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

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

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

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.

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