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Mexico in a screening program to identify individuals with latent tuberculosis infection.

The 1996 Statistical Report of the Immigration and Naturalization Service shows that in 1996 421,405 immigrants were admitted to the United States as new arrivals. A total of 187,079 immigrants were admitted from 24 of the high-tuberculosis-risk countries. No immigrants were reported separately in the 1996 statistical report as arriving from the other 29 high-tuberculosis-risk countries, but 4,360 newly arrived immigrants were reported as arriving from other countries in the African, American, Eastern Mediterranean, Southeast Asian, and Western Pacific regions. However, these individuals are not included in further analyses since the precise country of origin could not be ascertained. An additional 52,946 newly arrived immigrants from Mexico were included in the analysis, yielding a total of 240,025 newly arrived immigrants from high-tuberculosis-risk countries and Mexico.

ESTIMATING THE NUMBER OF TUBERCULOSIS CASES

After estimating the number of immigrants to be included in a screening program, the second step is to estimate the number of tuberculosis cases that will occur among these immigrants. Two methods were used to estimate the number of tuberculosis cases. One uses the risk of developing tuberculosis given a positive tuberculin skin test in a population at high risk for infection and with a mixture of old and new infections. The other method uses country- and region-specific annual incidence rates estimated for newly arrived immigrants during the first 5 years in the United States.

The risk of tuberculosis given a positive tuberculin skin test in a population at high risk for infection and with a mixture of old and new infections can be calculated from data for the placebo group of the U.S. Public Health Service trial of isoniazid preventive therapy among household contacts. Patients were enrolled in this trial from 1956 to 1959. Even though nearly 90 percent of these individuals were enrolled as household contacts of individuals with new active cases of tuberculosis, this was very early in the era of tuberculosis chemotherapy (isoniazid was first discovered in 1952) and it is likely that this population had a mixture of recent and old tuberculosis infections. There were 121 cases of tuberculosis among the 4,992 individuals with a tuberculin skin test induration of 10 millimeters or more, for a 5-year risk of tuberculosis in the placebo group of 2.42 percent. By using the individual country estimates of the prevalence of infection and the number of newly arrived immigrants, a total of 87,287 individuals would be infected. With a risk of 2.42 percent, 2,112 cases would be expected to arise in this group in 5 years.



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