Efficacy trials involve thousands of people and end not after a set period but after a set number of infections.

Small-scale efficacy trials were carried out in the 1970s and 1980s, and attention grew in the 1990s following a meta-analysis.

Adequate safety and efficacy trials must be carried out.

As of late 2010, those trials, while promising, were not expected to be finished until late 2015, with efficacy trials running in parallel from 2009 to 2019.