Can Clade I Monkeypox Spread on a Plane? U.S. Contact Tracing, 2024–2025

Monkeypox is a rash illness caused by Monkeypox virus (MPXV), which comes in clades (I and II) and subclades. It usually spreads from person to person through close, sustained physical contact.

Background

• Clade II spread worldwide from 2022; about 37,500 U.S. cases have been reported, and it still spreads domestically. CDC traced airline contacts of clade II travelers from 2021, but stopped routine investigations in February 2023 after finding no in-flight spread: a 2024 report found no secondary cases among 1,538 people who had contact with 113 infected travelers on 221 flights, though outcomes were missing for about a third.
• Clade I outbreaks began in Central Africa in 2023–2024 and spread across the continent, with about 150 travel-related cases outside Africa. Limited data suggested clade I might spread more easily than clade II. It has historically been deadlier, though recent outbreaks have had lower death rates — some similar to clade II.

Little was known about clade I and air travel, so CDC investigated.

The travelers

From November 2024 to January 2025, CDC learned of three men aged 20–40 who flew on five flights while infectious with confirmed clade I monkeypox. Each flew separately into the United States; two took domestic connecting flights. During travel they reported skin lesions or rash (two), fever (one) and malaise (one).

Tracing the contacts

Contacts were crew who served the travelers and passengers seated within two seats (6 ft / 1.8 m). CDC identified 80, all judged at uncertain to minimal risk, with 9 to 24 per flight. It excluded 20: 11 who knew the traveler outside the flight, 5 who had already left the country and 4 without a U.S. address.

The remaining 60 were referred to state and territorial health departments; 57% were women, with a median age of 43. Health departments reached 29 (48%) for interviews. One reported fatigue, aches and a skin lesion after the flight, but the lesion tested negative.

No secondary cases were found.

What CDC concluded

As with clade II, the risk of catching clade I monkeypox on a commercial flight appears low — though the sample was small and fewer than half of contacts were interviewed. In 2025, CDC stopped airline contact investigations for all monkeypox clades and subclades.

Still:

• People with monkeypox should delay travel until their illness has resolved.
• Travelers to places with outbreaks should be told about the risks.
• Vaccinating groups at risk has proven effective at reducing spread and severity.

Sources

Based on Lugo Robles R, Gertz AM, Alvarado-Ramy F, et al., "Notes from the Field: Contact Tracing for Monkeypox virus Clade I Cases Associated with Air Travel — United States, November 2024–January 2025," MMWR Vol. 75, No. 15, CDC; a work of the United States government in the public domain.