Tuberculosis attributable to reinfection occurs four times as often as new tuberculosis in South Africa, according to a report in the June issue of the American Journal of Respiratory and Critical Care Medicine.

Researchers have hypothesized that infection with one strain of Mycobacterium tuberculosis will protect against subsequent reinfection with another strain, the authors explain, suggesting that the reinfection rate among cured patients should be lower than the rate of new infection in a given population.

Dr. Suzanne Verver from KNCV Tuberculosis Foundation, The Hague, and colleagues investigated the incidence of tuberculosis attributable to reinfection among successfully treated patients in Cape Town, South Africa. Follow-up periods ranged from 3 to 8.5 years (median 5.2 years) and 612 subjects were included in the analysis.

Eighteen percent of the patients had a recurrence of tuberculosis, the authors report, including 14% of successfully treated patients and 28% of patients who did not complete treatment.

Three quarters of the recurrences in successfully treated patients tested with DNA fingerprinting represented reinfections, the report indicates.

The rate of reinfection disease was 2.2 cases per 100 person-years, the researchers note, compared with a crude incidence rate of 313 per 100,000 and an age-adjusted incidence rate of 515 per 100,000 for new tuberculosis infection.

Risk factors for recurrent infection were not identified, the results indicate. Age, sex, smear positivity, and being a new or retreatment patient at enrollment were not associated with reinfection.

“This result challenges the hypothesis that, in immunocompetent persons, infection with one strain of M. tuberculosis protects against disease attributable to subsequent reinfection with another strain,” the authors conclude.

“People who have been treated successfully for TB are at higher risk of developing TB from reinfection than the general population,” the investigators add. “This suggests that a subgroup of individuals is intrinsically vulnerable to TB.”

“Further study is needed to find out whether the high risk of reinfection disease is attributable to a high risk of reinfection or a high risk of breakdown to disease,” the researchers write.

Am J Respir Crit Care Med 2005;171:1430-1435.