While prevention of infectious disease has been the primary goal for vaccines, studies also have raised the specter of postexposure effectiveness for many of them. Yet, according to a new review, research in that area is very limited.

“Many available vaccines have demonstrated post-exposure effectiveness, but no published systematic reviews have synthesized these findings,” write researchers from Dartmouth and the Harvard School of Public Health.

“While preexposure vaccination protects uninfected individuals from infection, post-exposure vaccination serves to modify or prevent clinical disease among those who are already infected,” the authors add. “As a result, post-exposure trials must operate within a constrained timeframe: participants must be identified and treated between exposure and symptom onset. Measurable benefits may occur if the vaccine stimulates an immune response faster or larger than that provoked by the natural infection alone.”

For the article published in Epidemiologic Reviews, the study team searched the PubMed database for clinical trials and observational human studies concerning the postexposure vaccination effects, targeting infections with FDA licensed vaccine plus dengue, hepatitis E, malaria, and tick borne encephalitis, which have licensed vaccines outside of the United States.

Excluded were any studies involving animal models, serologic testing, and products still in the pipeline. Researchers evaluated eligible studies by definition of exposure, while rating on a scale of one to four attempts to distinguish pre- and postexposure effects. Of 4,518 articles screened, 14 clinical trials and 31 observational studies were included in the review, representing 45 eligible articles on seven of the 28 vaccine-preventable diseases. 

For secondary attack rate, results determined the following medians for postexposure vaccination effectiveness: hepatitis A: 85% (interquartile range: 28; 5 sources); hepatitis B: 85% (IQR: 22; 5 sources); measles: 83% (IQR: 21; 8 sources); varicella: 67% (IQR: 48; 9 sources); smallpox: 45% (IQR: 39; 4 sources); and mumps: 38% (IQR: 7; 2 sources). 

Researchers noted that for case fatality proportions resulting from rabies and smallpox, the vaccine efficacies had medians of 100% (IQR: 0; 6 sources) and 63% (IQR: 50; 8 sources), respectively, for postexposure use. 

“Although mainly used for preventive measures, many available vaccines can modify or preclude disease if administered after exposure,” the authors concluded. “This post-exposure effectiveness could be important to consider during vaccine trials and while developing new vaccines.”

“Since the advent of variolation in the early second millennium, vaccination has been considered a way to prevent healthy individuals from acquiring disease. However, in order to implement informed trials and curb future outbreaks and epidemics, post-exposure effectiveness must be better understood,” they add, pointing out that the rabies vaccine has seen near total efficacy in exposed individuals for the past century. 

The researchers also cite the smallpox vaccine, which “provided well-documented post-exposure prophylaxis (PEP) until the disease was eradicated in 1980, with recommendations dating back to the mid-19th century.”

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