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The goal behind PAHO's recommended vaccination strategy for measles eradication is to maintain the number of susceptible population at the lowest possible level. After the countries have carried out their mass catch-up campaign and progress towards measles eradication, implementing the other two elements of the strategy, keep-up and follow-up vaccination are extremely important in maintaining the number of susceptibles at the lowest level. Obtaining vaccine coverage better than 95% is imperative for these vaccination activities to be effective.
As countries reach these milestones, the number of measles cases should dramatically drop. At this point, timely reporting and rapid investigation of each suspected measles case becomes critical to detect further circulation of measles virus in an area. Rapid case investigation of each suspected measles cases will allow health workers to determine the source of infection and define the chain(s) of transmission when more than one case occurs in a specific geographic area. Establishing the source of infection is necessary to determine whether the case is due to an importation, or it will confirm that the case is due to indigenous transmission. If there is more than one case of suspected measles in a defined area, establishing the chain(s) of transmission is extremely important, because one can arrive at the source for continued measles transmission, take corrective actions and institute the necessary changes to avoid future program failure.
Figure 1 below shows a measles outbreak recently detected in Uruguay. As can be seen, there is a void of epidemiological information and data between the first measles case, which was imported from a neighboring country in week 36, 1998, and the subsequent detection of additional cases commencing in week 50. In 1997, Uruguay reported only one confirmed measles case due to an importation. In week 1 of 1999, another importation was detected along with the other cases for the same period. Because surveillance was not optimal, the reasons for continued transmission of the measles virus was not established. Therefore, the source for the cases occurring at the beginning of week 50 is unknown. However, by the time the second imported case of measles occurred and was hospitalized, measles surveillance was heightened and the surveillance team was able to identify 23 cases that formed four chains of transmission (see Figure 2.) It was determined that in all but one of the four chains, a health worker was either a person transmitting measles infection to others, or the receptor of the infection.
There are 11 cases not shown in Figure 2 that could not be associated with any chain of transmission. However, the investigation of these cases has shown that they had either traveled to infected areas (mainly by bus), or were employed in businesses associated with tourism, and had been in contact with tourists coming from neighboring endemic countries. The date of onset for these 11 cases all occurred within weeks 1- 7, 1999. Furthermore, of the 35 cases only five cases occurred in children between 0-5 years of age, and there were no cases in the 6-20 years age group. This indicates that the catch-up and the follow-up campaigns implemented by the Ministry of Health were effective.
From Figure 1 the measles outbreak appears short-lived. This is due to the fact that Uruguay has maintained its pool of susceptibles to an absolute minimum by maintaining vaccine coverage greater than 95% in all phases of the measles eradication strategy (catch up, keep up, and follow up). Uruguay obtained 95% coverage in their last follow up campaign carried out in November 1998, and their routine immunization program (keep up). In order to control the outbreak, the Ministry of Health implemented the following PAHO recommendations: 1) strengthening of surveillance activities in the entire country by alerting departmental health authorities; 2) measles vaccination for all children ages 6-11 months with a booster at 15 months in the entire country; 3) request that all children 14 and under who were not vaccinated during the last mass campaign be vaccinated against measles; 4.) vaccination of all people working in border crossing areas; 5.) vaccination of public and private health workers in the department of Montevideo and Maldonado.
From the above, it can be concluded that PAHO's strategy, if implemented in full and coupled with aggressive investigation of suspected measles cases, will assure that measles virus transmission is tracked down until it has nowhere to hide.
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Source:Ministry of Public Health of Uruguay, Division of Epidemiology, Prevention and Control of Diseases, National Immunization Program.
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