Study: Low risk of smallpox virus in blood donated by vaccine recipients

Current blood donation guidelines are likely sufficient to protect donation recipients from contracting smallpox from donors vaccinated against the disease with the live vaccinia virus, according to a recent study.

Researchers with the National Institute of Allergy and Infectious Diseases and the National Institutes of Health Clinical Center found rare instances of vaccinia virus DNA in blood samples taken from 28 NIH employees. The employees had received the Dryvax smallpox vaccine as a recommendation for their positions. The vaccinia virus DNA found in the blood was not the infectious virus.

The team used two different testing methods for the blood samples and found that four out of 200 specimens tested positive for the virus DNA. The samples were considered weakly positive and did not contain detectable infectious virus. All throat swabs were negative for vaccinia virus DNA.

The study findings contrast with the conclusions of an earlier study that recommended an extended time between when individuals receive smallpox vaccinations and when those people are eligible to give blood.

"Like the earlier study, we also found evidence of the smallpox vaccine virus DNA in the blood," lead researcher Jeffrey I. Cohen said. "However, the earlier study did not look for infectious virus in the blood. We were able to look at the same samples that were positive for viral DNA and found no infectious virus. Thus, we feel that as long as the current guidelines for donating blood after smallpox vaccination are followed, the risk of transmitting the vaccine virus through blood transfusions should be exceedingly small."

Current guidelines recommend that recipients of Dryvax who develop no complications from vaccination should defer from blood donations until 21 days after or after the vaccine scab has spontaneously separated, whichever is longer. Individuals who experience vaccine complications should defer from blood donations until 14 days after the cessation of all complications.

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National Institutes of Health

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