The Neutralization Assay (NAb) is used to measure the ability of antibodies to block entry of virus into target cells. Neutralizing antibodies are considered important because a) pre-existing neutralizing antibodies have been shown to prevent HIV infection through a variety of routes in non-human primates; and b) a rapid secondary antibody response to infection (primed by prior vaccination) may help to control infection and decrease transmission.

Viruses can be classified into tiers based on the ease with which they are neutralized, with Tier 1 being very easy to neutralize and Tier 3 very difficult to neutralize. This classification is based on neutralization assays performed with many donor samples.