A vaccine is an antigenic preparation used to produce active immunity to a disease, in order to prevent or ameliorate the effects of infection by any natural or 'wild' strain of the organism. The term derives from vaccinia, the infectious viral agent of cowpox, which, when administered to humans, provided them protection against smallpox.
Vaccines may be living, weakened strains of viruses or bacteria which intentionally give rise to inapparent-to-trivial infections. Vaccines may also be killed or inactivated organisms or purified products derived from them.
There are three types of traditional vaccines:
A number of innovative vaccines are also in development and also in use:
In recent years a new type of vaccine, created from an infectious agent's DNA called DNA vaccination, has been developed. It works by insertion (and expression, triggering immune system recognition) into human or animal cells, of viral or bacterial DNA. These cells then develop immunity against an infectious agent, without the effects other parts of a weakened agent's DNA might have. As of 2003, DNA vaccination is still experimental, but shows some promising results.
The immune system recognizes vaccine agents as foreign, destroys them, and 'remembers' them. When the virulent version of an agent comes along, the immune system is thus prepared to respond, by (1) neutralizing the target agent before it can enter cells, and (2) by recognizing and destroying infected cells before that agent can multiply to vast numbers.
Vaccines have contributed to the eradication of Smallpox, one of the most contagious and deadly diseases known to man. Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are no where near as common as they were just a hundred years ago. As long as the vast majority of people are vaccinated it is much more difficult for an outbreak of disease to occur, let alone spread. This effect is called herd immunity.
See article: Vaccine controversy
Opposition to vaccination, from a wide array of vaccine critics, has existed since the earliest vaccination campaigns: .
Prior to 1997, a number of vaccines, including those given to very young children, used thimerosal, a preservative that metabolizes into ethylmercury. It is used in some influenza, DTP (diphtheria, tetanus and pertussis) vaccine formulations.
In the late 1990s, controversy over vaccines escalated in both the US and the United Kingdom when a study, published in the respected journal Lancet, by Dr. Andrew Wakefield claimed a possible link between bowel disorders, autism and MMR vaccine, and urged further research . His report garnered significant media attention, leading to a drop in the uptake of the MMR vaccine in the UK and some other countries. The study garnered criticism for its small sample size, and for failing to use healthy controls. In response to the controversies a number of studies with larger sample sizes were conducted, and failed to confirm the findings EG:  . In 2004, 10 of the 13 authors of the original Wakefield study retracted the paper's interpretation, stating that the data were insufficient to establish a causal link between MMR vaccine and autism . Also in 2004, the United States' Institute of Medicine reported that evidence "favors rejection" of any a link between vaccines containing thiomerosal, or MMR, and the development of autism .
Some people refuse to immunize themselves or their children, because they believe vaccines' adverse side effects outweigh their benefits. A variation of this reasoning is that not enough is known of the adverse effects to determine whether the potential benefits make the risks worthwhile. Since most people are vaccinated against contagious and potentially fatal diseases, the chances of someone who is not vaccinated becoming ill is a good deal smaller than it might be if their opinion was held by more people. Thus it could be argued that they reap some of the benefits of vaccines, through herd immunity, without assuming the risks those who choose to vaccinate do.
Advocates of routine vaccination argue that side effects of approved vaccines are either far less serious than actually catching the disease, or are very rare, and argue that the calculus of risk/benefit ratio should be based on benefit to humanity rather than simply on the benefit to the immunized individual. The main risk of rubella, for example, is to the fetuses of pregnant women, but this risk can be effectively reduced by the immunization of children to prevent transmission to pregnant women.
One challenge in vaccine development is economic: many of the diseases most demanding a vaccine, including HIV, Malaria and Tuberculosis, exist principally in poor countries. Pharmaceutical firms and biotech companies have little incentive to develop vaccines for these diseases because there is so little revenue potential. Most vaccine development to date has relied on "push" funding by government and non-profit organizations, of government agencies, universities and non-profit organizations. To date, there has been very little involvement of private industry on a commercial basis.
Many researchers and policymakers are calling for a different approach, using "pull" mechanisms to motivate industry. Mechanisms such as prizes, tax credits, or advance market commitments could ensure a financial return to firms that successfully developed an HIV vaccine. If the policy were well-designed, it might also ensure that people have access to a vaccine if and when it is developed.