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The Centers for Disease Control and Prevention’s (CDC) 2021 STD Surveillance Report indicates cases of sexually transmitted diseases decreased during the early months of the pandemic, but most resurged by the end of the year and into 2021.

The CDC report also highlighted how STD rates (number of infections per 100,000 people) vary by state, age, and sex. Here's a deeper look at that data.

What are STDs?

STDs, or sexually transmitted diseases, are infections that are primarily spread through sexual contact. These diseases affect both men and women and are caused by bacteria, viruses, parasites, or other microorganisms. STDs can be transmitted through vaginal, anal, or oral sex, or from mother to child during childbirth or breastfeeding.

STDs are also commonly referred to as STIs (sexually transmitted infections).

Which STDs are tracked by the CDC?

The CDC tracks reported cases of chlamydia, gonorrhea, syphilis, and congenital syphilis.

Human papillomavirus, or HPV, is the most common STD in the nation, but most people with the infection have no symptoms, and no national case-reporting system for HPV infections exists.

Of the STDs tracked by the CDC, chlamydia makes up the largest proportion of cases in the US, with over 1.6 million cases (496 cases per 100,000 people) reported to the CDC in 2021.

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It's also worth noting that HIV data is collected in a separate CDC Surveillance Report. There were 36,136 cases reported in 2021.

Which states have the highest STD rates?

In 2021, the five states with the highest combined rates of chlamydia, gonorrhea, and syphilis cases per 100,000 people were Mississippi, Louisiana, Alaska, South Carolina, and South Dakota.

State STD cases per 100,000 people
Mississippi 1,266
Louisiana 1,160
Alaska 1,091
South Carolina 1,052
South Dakota 1,009
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How do STD rates vary by age and sex?

Younger adults are more likely to be affected by STDs, but differences in infection rates between men and women vary by type of STD.

Women were more likely than men to report chlamydia infections. However, men were more likely to report both gonorrhea and syphilis infections.

Infection rates peaked between the ages of 20–34, and nearly half (46%) of all new STDs occur among people between the ages of 15–24. Adults over 35 were less likely than younger adults to report chlamydia, gonorrhea, and syphilis infections.


Women aged 20–24 made up the largest group in the United States of those reporting chlamydia infections. Overall, women and girls aged 10–34 outpaced men in rates of chlamydia infections.

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While men reported higher rates of gonorrhea than women above the age of 25, women aged 20–24 made up the largest group in the US reporting gonorrhea infections in 2021.

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The highest rates of syphilis (both primary and secondary syphilis infections[1]) were reported by men across all age groups. The highest rate of syphilis was reported by men aged 25–29.

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Where did this data come from?

This data was gathered from the CDC's 2021 STD Surveillance Report. In this surveillance report, the CDC warned that the data may be affected by the COVID-19 pandemic, and should be interpreted cautiously.

Learn more about the state of health in the US, rising STD rates during the pandemic, and get the data directly in your inbox by signing up for our email newsletter.

Sexually Transmitted Disease Surveillance 2021
Last updated
April 11, 2023

Primary syphilis refers to the initial stage of the infection caused by the bacterium Treponema pallidum. Secondary syphilis refers to the more advanced stage of the infection, indicated by an increase in the amount of circulating Treponema pallidum in the blood.