## 4 Discussion  
It is clear from these results that PrEP can be a highly effective tool in mitigating and controlling the spread of HIV infections. Proper PrEP usage is ideal but even under the pessimistic scenario, the projections for HIV infections are still significantly reduced when the average number of sexual partners is two. The importance of the number of average sexual partners per year is more prominent in the pessimistic case and suggests that improper PrEP usage and an increased number of partners minimize the long-term effectiveness of PrEP for mitigating HIV management.  
We emphasize that our model and its results are based on U.S. data as a whole. Policy decisions, both in the U.S. and other countries, should include the consideration of subgroups involving educational level, race, ethnicity, and economic status. In addition, because PrEP usage does involve minimal monitoring, ease of access is also important.  
Our model does not currently discriminate between the various subgroups that are included in the high-risk population. However, the model may be modified to selectively analyze a chosen age range, ethnicity, or a specified income range, for example. This would require appropriate parameters for each subgroup. The model is only limited by the availability of data. We note that the CDC case surveillance data will allow for breakdowns by age, race/ethnicity, gender, and transmission categories with the caveat that some data can be suppressed due to agreements between the CDC and state level HIV surveillance initiatives [11].  
Clearly, policy decisions need to be based on the cost and effectiveness of different methods of preventing HIV infections. When used properly, condoms and PrEP are close to being equally effective. Condom usage is inexpensive and does not require monitoring. In  
Figure 5: This plot shows total susceptible and total infected population percentages as a function of PrEP usage at the 50-year mark for the optimistic (black and red) and the pessimistic (green and blue) scenarios. This uses the assumption of two partners/year.  
addition, condoms may be cost-effective in less well-developed countries or among lower income groups. PrEP also requires access to care which could be an issue in some African countries, for instance. However, there is widespread reluctance to use condoms. To illustrate, in the U.S. only about 35% of the high-risk population uses condoms consistently, and this number has been dropping in recent years. The percentage of men in the 18-24 age range using condoms is even lower, suggesting that they might particularly benefit from a push to use PrEP [17; 19]. This is particularly important for young black men who have a 25% chance of being infected with HIV by age 25 [20]. There are many other countries where consistent condom use is near or below 50%. This includes, for example, Namibia at 46% [21], Bangladesh at 34-50% [22], and Thailand at 55% [23].  
We note that, in the U.S. after the Affordable Care Act, the use of PrEP and its associated costs are mandated to be free under most insurance plans, including Medicare and Medicaid. This suggests that it is reasonable to encourage PrEP use within the U.S. as economic considerations have been removed.  
This exploration utilizes the most recent data available from the CDC case surveillance data and projections can be improved upon as more salient data are collected, allowing for a better evaluation of some of the key parameters. Future work should include allowances for growth in the percentage of PrEP usage over time as more physicians and clinicians become more knowledgeable about PrEP and raise awareness with patients that are at risk of HIV infection.