Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
236
The eradication of smallpox is a medical success story.
It has been more difficult to repeat this success with
other infectious diseases. This is partly because of the
more unstable political situation since the late 1970s
and 1980s, particularly in Africa, Latin America and
parts of Asia such as Pakistan. Public health facilities
are difficult to organise in developing countries with
poor infrastructure, few trained personnel and limited
financial resources. They are almost impossible to
maintain during periods of civil unrest or during a war.
Preventing measles
Measles is a preventable disease and one that could be
eradicated by a worldwide surveillance and vaccination
programme. However, a programme of one-dosevaccination
has not eliminated the disease in any country,
despite high coverage of the population. This is explained
by the poor response to the vaccine shown by some
children who need several boosters to develop full
immunity. In large cities with high birth rates and shifting
populations, it can be difficult to give boosters, follow up
cases of measles and trace contacts. Migrants and refugees
can form reservoirs of infection, experiencing epidemics
within their communities and spreading the disease to
surrounding populations. This makes measles a very difficult
disease to eradicate, even with high vaccination coverage.
Measles is highly infectious and it is estimated
that herd immunity of 93–95% is required to prevent
transmission in a population. As the currently available
vaccine has a success rate of 95%, this means that the
whole population needs to be vaccinated and infants
must be vaccinated within about eight months of birth.
Many countries achieve up to 80% or more coverage with
measles vaccination (Figures 11.17 and 11.18).
The Americas have been free of endemic measles since
2002 with any cases being the result of someone bringing
in the disease from somewhere else in the world. With an
estimated coverage of only about 75% in Africa, India and
South-East Asia, it is likely that the disease will still persist
for many years to come.
Figure 11.17 The success of immunisation programmes relies
on people, such as these Red Cross workers in Nairobi, Kenya,
ensuring that all families know when and where vaccinations
are available.
QUESTION
11.19 a Distinguish between herd immunity and
ring immunity.
b Explain the biological reasons for the difficulty
in developing successful vaccines for cholera,
malaria and TB.
Number of cases in millions
5
100
90
4
80
70
3
60
50
2
40
30
1
20
10
0 0
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
number of cases
Year
immunisation –
official coverage
immunisation –
WHO/UNICEF estimate
Figure 11.18 The global measles immunisation programme coordinated by the WHO, which has caused the number of cases
reported each year to decrease significantly.
Immunisation coverage as percentage
of the population