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Fewer injections of anthrax vaccine may provide protection

SAN FRANCISCO – A three-dose schedule of intramuscular primary injection with triennial boosters may be effective in protecting against anthrax, according to findings presented here by Jennifer Wright, DVM, MPH, of the CDC.

Wright and her colleagues randomly assigned 1,563 adults at five sites in the United States to one of six regimens, all of which involved eight injections of anthrax vaccine absorbed or saline placebo:

• eight subcutaneous injections at week 0, 2 and 4 and at month 6, 12 and 18 plus 30- and 42-month boosters (8SQ)
• eight intramuscular injections at week 0, 2 and 4 and at month 6, 12 and 18 plus 30- and 42-month boosters (8IM)
• intramuscular injections at week 0 and 4, month 6, 12, 18, 30 and 42 (7IM)
• intramuscular injections at week 0 and 4, month 6, 18, 42 (5IM)
• intramuscular injections at week 0 and 4, month 6 and 42 (4IM)
• Saline placebo injections per the eight subcutaneous or intramuscular schedule

Saline substitutions were added for dropped anthrax vaccine absorbed doses in groups receiving reduced doses.

The researchers analyzed sera for anti-protective antigen antibody levels. They also conducted non-inferiority analyses for antibody levels at four weeks after doses at week four and at months six and 42.

Participants who received four intramuscular injections maintained measurable antibody levels for 36 months between the six-month priming dose and the 42-month booster. Significantly higher antibody levels were observed in the four-injection group in comparison with all other groups.

“The results demonstrated that four intramuscular injections over 43 months provide an immune response that is higher, actually double, that of the response provided by eight subcutaneous injections,” Wright said. “We have demonstrated that the vaccine is safe, that it works better with half the number of doses as originally licensed and that circulating antibody produced by the vaccine is still measurable three years following the six-month injection, which was previously believed to be untrue.”

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