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Measuring Vaccine Safety and Efficacy in Clinical Trials

Specimens (fluids and cells) collected from clinical trial participants are routinely analyzed in the laboratory using highly sensitive assays (or tests) in accordance with federal government regulations and Good Clinical Laboratory Practice guidelines. These assays, which are conducted during all phases of clinical trials, allow scientists to monitor the safety and well-being of the volunteers and to measure the immune response induced by the vaccine. Examples of routine safety assays include blood and urine sample analyses that measure blood cell counts, hemoglobin levels, and kidney and liver function; as well as screening for infection with syphilis, HIV, and/or hepatitis B and C viruses. In addition, diagnostic assays are also performed during an HIV vaccine clinical trial to distinguish whether a volunteer tests positive for HIV due to the vaccine (also known as vaccine-induced seropositivity) or due to natural HIV infection. The vaccines being tested cannot cause HIV infection because they do not contain the HIV virus. Vaccine-induced seropositivity, or VISP, occurs because vaccines cause the body to produce antibodies to components of the vaccine that resemble HIV.

Assays that measure the vaccine’s potential efficacy are known as “endpoint assays.” These tests are important because they provide useful information on whether a vaccine candidate should continue on for further clinical testing. There are currently four optimized and validated assays that are being used to evaluate vaccine efficacy, each with different requirements for testing such as type of specimen (whole blood or peripheral blood mononuclear cells), fresh or cryopreserved (frozen) samples, and length of incubation time.

  • IFN-γ ELISPOT assay—measures the amount of interferon gamma (IFN-γ), a protein produced by T cells following exposure to HIV antigens or peptides.
  • Intracellular Cytokine Staining (ICS) assay—uses a device (flow cytometer) to isolate T cells from other lymphocytes and measure cytokine production following exposure to HIV antigens or peptides.
  • Virus Neutralization assay—measures serum or plasma levels of neutralizing antibodies aimed at blocking HIV entry into target cells.
  • ELISA binding antibodies—detects whether a patient’s blood, serum, or plasma contains antibodies that can bind to HIV. 

Last Updated July 31, 2012

Last Reviewed July 31, 2012