CDC Releases Updated Anthrax Vaccine Recommendations

In 2001 the United States witnessed a string of attacks in which letters containing Bacillus anthracis were mailed to a variety of public figures. Ever since, anthrax has been known as a serious bioterror threat, which on rare occasions, can be acquired through the environment as well.

The US Centers for Disease Control and Prevention (CDC) has released updates to CDC Advisory Committee on Immunization Practices (ACIP) recommendations on Anthrax vaccination in the Morbidity and Mortality Weekly Report. New recommendations include changes to booster dose procedures and large-scale emergency response protocols.

The report summarizes data on estimated efficacy using safety information published since the last ACIP recommendations were made in 2009. The report also provides updated recommendations for anthrax vaccine adsorbed for both preexposure and postexposure prophylaxis, gives guidance regarding preexposure vaccination for emergency responders, and summarizes data on investigational anthrax vaccine AV7909.

Updates to the recommendations include changes to the booster dose protocol. First responders that are not at high risk can opt-in to preexposure vaccination. Previously, those not at high risk were recommended for annual vaccination. In the updated recommendations, those who have received the initial 3 dose priming and 2 dose booster series can receive an additional booster dose every 3 years.

Updates pertaining to a large-scale emergency response include changes to route of administration and recommendations on dose-sparing if the anthrax vaccine supply is insufficient.

Based on the recommendations, the subcutaneous route of administration of anthrax vaccine is preferred to intramuscular administration because it results in higher antibody concentrations by week 4. This leaves survival estimates, based on the correlates of protection model, 3.8% higher by week 4. On the other hand, by week 9, predicted survival is no longer significantly different.

The updated recommendations also allow for the use of intramuscular administration of anthrax vaccine if significant material challenges arise preventing subcutaneous administration.

In a large-scale accident or aerosolized bioterror attack that releases spores over a densely populated region, hundreds of thousands of people may require vaccination. The report notes that most health care providers have more experience administering vaccines by the intramuscular route, and it is easier to train new vaccinators in the method than in subcutaneous administration.

The authors of the report also pointed to the different reactogenicity between routes of administration, citing more adverse effects such as injection site warmth or swelling in the subcutaneous route.

“Because the preponderance of injection site adverse events was associated with the SC [subcutaneous] route, concern has been raised that using this route might decrease the likelihood of patients completing the second and third doses of AVA [Anthrax Vaccine Adsorbed.]”

Due to the fact that a large-scale anthrax event may require pre-exposure vaccination for more people than could be vaccinated with the available stockpile, alternative dose-sparing regimens are included in the recommendations. The 2 full-dose strategy that the authors provide could expand the existing supply of vaccines by 50%, and the 3 half-dose strategy could expand the supply of vaccines by 100%.

A second-generation investigational anthrax vaccine, AV7909, is also discussed in the updated recommendations. AV7909 consists of the existing anthrax vaccine combined with CpG 7909, a synthetic immunostimulatory oligodeoxynucleotide.

According to the report, CDC has submitted a pre-emergency use authorization request to allow emergency use of AV7909 for post-exposure treatment when the supply of the currently licensed anthrax vaccine is insufficient.

The report calls for future research of immunogenicity and safety in populations such as children or older adults, evaluation of the interchangeability of anthrax vaccine adsorbed and AV7909, and the optimal duration of antimicrobial use in in postexposure contexts.

While anthrax has faded from headlines in recent years, these updated recommendations reflect ongoing seriousness about the possibility of a mass casualty event and strategies to increase the number of people receiving effective treatment in the event of a major incident involving anthrax.