updated 8/3/2005 5:43:32 PM ET 2005-08-03T21:43:32

Screening blood donations for the West Nile virus to prevent its spread is working, though a few contaminated units have been missed, according to reports on the first two years of testing.

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The nation’s blood supply has been screened for West Nile virus since the summer of 2003, after it became apparent that the mosquito-borne illness could be passed on through transfusions.

Since then, West Nile infections have been found in 1,039 of the 27 million blood donations screened, according to the Centers for Disease Control and Prevention. That means about 1,500 transfusions of tainted-blood products were prevented, said the CDC’s Dr. Lyle Petersen.

“Screening has markedly improved the safety of the blood supply. There remains a small residual risk, but it’s very, very small,” Petersen said.

He estimated that screening has reduced the risk of getting an infection through a blood transfusion by 90 percent. The virus that is missed is present in very low levels, he said.

Petersen and Dr. Jay S. Epstein of the Food and Drug Administration wrote an editorial that accompanied the two reports published in Thursday’s New England Journal of Medicine.

The number of confirmed cases of West Nile infections through blood transfusions dropped significantly once screening began. There were 23 confirmed cases in 2002, six in 2003 and only one last year, according to CDC figures.

Petersen said the actual number of cases is probably higher because not all cases are detected and the cause of others cannot be confirmed.

Blood screening is initially done in pools of 16 donations. If the pool tests positive, then the donations are tested individually. Blood banks switch over to the more sensitive individual testing in areas where there is an outbreak.

The editorial noted that one-third to one-half of the units that tested positive during individual testing had been missed during screening in a pool.

West Nile was first reported in the United States in New York in 1999 and moved across the country. The American Red Cross reported in the journal that about half of its 104 positive donations last year came from four counties in Southern California, one of the hardest-hit areas.

Petersen said people should be more worried about getting bitten by mosquitoes than receiving a tainted transfusion.

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