> to my knowledge there have only been two recorded infections in people who were reliably taking PReP, both of which were due to unusual, rare mutations. The

> to my knowledge there have only been two recorded infections in people who were reliably taking PReP, both of which were due to unusual, rare mutations. The number is actually six, but given the number of people who've been taking PrEP since it was FDA-approved in 2012, that's still an impressive number. The reason emtricitabine and tenofovir disoproxil fumarate work well together is because the primary known mutation that makes HIV resistant to the latter also increases its susceptibility to the former. There's a analogous situation in reverse with the primary mutation that provides resistance to emtricitabine. PrEP is effective because strains of HIV that are resistant to both drugs are very rare, and also because most people who have resistant strains of HIV are on ARV treatment, which suppresses the viral load to the point where they cannot infect anyone else anyway (even without PrEP).