Analysis of Time to Event Data to Estimate Vaccine Effectiveness
Arunga, Geoffrey O.
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There has been a continued undervaluation and underutilization of vaccination as a way of preventing vaccine preventable diseases and thus consequently reduce morbidity and mortality associated with the particular diseases. Currently vaccination is the most potent tool to prevent diseases and prevent deaths because there has been a dramatic decrease or wiping out altogether of diseases attributed to increased vaccination against the diseases. The undervaluation and underutilization of vaccines stems from the underestimation of the severity of the vaccine preventable diseases, underestimating the benefits of vaccination and concerns regarding the side effects of vaccines. The undervaluation and underutilization of vaccines has resulted in a drop in the investment in research and development of new vaccines to combat diseases that are considered preventable in humans especially amongst the developing countries like Kenya; thus worsening an already worse situation. Though there are costs incurred when eliminating a disease through vaccination, there are great amounts of savings to be made if vaccination is undertaken to prevent a disease amongst a population. The savings made are counted in human lives saved, cost of vaccines not needed hence not bought and the cost of surveillance activities not carried out pertaining to the disease. A population can be free of a disease because of high or increasing vaccination rates against a particular disease, which can also result in the development of herd immunity within the population under consideration, where a member of the population, whether vaccinated or not vaccinated, is at a lowered risk of getting infected with a particular disease because there are fewer members of the population who are susceptible or infected due to increased vaccination within the population. Typically vaccines contain the same antigens that cause diseases but the antigens in vaccines are either killed or greatly weakened. Vaccine antigens are not strong enough to cause disease but they are strong enough to make the immune system produce antibodies against them. Memory cells prevent re-infection when they encounter that disease again in the future. Through vaccination, humans develop immunity without suffering from the actual diseases that the vaccinations are designed to prevent. There has been widespread interest in the level of protection offered by vaccines. This level can be assessed using two ways namely, estimates of efficacy from randomized control trials (RCTs) and also as estimates of effectiveness from observational studies. Sometimes the efficacy estimates from the RCTs are higher than the effectiveness estimates from observational studies and this is mainly due to the fact that RCTs are conducted in controlled environments. Observational studies are normally more ethical and feasible to carry out as compared to RCTs and would thus be better in a population to evaluate the effectiveness of a vaccine. Vaccine effectiveness (VE) is a measure of how well the vaccine works to protect against infections and illness when they are used in routine circumstances in a population, and it is normally easily estimated using the observational studies which are conducted in community settings (populations) and the researchers involved have no control over those who chose to be vaccinated or not to be vaccinated during the study. Vaccine effectiveness studies are subject to various forms of bias, which are typically much, more than RCTs; thus caution should be exercised when analyzing and evaluating the results of these studies. Typically the three types of biases that .affect the interpretation of results from VE studies are confounding bias, selection bias and information bias. Factors that raise or lower the apparent attack rates in either the vaccinated or non-vaccinated groups will bias the vaccine effectiveness estimates and should be considered and taken care of in the study design and analysis as any potential biases which are typically inherent in observational studies. Despite the above challenges of using observational studies to estimate the effect of vaccines, they are the best way of estimating the effects hence when one considers and takes care of the potential biases that may arise then the treatment effects (effectiveness) of the vaccines will be accurately estimated using observational studies. From the results obtained in this study it's seen that those who got vaccinated had massively higher hazards as compared to those who were not vaccinated before the vaccination campaign and that might be the reason why they got vaccinated and thus consequently their hazards decreased greatly after the vaccination campaign. Hence it was observed that the vaccine offered some level of protection to those who were vaccinated but this wasn't statistically significant. Going forward there should be greater encouragement and research geared towards the use of observational studies to estimate effectiveness of vaccines, this would make this information readily available to the population: With more accurate and available information on the efficacy, safety, potential side effects and effectiveness associated with a vaccine, a population would be more willing to consider receiving the vaccination under consideration. Overall the role of vaccination in the greater realm of public health cannot be underestimated in the world, be it the developed or developing world.
CitationGeoffrey Odiwuor Arunga (2013). Analysis Of Time To Event Data To Estimate Vaccine Effectiveness. Master Of Science In Biometry
University of NairobiSchool of Mathematics