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Vaccines help the body remember a microorganism to be able to fight it off when the need comes. However, vaccines are not fool proof and do not guarantee complete protection from a disease.
This could be due to various reasons. Sometimes this is because the host's immune system simply doesn't respond adequately or at all. This could be in diseased persons with lowered immunity e.g. in diabetics, those on steroids or other immunity suppressing drugs or those with HIV infection.
The reason for non-development of immunity to a disease could also be because the host's immune system does not have a B cell capable of generating antibodies against the antigen or microbe or the immune system may not be strong enough to fight off the infection.
The efficacy of a vaccine is different from its effectiveness. It is dependent on several factors such as:
The mathematical deduction of protective vaccine efﬁcacy is nearly 100 years old, having been proposed by Greenwood and Yule in 1915 for inactivated whole cell cholera and typhoid vaccines.
Vaccine efﬁcacy is best measured by double-blind, clinical trials. These explore the “best case scenarios” of vaccine protectiveness under controlled conditions and are commonly required before a new vaccine is licensed by the Food and Drug Administration and other global regulatory authorities.
The outcome of efficacy is measured by parameters like - proportionate reduction in disease attack rate (AR) between the unvaccinated (ARU) and vaccinated (ARV) individuals in the clinical trial. This can give the relative risk of getting the disease (RR) of the disease after use of the vaccine.
Efficacy = (ARU-ARV)/ARU X 100
Efficacy = (1-RR) X 100
While advantages of knowing the vaccine efficacy means it has been tried in strict clinical conditions, the disadvantages that it has not been tried on larger general populations. Vaccine efﬁcacy studies can measure outcomes beyond disease attack rates, including hospitalizations, medical visits, and costs.
Vaccine effectiveness was initially termed “ﬁeld efﬁcacy”. Essentially, vaccine effectiveness is a “real world” view of how a vaccine reduces disease in a population. This vaccine may already have been proved to be efficacious in clinical trials. This measure can assess the net balance of beneﬁts and adverse effects of a vaccination program rather than the vaccine alone in field conditions.
Vaccine effectiveness is proportional to vaccine potency or vaccine efficacy but is primarily affected by how well target groups in the population are immunized, difficulties in storing, administering, cost, accessibility, availability, stability and manufacturing of the vaccine.
Effectiveness is expressed as a rate difference. It uses odds ratio (OR) for developing infection despite vaccination and can be derived as:
Effectiveness = (1-OR) X 100.