As syphilis levels hit 70-year high, sexually transmitted infection epidemic shows 'no signs of slowing'

illustration of the spiral shaped bacteria that cause syphilis
Syphilis, the sexually transmitted infection, is caused by corkscrew-shape bacteria. (Image credit: CHRISTOPH BURGSTEDT/SCIENCE PHOTO LIBRARY via Getty Images)

More than 2.5 million cases of chlamydia, gonorrhea and syphilis were reported in the U.S. in 2021 — nearly 6% more than were reported in 2020. 

Syphilis rates, in particular, leapt to levels not seen since the 1950s. More than 176,000 cases of syphilis were reported in 2021, up from nearly 134,000 in 2020. Rates of the bacterial sexually transmitted infection (STI) reached historic lows in the early 2000s — when they hovered around 30,000 per year — but they have climbed steadily since then, according to finalized data released by the Centers for Disease Control and Prevention (CDC) on Tuesday (April 11).

Out of the total reported syphilis cases, 2,855 were congenital syphilis, which occurs when syphilis-causing bacteria pass through the placenta during pregnancy. These cases resulted in 220 congenital syphilis-related stillbirths and infant deaths. Overall, the nationwide rate of congenital syphilis increased more than 30% between 2020 and 2021. Rates of the disease are 464% higher than they were in 2001.  

The resurgence of syphilis in the U.S. "remains a significant source of concern," Dr. Leandro Mena, director of CDC's Division of STD Prevention, wrote in the report. However, the 2021 rates of chlamydia and gonorrhea far outpaced those for syphilis.

Related: 900% increase in babies born with syphilis in Mississippi may be part of nationwide trend

More than 1.6 million cases of chlamydia were reported in 2021, making the STI the most commonly reported of the three. Chlamydia rates were up about 4% from 2020, signaling a shift towards pre-pandemic levels but not a full rebound. (Case rates dipped from 2019 to 2020, likely due to a decrease in screening related to the COVID-19 pandemic. Chlamydia is often asymptomatic and so cases may go uncounted when screening falters, the CDC report states.)

More than 710,000 cases of gonorrhea were reported in 2021, an increase of 4.6% from 2020. Since hitting a historic low in 2009, gonorrhea rates have increased by 118%.

It's estimated that about half of the 2021 infections showed resistance to at least one antibiotic drug, but nearly all circulating gonorrhea strains in the U.S. remain vulnerable to ceftriaxone, the recommended treatment for uncomplicated gonorrhea. Massachusetts health officials recently identified a "concerning" gonorrhea strain with "reduced susceptibility" to ceftriaxone, but even that strain could be killed with high doses of the drug. The CDC says it's only a matter of time before a strain becomes fully resistant to available treatments.

"The U.S. STI epidemic shows no signs of slowing," Mena said in the report. "The reasons for the ongoing increases are multifaceted — and so are the solutions." 

Why are sexually transmitted infections on the rise in the US? A perfect storm of factors, including pandemic-related disruptions, a reduced public health focus on sexual health, increased opioid use and changes in sexual behavior, such as decreased condom use. 

Nicoletta Lanese
Channel Editor, Health

Nicoletta Lanese is the health channel editor at Live Science and was previously a news editor and staff writer at the site. She holds a graduate certificate in science communication from UC Santa Cruz and degrees in neuroscience and dance from the University of Florida. Her work has appeared in The Scientist, Science News, the Mercury News, Mongabay and Stanford Medicine Magazine, among other outlets. Based in NYC, she also remains heavily involved in dance and performs in local choreographers' work.