There are a lot of factors that determine the success rate of a post exposure prophylaxis which include:

+Duration of exposure

+Individual side effects

+Individual's state of health

+many more!

A PeP is really only effective within a certain time frame (within 3 days) of being possibly exposed to the virus. During this time, it really depends on if the virus will activate itself or if the person is able to produce enough antibodies to keep the virus at bay until the PeP is able to administered.

Secondly, the PeP is not for everyone particularly for those who have poor kidneys. Due to the intensity of the drug, potential side effects include potential negative effects to the kidney. Not everyone will experience side effects so it depends on the person.

Leading up to this is also the individual's current state of health. If the particular individual is not healthy and/or suffering from another ailment, it may not be wise to administer the PeP since it could worsen the individual's condition. The reality is, the PeP is a very powerful drug and should only be administered under the guidance of a health professional who has been schooled in PeP and PreP use.

Additionally, it has been difficult to measure the success rate of the PeP due to it's very nature of being a "PeP." If the drug is successful, it is hard to be able to test if the individual was initially exposed to the virus because the PeP would have destroyed it. However, if the drug was not successful then the virus would be present and the person would have the virus. Thus how would we know if the person was exposed at all unless we are able to trace it the source? However, even if the source was positive for the virus (for example two individuals engaging in unprotected sex), we wouldn't know if the source was absolutely contagious or if the virus was present in the fluid at the time of contamination. The only way to find out is to test on animals and measure the rate of success on animals.