Can Mpox Spread on a Plane? Contact Tracing After 221 Flights

Monkeypox virus (MPXV), which causes mpox, spreads between people through direct contact with lesions, scabs or saliva; through respiratory secretions; indirectly from contaminated objects; through skin-piercing injuries; and across the placenta in pregnancy. Since May 2022, about 33,000 U.S. mpox cases had occurred; most patients had painful skin lesions, typically in the anogenital area, and some had life-threatening complications or long illness.

Before 2021, little was known about the risk of catching mpox on a commercial flight, and two earlier investigations had found no cases among passengers seated near infected travelers. When a large global outbreak of clade IIb mpox — mainly linked to male-to-male sexual contact — was recognized in May 2022, hundreds of infectious people traveled by air. CDC investigated.

How contacts were identified

When a health agency learns that someone with certain communicable diseases flew while infectious, CDC decides whether to start an aircraft contact investigation and requests the flight manifest. A person with mpox counted as infectious from symptom onset until all lesions had crusted, the crusts had fallen off and fresh skin had formed.

CDC defined an exposure risk zone:

• flights of 3 hours or less: passengers within 3 feet (one seat in any direction);
• flights over 3 hours: within 6 feet (two seats in any direction);
• two July 2021 flights with a passenger who had an extensive rash with pus: everyone who may have used the same lavatory;
• crew serving the infected passenger's cabin; and, if a crew member was infectious, fellow crew who worked more than 3 hours with them. Passengers were not counted as contacts of infected crew, because crew typically wore gloves and masks and had only brief contact with any one passenger.

Contact details went to health departments where travelers lived, for symptom monitoring for 21 days after the flight — the longest known incubation period. Under the International Health Regulations, CDC also notified other countries about infectious travelers and contacts heading there.

Results

In all, 113 infectious people (100 passengers and 13 crew members) traveled on 221 flights. Of 1,538 contacts, outcomes were known for 1,046 (68%), and none acquired mpox from a U.S. flight.

CDC also notified 41 countries about 299 international travelers — 84 people with mpox and 215 contacts. Of the 21 countries that reported back, one said a flight contact developed mpox within 21 days, but there was not enough information to judge whether it was caught on the flight; privacy laws there blocked further inquiry.

What it means

The results match an investigation by Australia's Victorian Department of Health of 15 international flights in May–October 2022, which used a wider contact zone and found no secondary cases. The risk of catching mpox on a plane, even when seated near an infectious person, appears very low — probably in part because the direct contact with lesions behind most transmission rarely happens on flights. Based on this evidence, CDC stopped routine aircraft contact investigations for mpox in August 2022.

Clade I mpox has historically been more transmissible than clade II, but both spread the same ways, mainly through close physical or intimate contact such as sex or shared bedding. Limited aircraft investigations could be considered for the first clade I cases in recent air travelers to confirm the risk is similarly low.

Aircraft contact investigations take heavy resources that could go to more productive prevention, especially during a large outbreak. This is the largest published series of mpox aircraft contacts, and it indicates that flying with someone who has clade II mpox is not in itself an exposure that warrants routine contact tracing. CDC still recommends that people with mpox isolate and delay travel until they are no longer infectious.

Limitations: outcomes were missing for about a third of contacts; passengers of infected crew weren't counted; and all cases were clade II.

Sources

• Delea KC, Chen TH, Lavilla K, et al. "Contact Tracing for Mpox Clade II Cases Associated with Air Travel — United States, July 2021–August 2022." Morbidity and Mortality Weekly Report 73(35), Centers for Disease Control and Prevention.