Estimating the Size of the U.S. Population at Risk of Severe Adverse Events from Replicating Smallpox Vaccine

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To quantify the population at risk of serious adverse reactions to replicating smallpox vaccine.

Design and Sample:

Conditions known or suspected to carry risk were identified via Centers for Disease Control and Prevention planning documents, other federal publications, and peer-reviewed literature. Conditions identified were categorized as historically recognized risks or more recently recognized immunocompromised states that may pose risk. Major historical risk factors were as follows: eczema/atopic dermatitis, pregnancy, HIV, and primary immunodeficiency. More recently identified states were as follows: rheumatoid arthritis, inflammatory bowel disease, dialysis, bone marrow transplant recipients within 24 months post-transplant, solid-organ transplant recipients within 3 months post-transplant, age under 1 year, and systemic lupus erythematosus.


The estimated prevalence or absolute number of affected individuals for each condition was ascertained from peer-reviewed studies, vital statistics, and registry databases.


An estimated 48,121,280 to 50,028,045 individuals (15.2–15.8% of the U.S. population) are potentially contraindicated to replicating smallpox vaccine. This rises to 119,244,531 to 123,669,327 (37.4–38.8%) if household contacts are included.


These figures are significant and larger than the only previously published study. Understanding this number allows for improved clinical utilization, equitable attention to the health needs of a vulnerable population, and strategic vaccine stockpiling.

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