We assessed the risk of pneumonia due to Pneumocystis carinii in 1665 participants in the Multicenter AIDS Cohort Study who were seropositive for human immunodeficiency virus type 1 (HIV-1) but did not have the acquired immunodeficiency syndrome (AIDS) and were not receiving prophylaxis against P. carinii. During 48 months of follow-up, 168 participants (10.1 percent) had a first episode of P. carinii pneumonia. The risk was greatly increased in participants with CD4+ cell counts at base line of 200 per cubic millimeter or less (relative risk, 4.9; 95 percent confidence interval, 3.1 to 8.0). Although most participants (60.7 percent) described no HIV-1—related symptoms at the clinic visit at which a CD4+ cell count of 200 per cubic millimeter or less was first noted, this finding during follow-up was also associated with an increased risk of P. carinii pneumonia. The development of thrush or fever significantly and independently increased the risk of P. carinii pneumonia in these patients (adjusted relative risks, 1.86 and 2.15 for thrush and fever, respectively). Most participants with CD4+ cell counts above 200 per cubic millimeter who had P. carinii pneumonia within six months were symptomatic. We conclude that P. carinii pneumonia is unlikely to develop in HIV-1—infected patients unless their CD4+ cells are depleted to 200 per cubic millimeter or below or the patients are symptomatic, and therefore that prophylaxis should be reserved for such patients. (N Engl J Med 1990;322:161–5.) PNEUMONIA due to infection with Pneumocystis carinii is the most common of the opportunistic diseases included in the diagnosis of acquired immunodeficiency syndrome (AIDS) in the United States. Furthermore, it is estimated that 75 percent of patients with AIDS have P. carinii pneumonia at some time during the course of their illness.1 Given current estimates of the number of persons infected with human immunodeficiency virus type 1 (HIV-1) in the United States, more than 40,000 cases of P. carinii pneumonia can be expected in the current calendar year and more than 60,000 in 1992. The Multicenter AIDS Cohort Study is.
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