What does that mean, exactly? Well:

• Without PrEP, your risk of contracting HIV as a result of a single sexual encounter with an HIV-positive partner is estimated at 1.43% [Notes 1, 2, 3].
• If you are on PrEP, that risk is reduced by 99% to 0.0143%.

To put that another way: without PrEP, the risk is 1 in 70, but with PrEP it’s 1 in 7,000.

The risk is substantially higher (about 26 times higher) from a sexual encounter with someone who recently became HIV-positive [Note 2]. For about 12 weeks after becoming infected, the viral load can be very high and it is much easier for someone to transmit HIV to a sexual partner. In this case, without PrEP, the risk may be as high as 37% (about 1 in 3). If you are on PrEP, though, even this risk is reduced by 99% to 0.37% (about 1 in 270). People who recently contracted HIV may not yet know that they are HIV-positive and so may identify themselves as HIV-negative.

Note: The 99% figure, and most of the other numbers here, have been confirmed only for cisgender men who have sex with men (MSM) and transgender women engaging in anal sex; for more details, see this article for more details.

These estimates are not relevant if your HIV-positive sex partner has maintained an undetectable viral load for 6 months or longer. In that case, there is effectively no risk of sexual transmission of HIV, even if you are not on PrEP [Note 4].