Case Study 1 - Outbreak of Influenza in a Kentucky Nursing Home

The occurrence of disease is ultimately attributable back to a cause, or causes. According to Fleming (2008), there are different determinants of disease, with vaccination named as one significant factor (p. 20). In the case study provided, a New York Nursing Home is faced with an outbreak of Influenza A, with certain segments of the population contracting the disease despite a positive history of vaccination (Last Name, 2014, p.1). Epidemiological managers for a Kentucky Nursing Home now need to evaluate the proportion of sick residents using various data, as well as calculate the risk factor for the disease among vaccinated and non-vaccinated patients (Last Name, 2014, p. 1). While other host factors such as age and gender also form the basis for any Epidemiologic Triad, the calculations provided are based solely on the number of vaccinated versus unvaccinated patients (Fleming 2008, p. 20). These calculations yielded important data for consideration.

Calculating the Proportion of Sick Residents

Incidence proportion is calculated by using the number of cases (numerator) divided by the population (denominator) (“Lesson 3”, 2012). If 75 of the residents developed influenza-like illness (ILI), in order to calculate what proportion of the residents became sick, the proportion is calculated using a numerator of the 75 residents that developed ILI, and a denominator of the 400 residents of a New York Nursing Home. The resulting proportion is calculated as follows: (75/400)*100 = 18.8%.

Calculating the Proportion

In examining the total number of residents with ILI, if 40 developed pneumonia, 25 required hospitalizations and two died, in order to determine what proportion of those with ILI developed pneumonia, the proportion is calculated using a numerator of the 40 residents with ILI that developed pneumonia, and a denominator of the 75 residents that developed influenza-like illness. The resulting proportion is calculated as follows: (40/75)*100 = 53.3%.

In calculating what percent of those patients with ILI and pneumonia were hospitalized the proportion is calculated using a numerator of the 25 residents with ILI that required hospitalizations, and a denominator of the 40 residents that developed influenza-like illness and pneumonia. The resulting proportion is calculated as follows: (25/40)*100 = 62.5%.

In calculating what proportion of those patients with ILI died, the proportion is calculated using a numerator of 2 residents with ILI that died, and a denominator of the 75 residents that developed ILI. The resulting proportion is calculated as follows: (2/75)*100 = 2.7%.

Calculating the Proportion

In examining the 375 residents who were vaccinated, if 60 developed ILI, and of the 25 residents who were not vaccinated 20 of those residents developed ILI, in order to calculate the percentage of vaccinated residents that developed ILI, the proportion is calculated using a numerator of 60 residents who were vaccinated that developed ILI, and a denominator of the 375 residents that were vaccinated. The resulting proportion is calculated as follows: (60/375)*100 = 16.0%.

In calculating what proportion of unvaccinated residents developed ILI, the proportion is calculated using a numerator of 20 residents who were unvaccinated that developed ILI, and a denominator of the 25 residents that were not vaccinated. The resulting proportion is (20/25)*100 = 80%.

The ratio between the percentage of unvaccinated residents and vaccinated residents is called the “relative risk” (Fleming, 2008, p. 37). In order to calculate how much higher the rate of ILI is among the population that was unvaccinated related to those who were, the relative risk is calculated using a numerator of the percent of unvaccinated residents developed ILI (80%), and a denominator of the percent of vaccinated residents developed ILI (16%). The relative risk is calculated as follows: (80/16) = 5. This means that the unvaccinated group is five times more likely to develop ILI over the unvaccinated group.

Calculating the Proportion

In examining the 375 vaccinated residents, if 35 developed pneumonia subsequent ILI compared to 15 residents among the 25 who were unvaccinated, in order to determine what percentage of vaccinated people acquired pneumonia post-ILI, the proportion is calculated using a numerator of 35 vaccinated residents who developed pneumonia post-ILI, and a denominator of 375 vaccinated residents. The resulting proportion is calculated as follows: (35/375)*100 =9.3%.

In calculating what percentage of unvaccinated inhabitants developed pneumonia subsequent to ILI, the proportion is calculated using a numerator of 15 unvaccinated residents who developed pneumonia after ILI, and a denominator of 25 unvaccinated residents. The resulting proportion is calculated as follows: (15/25)*100 = 60%.

In calculating the risk factor regarding how much higher the incidence of pneumonia after ILI is among the population that was unvaccinated, compared to those who were, the risk factor is calculated using the numerator of the percent of vaccinated residents developed pneumonia following ILI (9.3%), and a denominator of the percent of unvaccinated residents developed pneumonia following ILI (60%). The resulting relative risk is calculated as follows: (60/9.3) = 6.45. This means that the unvaccinated group is six point four five times more likely to develop pneumonia following ILI over the unvaccinated group.

Calculating the Proportion

According to Fleming (2008), “vaccine efficacy is calculated as the difference between illness rates (unvaccinated –vaccinated) divided by the unvaccinated illness rate” (p. 38). In order to calculate the vaccine efficacy for averting ILI and pneumonia, the vaccine efficacy is calculated using a numerator of the percentage of unvaccinated residents developed ILI (80%), and a denominator of the percentage of vaccinated residents developed ILI (16%). The efficacy is calculated as follows: (0.80-0.16)/0.8 = 0.8 (80%). The vaccine efficacy in inhibiting ILI is 80%.

In order to calculate the vaccine efficacy for preventing pneumonia, the vaccine efficacy is calculated using a numerator of the percentage of unvaccinated residents that developed pneumonia following ILI (60%), and a denominator of the percentage of vaccinated residents that developed pneumonia following ILI (9.3%). The efficacy is calculated as follows: (0.60-0.093)/0.6 = 0.845 (84.5%). The vaccine efficacy in preventing ILI is 84.5%.

References

Fleming, S. T. (2008). Managerial epidemiology concepts and cases (2nd ed.). Chicago: Health Administration Press.

Last Name of Professor, First Initial. (2014). Outbreak of Influenza in a Kentucky Nursing Home. City of Publication: Name of Class.

Lesson 3: Measures of Risk. (2012, May 18). Centers for Disease Control and Prevention. Retrieved from http://www.cdc.gov/osels/scientific_edu/SS1978/Lesson3/Section2.html