The vaccine schedule should be followed as closely as possible. However, if a person is late for a dose, the next dose should simply be given as soon as possible. Then subsequent doses should be given according to the standard dosing intervals from the most recent dose. This applies to anthrax vaccine, as well as other vaccines, according to the Centers for Disease Control & Prevention. http://www.cdc.gov/mmwr/PDF/rr/rr4915.pdf
If an annual booster has not been administered on time, administer the booster dose at the earliest possible date, adjusting the subsequent booster schedule accordingly. Once the primary series of five doses is complete, the primary series is never repeated.
The current FDA-licensed schedule calls for doses to be administered intramuscularly in the deltoid according to the following schedule (the first dose is considered "week 0"): 0, 4 weeks; 6 months, 12 months, and 18 months.
Recipients receive the first shot, then the second shot four weeks later, and then the third shot five months after the second shot. Six months after the third shot, recipients receive the fourth shot. Six months after that, they receive the fifth shot. The entire primary series takes 18 months to complete. Annual booster doses of the vaccine are required for ongoing protection.
On 11 Dec 08, the Food and Drug Administration (FDA) approved two updates to the package insert for the anthrax vaccine. The route of administration was changed from a subcutaneous injection over the deltoid to an intramuscular injection in the deltoid. The FDA also approved a change in the vaccination series by removing the 2-week dose. The new schedule is now 0, 4 weeks, and 6 months, 12 months, 18 months, and annual boosters.