the
serious complications) were and are on college campuses, and occurred
*not* because of failure to vaccinate, but because the vaccine that
was given to kids between '69-'76 turned out not to give total
immunity. Many kids who were vaccinated were victims of measles,
before people became conscious that it was necessary for many teens
and young adults to be re-vaccinated.

I had measles my first year in college, after vaccination at the
appropriate age. After that outbreak my college began requiring
re-vaccination. But it is not technically correct to blame the
measles outbreak on the failure of parents to vaccinate.

It's true that that's what happended in that case, but it's not true for
other cases, in which failure to vaccinate has been a major factor:

"The nation [U.S.] has experienced a marked increase in measles cases
during 1989 and 1990. Almost one half of all cases have occurred in
*unvaccinated* preschool children." (JAMA. 1991 Sep 18. 266(11).
P 1547-52.)

"Beginning in October, 1990, a large measles outbreak involving
predominantly *unvaccinated* preschool age children occurred in
Philadelphia. By June, 1991, 938 measles cases had been reported to
the Philadelphia Health Department. In addition to these cases, 486
cases and 6 measles-associated *deaths* occurred between November 4,
1990, and March 24, 1991, among members of 2 Philadelphia church
groups that do not accept vaccination." (Pediatr-Infect-Dis-J. 1993 Apr.
12(4). P 288-92.)

"In 1989 and 1990 the United States experienced a measles epidemic with
more than 18,000 and 27,000 reported cases. Nearly half of all persons
with measles were *unvaccinated* preschool children under 5 years of age."
(Am-J-Public-Health. 1993 Jun. 83(6). P 862-7.)

Measles is bad, but I'm more concerned myself about pertussis
(whooping cough):

"From 1980 through 1989, 27,826 cases of pertussis were reported to
the Centers for Disease Control....Infants less than 2 months of age
had the highest reported rates of pertussis-associated hospitalization
(82%), pneumonia (25%), seizures (4%), encephalopathy (1%), and *death*
(1%)." (Clin-Infect-Dis. 1992 Mar. 14(3). P 708-19.) [Many of these
infants would not have caught the disease if enough older children were
appropriately vaccinated.]

"Two large *epidemics* of pertussis occurred in Britain during 1977-79
and 1981-83." (Commun-Dis-Rep-CDR-Rev. 1992 Dec 4. 2(13). P R155-6.)

This explains the herd immunity concept rather well:

"The epidemiology of whooping cough [pertussis] in Denmark is
described on the basis of the notified cases of the disease. The
frequency of whooping cough has decreased to approximately one
sixteenth of the previous level in children following the
introduction of vaccination for whooping cough in 1961....deaths
from whooping cough still occurred in the eighties, all of these
among *unvaccinated* infants. The risk of whooping cough in an
*unvaccinated* child is approximately one sixth of the risk prior
to introduction of vaccination. In a vaccinated child, the risk,
as judged from the notified cases, is one twentieth of the risk
during the time prior to introduction of vaccination. In all age
groups "herd immunity" is considered to have contributed
considerably to the reduced incidence. The incidence in Denmark
is, however, high compared with the incidence in some other
industrialized countries. A vaccination programme with more
numerous whooping cough vaccinations...may be recommended on the
basis of the strategy aimed at keeping the incidence of whooping
cough, and thus the risk of exposure, as low as possible."
(Ugeskr-Laeger. 1990 Feb 26. 152(9). P 597-604.)

Paula Burch
not speaking for Baylor College of Medicine


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