Elizabeth Tenney Infectious Disease Epidemiology Homework 2 Texarkana Epidemic Measles in a Divided City Question 1a: Any outbreak of measles is cause

Transcription

1 Elizabeth Tenney Infectious Disease Epidemiology Homework 2 Texarkana Epidemic Measles in a Divided City Question 1a: Any outbreak of measles is cause for concern, especially one with such a large increase in and number of cases. The fact that there were cases in previously immunized children is alarming. The high amount of people who travel to and from the town could cause the outbreak to spread quickly. Question 1b: The objectives of the initial phase of my investigation would involve putting together an investigative team, defining roles, and confirming the existence of the outbreak. With confirmed cases of measles, time would be an important factor and the team would have to work quickly. Question 2a: The case presents with a 4-5 day prodromal period with high fever, coryza, cough, and conjunctivitis followed by the appearance of a bright maculopapular rash. The temperature usually returned to normal 2 to 3 days after appearance of the rash, while the rash persisted for 5-7 days. The case resides in or has traveled to or been in contact with someone who resides in or has traveled to or from the city of Texarkana in October of November of 1790.

2 Question 2b: A very sensitive case definition ensures that all true cases will be caught; yet it will also include individuals that do not have the disease. This is best suited for severe diseases with epidemic potential, where missing a case could have detrimental consequences. A very specific case definition will give a higher likelihood that all of the included in an investigation are true cases; yet it may miss individuals who have the disease but do not fit the case definition exactly. This is best suited in situations where epidemiologists want to investigate risk factors for transmission, such as a food outbreak, where false cases could lead to inaccurate conclusions. Question 2c: While the case definition used for this outbreak did not include specific symptoms for the cases, place, or time, and would be considered a sensitive case definition, it is appropriate for this outbreak because the measles are highly contagious. It would be better to catch all true cases, even if that means individuals who do not have the disease are included, rather than risk missing a case and potentially allowing the disease to spread. I would not have a problem using this as my case definition, but I would keep in mind that some individuals may not have the disease. Question 3: This epidemic curve best resembles a short point source epidemic, in which many individuals are exposed at the same time. One could conclude that many cases were exposed when schools and preschools opened, and that the vaccine campaigns were an effective effort to reduce the cases of measles. It seems as though the outbreak began in preschool-aged children, then spread to school-aged children, followed by the rest of the

3 community. Preschool-aged children are likely to have school-aged siblings, which could have been the cause of the spread of the disease. Since schools for older children tend to be larger, have larger classes, and have more interaction between children and rooms (recess, lunch, music rooms, etc.), it makes sense that more school-aged children were infected once the disease spread to that age-group. Subsequently, the infection would spread to the rest of the community via the children. Question 4a: Residence Urban/Rural Age Group # Cases Population Rate Bowie Co., Texas Rural 1-4 yr per 100, or yr per 100, or yr per 100, or Urban 1-4 yr per 100, or yr per 100, or yr per 100, Total 1-4 yr per 100, or yr per 100, or yr per 100, or Miller Co., Arkansas Total 1-4 yr per 100, or yr per 100, or yr per 100, or

4 Question 4b: The attack rate for Bowie County was much higher than the attack rate in Miller County (4.41 compared to 0.46 per 100). In Miller County, the attack rate was higher among children ages 1-4 compared to those ages 5-9, with attack rates of 0.71 and 0.18 per 100. For the Rural population in Bowie County, the attack rate was much higher among 5-9 year olds (school-aged children) than among 1-4 year olds (preschool-aged children), with attack rates of 5.49 and 1.92 per 100. In the Urban population in Bowie County, the attack rate was much higher among 1-4 year olds (preschool-aged children) than among 5-9 year olds (school-aged children), with attack rates of 7.89 and 2.43 per 100. Overall, however, attack rates were relatively the same between groups, with a slightly higher attack rate among 1-4 year olds (4.91 compared to 4.01 per 100). Question 5: Attack rate among the vaccinated populations: Miller County: Total population of children 1-9 = % vaccination prevalence = 5956 vaccinated children Number of cases among vaccinated children: 25 Attack rate = (25)/(5956) = , or 0.42 per 100 Bowie County: Total population of children = % vaccination prevalence = 6375 vaccinated children

5 Number of cases among vaccinated children: 27 Attack rate = (27)/(6375) = 0.097, or 9.69 per 100. Question 6a: Number of unvaccinated children = = 4810 Number of cases in the unvaccinated population: = 466 Attack rate in the unvaccinated population = (466)/(4810) = or 9.69 per 100. VE= ( )/(0.097) x100 = 95.67% Question 6b: With a vaccine efficacy of 95.67%, it is not likely that inadequate vaccine efficacy is responsible for the outbreak. Question 7: Possible causes for the failure of the vaccine to protect vaccinated children from acquiring the disease include improper vaccine administration, improper vaccine handling or storage, a mutation of the virus, and poor nutrition among the children causing weak immune systems and antibody responses. Question 8: The WHO recommended age for measles vaccination in developing countries is 9 months old. The risk of acquiring measles, complications at a given age, and vaccine efficacy at a given age are all factors that account for different recommendations in different countries.