OLDER AMERICANS RETAIN SOME SMALLPOX IMMUNITY



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Last Updated

18 Aug 2003

Source: Wall Street Journal, August 18, 2003

HEALTH

Baby Boomers, Older Americans Retain Some Smallpox Immunity

By MARILYN CHASE, Staff Reporter of THE WALL STREET JOURNAL

Baby boomers and other older Americans vaccinated against smallpox decades ago retain antibodies that could provide some immune defense against the virus in a bioterror attack, scientists said.

Researchers at Oregon Health & Science University in Portland, analyzed blood samples taken from more than 300 volunteers vaccinated 25 to 75 years ago, and found more than 90% maintain "substantial immunity" against smallpox. The U.S. halted mass vaccination when natural smallpox was eradicated in the 1970s.

Separately, vaccine maker Acambis PLC reported its new smallpox vaccine made with modern cell-culture techniques induces equally potent immune responses, with fewer potential side effects than the conventional Dryvax vaccine made by inoculating calf skin. The two upbeat smallpox reports appear in Monday's issue of the British journal Nature Medicine.

Antibody defenses continued as long as 75 years, while another immune component, antiviral T-cells, were undetectable in volunteers after 30 years, said Oregon researchers Mark Slifka and Erika Hammarlung. Though the study agreed with earlier findings, the doctors theorized that this partial protection could reduce the country's exposure to smallpox attack.

Dr. Slifka said in an interview that although T-cells wane, he believes vaccinated people derive "essentially lifelong immunity" from their antibodies, which he called "your first line of defense." Other experts were more cautious.

"Duration of antiviral immunity appears to be substantial," commented Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. However, he asked, "Does that mean there will be less infection or simply that you would handle the infection better? We don't know." Old vaccinations might shield their recipients against infection, or they might lessen risk of death if infection occurred.

"We don't know how long [immunity] lasts. It depends on how many times and when you were vaccinated," added Sharon Frey, vaccine researcher and associate professor of internal medicine at St. Louis University in Missouri. Both arms of the immune system are needed, she said, to form a strong defense: antibodies to block infection and T-cells to kill the virus in host cells.

Acambis, a vaccine maker based in Cambridge, England, and Cambridge, Mass., said a human trial in 60 volunteers showed those receiving ACAM1000 produced equivalent levels of antibodies and antiviral T-cells as those receiving Dryvax.

Although numbers were too small to assess incidence of rare but devastating side effects like rampant pox and encephalitis, or brain inflammation, animal tests suggested ACAM1000 is safer than Dryvax. Both smallpox vaccines are based on a vaccinia, a vaccine strain of pox virus cultured in calfskin. But Thomas Monath, chief scientific officer at Acambis in Cambridge, Mass., explained Acambis selected the least virulent clone, or copy, of vaccinia virus, then grew it up in cell-culture to lessen side effects and bacterial contaminants.

Meanwhile under a $438 million federal contract, Acambis and its partner, Baxter International Inc., scaled up and produced 155 million doses of safer second-generation vaccine -- designated ACAM2000 -- for the national drug stockpile. Larger studies of effectiveness are continuing in 1,200 volunteers, Dr. Monath said.

Further, Acambis and several other companies are working on even safer, third-generation smallpox vaccines using a milder strain of virus called MVA, or modified vaccinia Ankara, that might be used in people at high risk due to medical conditions like eczema and AIDS.