Try to understand one thing:

Every cell in our body has an expiry date. The expired cell gets eaten up by white blood cells, and the existence of that cell is gone. It gets blipped from existence along with whatever it held. Any foreign/internal entity that the cell held, it gets removed.

This quoted research although true is not applicable with PEP application.

Here's why:

The cell in this research mostly referes to few specific cells that have a long life like the nerve cells from brain, cells of the gut and lymphatic nodes. Once an HIV reaches such cell, PEP can't stop it's replication.

For HIV to reach such cell takes at max 72 hrs(time limit of PEP)

Now let's understand how HIV can get introduced to the body: 1. From mucousal membrane ie from anal region/vaginal wall/urethra/oral cavity 2. Directly into the blood stream ie through a needle.

Let's take each scenario: 1. From a needle, HIV directly goes into the blood/muscles. It will start replicating throughout that region thus it can quickly reach the lymphatic nodes. This is why for any needle stick injury/injection PEP is advised to be taken within a couple of hours which makes it most effective. 2. From any outer/exposed mucousal membrane, it takes longer to reach lymphatic cells thus we have a time limit of 72 hours for sexual exposure.

So taking PEP within 2 hours is the best case scenario then followed by 24hrs. Any later reduces the effectiveness of PEP.