Pre-exposure prophylaxis, commonly known as PrEP, has changed the landscape of HIV prevention. Available since 2012, the daily pill that decreases the risk of contracting HIV from sex by more than 90 percent and by more than 70 percent from injecting drugs has become the latest harm reduction development for people who have an elevated risk of HIV infection. And a recent study conducted by Yea-Hung Chen at al. suggests that this is for good reason.

In undertaking their study, Chen et al. sought to describe the pattern of PrEP use among men who have sex with men (MSM) in San Francisco over a period of fourteen years and to quantify how PrEP is associated with the use of other harm reduction methods and HIV risk factors.

The study’s primary finding was that the prevalence of PrEP use within the previous twelve month period among MSM increased from 0 percent in 2004 to 44.9 percent in 2017. This is a rapid and significant increase in the proportion of men who have sex with men choosing to use PrEP for HIV prevention and is unquestionably a public health success. Even more stunning is the 35.1 percentage- point increase from 2014 to 2017. 

However, the study also found that as PrEP use has become more common, the percentage of MSM who consistently use condoms has decreased in tandem. For example, in 2004, the prevalence of consistent condom use was 36.8 percent, but it steadily declined to 7.1 percent by 2017. The authors suggest that such a decline in consistent condom use is at least partially associated with the increase in PrEP use. This is primarily because they observed a 10.4 percentage-point decrease between 2014 and 2017—the precise period of the most rapid PrEP uptake.

And while on its face, decreased condom usage may seem alarming, despite such decreases, the study notes that the number of new HIV cases has also continued to decline, which confirms the effectiveness of PrEP in preventing HIV even without associated condom use. In fact, the authors cite a modeling study that suggests PrEP prevented about 40 new HIV infections in MSM living in San Francisco in 2014. Using that estimate, PrEP use alone is responsible for more than half of the decrease in the reported number of infections between 2013 and 2014.

From a purely financial standpoint, the prevention of these new infections is hugely beneficial. A 2016 study found that PrEP is a highly cost-effective treatment that costs only $27,863 per quality adjusted life year (QALY). This is well below the established willingness to pay threshold in the United States, which is $150,000/QALY. Additionally, a conservative estimate of the lifetime cost savings from preventing one new HIV infection is $379,668 (in 2010 dollars). This means that even without considering the other benefits of preventing new HIV infections, the economic impact of PrEP use is cause enough to encourage its continued uptake.

Despite the value of these results, however, at a certain point, the authors regrettably begin to veer away from demonstrated results and toward speculation: for example, in their suggestion that increasing rates of other sexually transmitted infections (STIs), such as chlamydia and gonorrhea, are a result of PrEP replacing consistent condom use. Although the study does find an association between decreasing rates of consistent condom use and increasing use of PrEP, the specific dataset used in the study simply does not support the conclusion that PrEP use is responsible for increasing infection rates of other STI’s.

While it is true that limiting the transmission of other STIs should rightfully be included in the conversation about sexual health, we must not lose sight of the overall impact. While chlamydia, gonorrhea and syphilis (the primary STIs mentioned by the authors) are all currently curable (and less harmful when left untreated), HIV is currently treatable but incurable. Purely from a harm reduction perspective, then, preventing new cases of HIV must be the highest priority. Since consistent PrEP use is as or more effective at preventing HIV than consistent use of condoms alone, its utility as a harm reduction strategy should therefore not be minimized.

Moreover, there are also important ancillary benefits associated with increasing PrEP use. For example, to remain on PrEP, patients must undergo STI testing every three to six months. This allows early detection and thereby minimizes the amount of time that an infected person can transmit the infection to partners. It also has the added benefit of increasing interactions with healthcare providers, which can improve all aspects of health, not just sexual health.

Overall, Chen et al. is valuable in its contribution to our understanding of how PrEP use affects other risk reduction behaviors. And, as the authors state, the most effective HIV harm reduction strategy is concurrent PrEP use and consistent condom use, which is consistent with the Centers for Disease Control recommendations. Of course, the main principle of harm reduction is that it “meets people where they are,” and by that token, encouraging any method of HIV-risk reduction (even if it doesn’t eliminate all other risks) is vital to improving health among MSM and the population as a whole.