The process of ensuring immunity to individuals by the administration of vaccines is known as immunization (WHO, 2010d). Immunization is a proven tool for controlling life-threatening diseases and is estimated to forestall over 2 million deaths annually (WHO, 2010d). Smith et al (2004) stressed that immunizations have averted vaccine-preventable diseases effectively and have reduced their fear in many societies. Greenwood et al (2007) further affirmed that immunization is a major means by which infections may be controlled and in some cases, eradicated. Due to its importance, Sutter & Cochi (2008) stated that immunizations are a critical part of public health practice.

Furthermore, Greenwood et al (2007) stated that the main rationale behind immunization is to produce a degree of resistance sufficient to prevent a clinical attack of the natural infection to the recipients without any harm and to prevent the spread of infection to susceptible individuals in a community.

Economically, immunization is the most cost-effective health intervention with proven strategies that makes it accessible to vulnerable and hard-to-reach populations of a society (i.e. populations with the lowest health status) (WHO, 2010d). Furthermore, it has clearly defined target groups and it can be achieved through outreach activities (Smith et al, 2004). In addition, there is both a personal gain from being immunized and a public health benefit to the population, for instance through the achievement of herd immunity in a community (Greenwood et al, 2007). Herd immunity on the other hand was explained by Mackett & Williamson (1995) as the level in which natural transmission of a particular infection that spreads from person-to-person is inhibited because most people in a community are immune. However, Greenwood et al, (2007) pointed out that herd immunity operates only for infections that spread from person-to-person.

Immunization can be attained through various techniques, most commonly vaccination (Miller, 2002). Vaccination on the other hand is the administration of antigenic materials (vaccines) to produce immunity against a disease and vaccines can ameliorate the effects of infection by pathogens and there is strong evidence for the efficacy of influenza, measles-mumps-rubella (MMR), polio, smallpox and chickenpox vaccines among others (Fiore et al, 2009; Chang et al, 2009).

The word vaccination was first used by Edward Jenner in 1976 (Dunn, 1996) and Louis Pasteur (a French chemist and microbiologist) who furthered the concept through a pioneering work in microbiology by developing a rabies vaccine now termed an antitoxin, which lead to a wide acceptance of vaccines and passage of compulsory vaccination laws in many countries in the 19th century (Stern and Markel, 2005).

Stern and Markel (2005) further pointed out that the 20th century marked the introduction of several effective vaccines which include; diphtheria, MMR, and polio vaccines. However, in terms of effectiveness, Grammatikos et al (2009) argued that vaccines do not guarantee complete protection from a disease and stated some of the reasons to be failure of the host’s immune system to respond adequately to the vaccine due to compromised immunity from either age, HIV infection, diabetes, steroid use or other factors alike or even lack of β-cells in the immune system. (A β-cell is defined by Albert et al (2002) as a cell capable of generating antibodies to antigens). However, Greenwood et al (2007) counter-argued that although the degree of resistance conferred may not protect against an overwhelming challenge, the exposure may help boost the recipient’s immunity.

There are several types of vaccines currently in use ranging from inactivated (killed) vaccines (for example influenza vaccine, Hepatitis A vaccine, rabies vaccine, and polio vaccine); live attenuated vaccines (example MMR, Bacillus Calmette-Guѐrin [BCG] for tuberculosis, and oral polio vaccine); toxoids (for example tetanus and diphtheria vaccines); subunit vaccines (for example Haemophilus influenzae type B vaccine); to experimental vaccines such as that of valley fever, stomatitis, and atopic dermatitis (Cates, 2000).

In relation to this, polio vaccine (which is the basis of this research) will be described in more detail in the subsequent heading.

2.1 POLIO VACCINE

Polio vaccine is described by Monsoon & Schoenstadt (2010) as a remedy that is administered to prevent polio which is highly effective in producing immunity to polio virus and protection from paralytic poliomyelitis, because approximately 90% or more recipients of the vaccine develop protective antibodies to all three poliovirus types after two doses and at least 99% become immune after three doses. The Global Polio Eradication Initiative (G.P.E.I) (2010a) reiterated that the development of polio vaccines was one of the foremost medical discoveries of the 20th century.

Two types of polio vaccines are available which include; a live attenuated or weakened oral polio vaccine (OPV) commonly called the Sabin vaccine which is administered orally, and an inactivated (killed) polio vaccine (IPV) commonly called the Salk vaccine which is administered via injection by a trained health worker (Sutter & Cochi, 2008; G.P.E.I, 2010a).

It is imperative to understand the individual types of polio vaccines and how they work alongside their merits and demerits.

2.1.1 Oral Polio Vaccine (OPV)

The efficacy of the OPV is bilateral in that it produces antibodies in the blood referred to as humoral or serum immunity to all the three types of polioviruses and it also elicits local immune response in the mucous membrane of the intestines (G.P.E.I, 2010a). Torok et al (2009) described the mucous membrane of the intestines as the primary site of poliovirus infection. G.P.E.I (2010a) reiterated that the disruption of person-to-person transmission is probably the main reason why OPV is opted for mass immunization campaigns.

Advantages of OPV include the following; it is administered orally, it can be given by volunteers – not necessarily health workers, it doesn’t require sterile injection equipments which is contrary to most vaccines, it is cost-effective because it is relatively inexpensive and at the time of writing it costs 8 US cents equivalent to 5 pence a dose (G.P.E.I, 2010a) (cost-effectiveness, considerations of need, efficacy, safety and ease of administration are major factors that influence governments to purchase large quantities of vaccines during National Immunization Days {Greenwood et al, 2007}). The above mentioned advantages make OPV the vaccine of choice for polio eradication which would not be feasible with IPV. (G.P.E.I, 2010a)

The disadvantage of OPV on the other hand was argued by Cono & Alexander (2002) that although OPV is safe and effective, it induces vaccine associated polio paralysis (VAPP) in either the vaccinated child or close contact in rare cases. G.P.E.I (2010b) further explained VAPP to be caused as a result of live attenuated poliovirus strains contained in the OPV while Cono & Alexander (2002) mentioned immune deficiency of the recipients to be among the causes. G.P.E.I (2010b) stressed that VAPP is familiar and accepted by most public health programmes across the globe but alleged that without OPV hundreds of thousands of children would be crippled annually. WHO (1998) stated that the emergence of VAPP led to combined immunization schedules using both OPV and IPV in many countries where the risk of wild poliovirus cases were low.

G.P.E.I (2010b) added further on the disadvantages of OPV by stating that in rare cases a strain of poliovirus in OPV may genetically change and circulate within a population leading to a vaccine-derived poliovirus (VDPV). Brooks et al (2007) defined vaccine-derived poliovirus (VDPV) as live, attenuated (weakened) strains of the virus contained in OPV which have been modified and reverted (i.e. reacquired original features) to a form that can cause paralysis in humans with the capacity for sustained circulation. VDPV are of three types: circulating vaccine-derived polioviruses (cVDPV), which are associated with person-to-person transmission and considered to be circulated in the environment; immunodeficiency related vaccine-derived polioviruses (iVDPV), which are isolated from immune-compromised patients who had prolonged infections after exposure to OPV, and ambiguous vaccine-derived polioviruses (aVDPV) which are isolated from a single immune-competent paralytic poliomyelitis patient with or without additional isolates from close contacts, healthy individuals, or the environment in the absence of paralytic cases. (G.P.E.I 2010b)

Sutter & Cochi (2008) further outlined t