New York ? Screening blood donations for the West Nile virus to prevent its spread has proved remarkably effective, though a few contaminated units have been missed, according to reports on the first two years of testing.

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.

The calculated odds of getting West Nile from a transfusion are 1 in 350,000. That compares with 1 in 1.8 million for HIV; 1 in 220,000 for hepatitis B; and 1 in 1.6 million for hepatitis C.