Which Contact Spreads Mpox? Condomless Receptive Anal Sex Carried the Highest Risk

Since May 2022, a global outbreak of clade II mpox — caused by monkeypox virus (MPXV) — has spread mainly through sexual contact, hitting gay, bisexual and other men who have sex with men (MSM) hardest. Spread through nonintimate contact, such as contaminated household objects, has been reported but is rare. Which kinds of contact carry the most risk? A CDC analysis looked at people who knew they had been close to someone with mpox.

The study

The data came from a vaccine effectiveness case-control study in 12 U.S. jurisdictions, August 2022–July 2023, of sexually active people aged 18–49 who identified as MSM or transgender.

• Case-patients: people with a confirmed or probable mpox (or orthopoxvirus) diagnosis, found through health department registries.
• Control patients: people seen at sexual health, HIV care or HIV PrEP clinics who hadn't had mpox, matched by timing and region.
• Participants answered a survey online or by phone, in English or Spanish; vaccination was checked against registries where possible.

Of 1,487 eligible respondents, 207 (13.9%) — 150 case-patients (32.8%) and 57 controls (5.5%) — reported close contact with someone with mpox, and they are the subject of this analysis.

Which contact mattered

Before adjustment, several behaviors with the person who had mpox were linked to higher odds of infection:

After adjusting for age, race, HIV status and PrEP or antiretroviral use, number of partners, mpox vaccination, the month, and all the other contact behaviors, only one remained linked: condomless receptive anal sex, with 5.4 times the odds of mpox (compared with people who had close contact but not that).

Sharing bedding, towels or clothing was linked only before adjustment. Contaminated linens and utensils have been implicated as a less common route, both where mpox is endemic and in this outbreak; shared bedding during sex may also play a part, and more study is needed.

Condoms help, but aren't enough

Condoms may reduce exposure at anal, genital and oral surfaces, but they can't block everything: the rash can appear elsewhere on the body, and the virus can also spread through saliva and respiratory secretions.

Prevention: more than one tool

• Vaccination. The mpox vaccine is highly effective — yet only one in four of the roughly two million people eligible in the U.S. had received both doses. Clinicians should keep promoting it.
• Safer sex strategies to reduce exposure.
• Talk with partners about any mpox signs or symptoms, and notice unexplained rashes or sores on a partner's body.
• Avoid close or intimate contact if you or a partner is infected or has an mpox-like rash.
• Remember mpox is still circulating.

Limits: participants volunteered and controls came from sexual health and HIV clinics, so they may differ from others; answers were self-reported about sensitive behavior; the survey couldn't capture frequency, partner type, group sex or substance use; fewer than 15% reported close contact, limiting power to detect less common risks; and 12 jurisdictions may not represent the whole country.

Sources

Based on Anna N. Chard, Alexandra F. Dalton, Alpha Oumar Diallo and others, "Risk of Clade II Mpox Associated with Intimate and Nonintimate Close Contact Among Men Who Have Sex with Men and Transgender Adults — United States, August 2022–July 2023," MMWR, volume 73 (as corrected), Centers for Disease Control and Prevention; a work of the United States government in the public domain.