单选题
Among the devastating consequences of AIDS
has{{U}} {{U}} 1 {{/U}} {{/U}}its epidemic spread in the
developing world. The disease has caused{{U}} {{U}} 2
{{/U}} {{/U}}suffering, debilitation, loss of life and disruption of
family, social and economic{{U}} {{U}} 3 {{/U}} {{/U}}Because of
the considerable expense and logistical difficulty in providing antiviral drugs
to populations{{U}} {{U}} 4 {{/U}} {{/U}}with the Human
Immunodeficiency Virus{{U}} {{U}} 5 {{/U}} {{/U}}the world, the
biomedical community is looking towards vaccines to help solve this compelling
problem. The search for an AIDS vaccine began more than 15
years ago with great{{U}} {{U}} 6 {{/U}} {{/U}}and high
expectations. With the{{U}} {{U}} 7 {{/U}} {{/U}}of the Human
Immunodeficiency Virus (HIV) as the cause of AIDS, it seemed that a{{U}}
{{U}} 8 {{/U}} {{/U}}would follow closely behind. But despite a
large concerted effort, the problem has proven more difficult than{{U}}
{{U}} 9 {{/U}} {{/U}}, and progress has not{{U}} {{U}}
10 {{/U}} {{/U}}the{{U}} {{U}} 11 {{/U}} {{/U}}hopes.
Here I review the{{U}} {{U}} 12 {{/U}} {{/U}}scientific
obstacles confronting the development of an effective HIV vaccine, and I
consider{{U}} {{U}} 13 {{/U}} {{/U}}strategies to overcome these
obstacles. It is instructive to consider the circumstances that
have{{U}} {{U}} 14 {{/U}} {{/U}}to past successes in vaccine
development. The smallpox vaccine is{{U}} {{U}} 15 {{/U}}
{{/U}}the most successful inventions in the history of{{U}} {{U}} 16
{{/U}} {{/U}}Why, 200 years ago, without the benefit of modern biotechnology,
did the smallpox vaccine succeed so readily while an AIDS vaccine{{U}}
{{U}} 17 {{/U}} {{/U}}elusive? The answer lies in an experiment of
nature that provided, to an astute observer, a clear direction for smallpox
vaccine development. In this classic story of{{U}} {{U}} 18
{{/U}} {{/U}}discovery, Edward Jennet noticed that milk maids who had
previously contracted cowpox were{{U}} {{U}} 19 {{/U}} {{/U}}to
smallpox infection. This observation was the critical event leading to the
finding that the cowpox virus cross-reacted immunologically with the smallpox
virus and could{{U}} {{U}} 20 {{/U}} {{/U}}be used to protect
against smallpox.