MERS After COVID-19: Fewer Reported Cases, Fewer U.S. Tests, Travel Back to Normal

Middle East respiratory syndrome (MERS) is caused by a coronavirus, MERS-CoV, that circulates in camels and occasionally jumps to people. First detected in humans in 2012, it can cause severe respiratory illness, with an estimated case fatality rate of about 35%. Most human infections come from contact with camels; person-to-person spread is limited, but has caused household clusters and outbreaks in health care facilities.

A CDC team led by Anastasia S. Lambrou and Erin South, working with WHO colleagues, reviewed MERS cases worldwide, U.S. testing and travel from the Arabian Peninsula for 2017–2023, comparing the years before COVID-19 (2017–2019) with the years since (2020–2023).

Background

• Where it occurs. Most camel and human cases have been in the Arabian Peninsula. There is evidence of possible camel-to-human spread in parts of Africa, and of camel infection in South Asia.
• Travel cases. Travelers have carried MERS to at least 17 countries outside the Arabian Peninsula, sometimes sparking human-to-human spread — most notably the large hospital outbreaks in South Korea in 2015.
• In the United States. Two unrelated cases were confirmed in 2014, both health care workers recently back from Saudi Arabia, with no onward spread. There have been none since.
• No vaccine or specific treatment. A few candidate vaccines were in preclinical and early clinical trials; care is supportive, with experimental treatments possible.

Since 2012, 2,608 cases had been reported to WHO by December 31, 2023 — 2,200 (84%) in Saudi Arabia.

Before and after the pandemic

"People tested" are persons under investigation (PUIs); each may give several specimens. Travelers are modeled estimates. The region counted here takes in Bahrain, Iran, Iraq, Israel, Jordan, Kuwait, Lebanon, Oman, Qatar, Saudi Arabia, Syria, the United Arab Emirates, the West Bank and Gaza, and Yemen. Source: CDC, MMWR.

MERS cases, U.S. testing and travel, 2017–2023. Credit: CDC, MMWR.

• Cases worldwide were steady before the pandemic, a median of 224 a year, then fell sharply; most are still reported by Saudi Arabia.
• U.S. testing ran at a median of 343 specimens from about 124 people a year before the pandemic, peaking each August and September alongside the Hajj pilgrimage to Mecca. It dropped sharply from April 2020, with none at all from April to August 2020, and has since run at a median of 39 specimens from 16 people a year. In 2023, 58 specimens were tested — 285 fewer than the prepandemic median. None tested positive in 2017–2023.
• Travel from the region held at a median of 2,808,009 a year before the pandemic, peaking in July and August. It fell to 906,783 in 2020, then climbed back to 2,878,642 in 2023, the most since 2019. About half of 2022–2023 travelers landed at 10 U.S. airports, and 30.9% at New York City-area airports.

Where travelers from the region arrived, 2022–2023. Credit: CDC, MMWR.

Why did cases fall?

It is not yet clear whether fewer infections are happening, fewer are being found, or both. Possible reasons:

• Missed cases. Health systems strained by COVID-19 may have identified fewer patients at risk and tested less.
• Pandemic measures that cut transmission.
• Changes in how people interact with camels.
• Cross-protection from antibodies against SARS-CoV-2.
• Changes in the virus itself — which is why continued sharing of MERS-CoV genome sequences matters.

What has not changed: data do not show less virus in camels, and human cases without camel contact still occur. Travel has recovered, yet U.S. testing has not; if the share of travelers meeting testing criteria were unchanged, testing should have risen with travel.

Updated testing criteria

In 2024 CDC updated its criteria for who should be tested, to account for SARS-CoV-2 as another cause of severe respiratory illness, the spread of multiplex tests that check for many pathogens at once, and evidence of camel-to-human spillover in Africa. The updated risk factors include:

1. direct camel contact in or near the Arabian Peninsula, even with mild illness;
2. direct camel contact with recent travel to parts of Africa, in patients with severe illness;
3. work handling infectious MERS-CoV material in a laboratory or research setting.

Clinicians should take a thorough travel history from anyone with an unexplained acute respiratory illness and work with their health department to test patients who meet the criteria (CDC's MERS testing guidance). Places whose airports receive many travelers from the region are strategic spots to strengthen testing and surveillance, within a One Health approach linking human, animal and environmental health.

Limitations

• Case data reported to WHO under the International Health Regulations (2005) vary in completeness and quality.
• U.S. testing records carry little detail about the people tested.
• Traveler numbers are modeled from ticket sales.
• Country of departure stands in for risk and says nothing about other risk factors.

Sources

Based on Lambrou AS, South E, et al., "Update on the Epidemiology of Middle East Respiratory Syndrome Coronavirus — Worldwide, 2017–2023," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.