Because the symptoms of lupus wax and wane and differ from person to person, it has been difficult to demonstrate in a clinical trial that a drug is effective against the disease.

The drug is likely to cost $20,000 a year or more, in line with biotechnology drugs for other autoimmune diseases like rheumatoid arthritis and multiple sclerosis. Analysts estimate Benlysta sales could reach a few billion dollars a year.

Human Genome executives said Monday that there were about 325,000 Americans with systemic lupus under the care of rheumatologists. They estimated that the 200,000 with the most severe disease would be candidates for Benlysta.

In the primary measure of the drug’s effectiveness, 43.2 percent of patients taking a high dose of Benlysta had a significant improvement in symptoms after one year compared with 33.8 percent of those getting a placebo. That suggests that only one of 10 people who take Benlysta will do better than they would on a placebo.

There were also some secondary measures for which the higher dose did not differ from a placebo in a statistically significant way in the second trial, like the assessment of doctors as to how their patients were faring. In the first trial, the differences were statistically significant for the most part.

The second trial, involving 819 patients, took place primarily in North America and Western Europe, while the first trial involved patients in Asia, South America and Eastern Europe. Because medical care in the United States and Western Europe is probably better than in those other regions, analysts say there could have been less room for Benlysta to make a difference.

The companies said the rate of side effects was not very different between those on Benlysta and those taking the placebo. However, three patients taking the drug died and five developed tumors, while none getting the placebo died and only one developed a tumor. Since twice as many patients got the drug as got the placebo, these differences might not mean anything.