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More than 6,400 units of blood for transfusions shipped during the past 13 months could have been contaminated with hepatitis or other viruses — and most may have already been used, according to the Japanese Red Cross Society.

The society said Tuesday that it managed to recall only 13 of the 6,419 units in question, meaning the remainder have likely been used, it said in a report submitted to the health ministry.

“The likelihood of a person infected (with a contagious disease) donating blood and the virus not being detected in the highly advanced screening tests is very low, ” society officials said. “The number of blood units that might have actually been contaminated with a virus is very low.”

Nevertheless, the society will call on people concerned to undergo medical checkups to make sure they have not contracted the viruses.

The society plans to track back to 1995, when the earliest data are available, targeting some 50,000 people.

The products have cleared a sensitive test, but the conventional screening technology’s inability to detect viruses during a certain period after a blood donor is infected might have allowed them to pass despite contamination, the society said.

It first came to light last month that the Red Cross had failed to check past donation records of blood donors who were found to have been infected with the hepatitis virus. The Health, Labor and Welfare Ministry has said it was unaware of the society’s actions, but some experts have criticized health authorities for the slip-up.

Tuesday’s finding came as the society worked to track past records of blood donations by people who tested positive for hepatitis B and C, AIDS and syphilis after offering their blood between July 13 last year and this July 21, it said.

Even if donated blood undergoes sensitive testing, there is a small window of time during which a virus can go undetected because it is only found in minute amounts in the blood, according to experts.