The study, published in the July 15 issue of The Journal of Infectious Diseases, examined 346 individuals who had participated in previous smallpox vaccination trials, 94 of whom developed fevers after vaccination.

The researchers at Washington University, in St. Louis; St. Louis University; and The Emmes Corp. analyzed 19 gene clusters -- called haplotypes -- linked to the body's response to viral infections.

The new study identified a total of eight haplotypes in four different genes that were associated with altered susceptibility to fever after vaccination, according to researcher Samuel L. Stanley Jr.

It is the first study to show that fever after smallpox vaccination is associated with specific gene clusters in the interleukin-1 (IL-1) gene complex on chromosome 2 and the interleukin-18 gene on chromosome 11. The interleukins, and especially the IL-1 gene complex, are groups of molecules associated with inflammation and immune responses.

The long-term goal is to determine genetic features that could be determined prior to vaccination, allowing practitioners to modulate the vaccination plan according to risk, according to Stanley.