The Value of Vaccines: An Overview

The value of vaccines

Vaccines are one of the greatest contributions made by the field of medicine towards improving public health, dramatically reducing the burden of infectious diseases worldwide.

Vaccines aim to activate the immune system to generate a memory response to a particular organism that will protect the individual form the disease following natural exposure to that organism. The earliest reports of vaccination against smallpox date back to 1000AD when the practice of variolation was first described in China. This involved rubbing fluid from the pustule of an infected individual into a scratch on the skin of a health individual. This practice spread to Europe via Turkey, and then onto England and America. Variolation was not without risks, at times leading to infection and death. In 1796, Edward Jenner made an improvement to the practice of variolation, ushering in the modern era of vaccination. He inoculated a boy with the fluid from a cowpox lesion on the hand of a milk maid. The cowpox, a less virulent organism, induced immunity that offered sufficient cross-protection to smallpox, thus establishing the practice of using an attenuated form of an infectious agent to induce immunity.

Attenuated and inactivated vaccines

Vaccines can be broadly divided into live attenuated vaccines and inactivated vaccines. Live attenuated vaccines contain living organisms that have been modified in order to remove their ability to cause disease, while maintain the elements that induce an immune response. This type of vaccine is more frequently developed for viruses than bacteria as viruses are more easily attenuated. Live attenuated vaccines induce both humoral and cellular immunity that should be long –lived, similar to immunity induced by a natural infection. As these vaccines contain live organisms, they need to be transported and sorted at fridge temperature (2-8?C). Maintenance of this cold chain is essential to ensure efficacy of the vaccine. A drawback of live attenuated vaccines is that they should not be administered to pregnant women, due to a theoretical risk to the fetus, or to severely immunocompromised individuals. If different live vaccines are being administered, they should be given either together or at least 30 days apart in order to ensure an optimal immune response. Examples of live attenuated vaccines include measles, rubella, mumps and varicella zoster.

Inactivated vaccines may consist of the whole organism that has been killed using heat, radiation or certain chemicals; or subunits (portions) of the organism that are known to induce immunity. The vaccines do not induce as robust an immune response as that induced by live attenuated vaccines, and generally require multiple injections and boosters in order to maintain protective immunity. These vaccines are, however, safe to administer to pregnant women and immunosuppressed individuals. Example of inactivated vaccines include hepatitis A, hepatitis B, influenza and HPV.

Conjugate vaccines

Conjugate vaccines are used for certain bacteria. The vaccine is composed of parts of the bacterial coat, but as these parts may not induce a good immune response, they are combined with the carrier protein that improves the immune response. An example of a conjugate vaccine is the pneumococcal vaccine.

Toxoid vaccines are prepared from inactivated or weakened forms of the toxin produced by certain bacteria. An example of a toxoid vaccine is the tetanus vaccine. The organism only forms part of the vaccine. The rest of the vaccine is made up of suspending fluid, preservatives, antibiotics and adjuvants. The suspending fluid may be saline or may contain proteins derived from the medium used for production of the vaccine, e.g. eggs or cell culture. Preservatives such as gelatine provide antiseptic properties.

Adjuvants are substances that increase the immune response to the vaccine. These various components may be responsible for hypersensitivity reactions to the vaccines.

Due to concerns regarding potential neurotoxicity, thiomersal, a mercury-containing preservative that was previously used in vaccines, has been removed from or reduced to trace amounts in all vaccines routinely used in children under the age of 6 years, except the inactivated influenza vaccine.

Impact on global health

The impact of global vaccination programmes cannot be ignored. The before and after figures speak for themselves. In the early 1960s, approximately one third of children in Africa died before the age of 5 years predominantly because of infectious diseases. In the subsequent half-century, widespread implementation of vaccination programmes has led to a marked reduction in morbidity and mortality attributed to vaccine-preventable infections. Smallpox was eradicated in 1979. Polio cases have decreased 99{c7b83ef3f28a5a4d1b92af1005aa96857b6821a19c5bf7bda4f75f8b16806b7f} since 1988. The number of deaths due to neonatal tetanus has been reduced by >90{c7b83ef3f28a5a4d1b92af1005aa96857b6821a19c5bf7bda4f75f8b16806b7f} since the 1980s. The measles vaccine prevented an estimated 15.6 million deaths from 2000 – 2013. The impact is not only in the developing world. For example, in 1921, more than 15000 Americans died of diphtheria. Only one case of diphtheria has been reported in the US since 2004. Overall, it is estimated that vaccines prevent 6 million deaths worldwide annually.

Protecting children and adults

Paediatric immunisation programmes have been implemented for decades and are almost universally accepted, with the exception of small groups of anti-vaccine campaigners. Adult vaccination programmes are the next frontier. Adult vaccinations can be divided into two groups; booster doses of childhood vaccines and vaccines for diseases relevant to specific age categories. It has become evident that immunity to certain vaccines, especially live attenuated bacterial vaccines, wane with time and booster doses are recommended to provide continued protection for adults. Examples of these include tetanus, diphtheria and pertussis. Certain vaccines are recommended for older adults in order to prevent diseases that are more common in the elderly, for example pneumococcal pneumonia and shingles (zoster). Vaccines can make an important contribution to the health of adults if there is increased uptake in their use by this sector of the population. Education regarding these vaccines is important for both health care workers and lay people in order to optimise their impact.

Future articles in this series will present individual vaccines as well as discussion of the concerns by anti-vaccine campaigners.

This article is an extract from the IPA magazine, the IPA magazine can be viewed by clicking on the link. IPA Magazine