Re: Why is there no vaccine against the common cold, gonorrhea, AIDS or herpes?

This is really a good question but the reasons are very complex. In 
general, there are several criteria for an effective vaccine. A successful 
vaccine must be safe, protective, give sustained protection, induce 
neutralizing antibody, and induce protective T cells (particularly for 
intracellular pathogens). Practical considerations such as low 
cost-per-dose, biological stability, ease of administration and low 
side-effects are important too. 

Influenza is caused by an RNA virus. Influenza often occurs in pandemics (a 
worldwide epidemic). When you receive a flu vaccine, you are actually 
getting a dose of the virus itself that has been chemically deactivated. In 
this way, your immune system is stimulated and antibodies that can be used 
against the virus are built up without causing disease. Vaccine makers 
actually grow the virus in chicken eggs. Since flu shots are made available 
in the Fall, and drug companies need about six months to produce the 
vaccine, they need to decide in the Spring what virus they are going to 
grow for next season's flu shots. In February or March a team of 
international influenza experts make their recommendation for next years 
vaccines based on last years findings. The flu vaccines may not be a 
perfect vaccine because of the at least six month delay. Since the virus 
could cause the very severe pandemics problem and the production of this 
virus is relatively easy and cheap, in my understanding, those might be the 
major reasons for this vaccination. However, because of its constant 
mutation by so called "antigenic shift (modifications in protein coat of 
virions)" and "antigenic drift (mutation of amine acids when passed through 
the population", it is very difficult to make a vaccine to prevent all the 
strains. We can only do our best to prevent pandemics. In the past years, 
the vaccination seems effective in the vaccinated populations.

The specific requirements for successful vaccination vary according to the 
nature of infectious organisms. Different organisms have different 
difficulties in the vaccine development. You mentioned cold (common cold), 
gonorrhea, AIDS and herpes. The common cold was mainly caused by 
Rhinoviruses (another RNA virus). But there are more than 100 different 
strains. Another group of virus, coronaviruses, are responsible for about 
15% of all colds in adults. A variety of other viruses are responsible for 
about 10% of common colds. Therefore, vaccination is not expected. A major 
limitation in the development of gonococcal vaccines has been the lack of 
an animal model. Several hundred male volunteer subjects are usually needed 
for an experimental infection model. This significantly delayed the 
development of the gonococcal vaccines. As you know, AIDS is caused by 
human immunodeficiency virus. HIV is a retrovirus. The genetic variability 
of HIV has thus far hampered the development of an AIDS vaccine. HIV 
replicates every 30 hours. Thus, a normal mutation rate allows new strains 
of HIV to occur within several weeks. Scientists are exploring many 
different ways to make an AIDS vaccine. One of the approaches uses 
anti-idiotypic antibodies. In this approach, antibodies to CD4, a HIV 
binding site on T lymphocytes, are used as an antigen. HIV binds to T 
lymphocytes by gp120 (a protein on HIV)-CD4 (a protein on T lymphocytes) 
interaction for initiating the infection. Antibodies raised to this 
idiotype (binding site ) should resemble the molecular configuration of 
CD4. In sufficient quantity, anti-idiotypic antibodies could then bind HIV 
particles by gp120-CD4 type interactions. Therefore, it blocks the virus 
attachment to intact CD4 T cells. Herpesviruses present difficult 
challenges in vaccine development because of their ability to evade immune 
clearance. Herpes simplex virus (HSV) produce not only a primary infection, 
but also latent and recurrent infections.  

I hope my brief and simplified explanations can answer your very complex 
question. When the 21st century is coming, we are proud of many vaccines, 
our greatest triumphs of modern immunology in the current century. 
Nevertheless, there are many important infectious diseases, as you 
mentioned, for which there is still no effective vaccine. Scientists are 
working very hard for the development of these vaccines.