Posts that viral respiratory disease 'kills four in 10 patients' misrepresent small study

"The US is experiencing an outbreak of a virus, with a 43 percent mortality rate after 100 days of infection," reads the caption on a video posted on Douyin, the Chinese version of TikTok, on May 31, 2023.

"The CDC has detected a virtually unknown 'deadly virus' called human metapneumovirus (hMPV) that has been on the rise since spring of this year," it goes on to say, adding the research was published by the US National Institutes of Health.

The post has been shared more than 8,000 times.

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Screenshot of the misleading Douyin video, captured on June 6, 2023

The claim circulated after the United States CDC released data in May showing hMPV cases had spiked across the country during the spring (archived link).

However, hMPV is not a "virtually unknown" virus as the post says. It was first identified by Dutch researchers in 2001 (archived link).

According to the American Lung Association, hMPV usually causes symptoms similar to the common cold that last roughly two to five days in most patients (archived link).

Pictures and videos with a similar claim were also shared on Twitter, WeChat, Zhihu and TikTok, as well as on Chinese news aggregator sites Sohu, Netease and Sina.

The Weibo hashtag "new US virus has 43% mortality rate after 100 days of infection" also racked up more than four million views.

These posts misrepresent data from a small study.

Immune-compromised patients

A keyword search found the study referenced in the circulating claim published in May 2013 and indexed on the National Institutes of Health's website (archived link).

The study says hMPV can cause "severe and even fatal disease in immunocompromised patients". It examined the infection in people needing hematopoietic cell transplantation (HCT), an infusion of cells for patients with diseases like cancer who need to improve marrow and immune functions (archived link).

The study included 23 patients needing or receiving HCT who had hMPV detected in samples between January 2006 and February 2011. Based on this group of patients with underlying conditions, the researchers found that "mortality rates at 100 days were 43% for both HMPV and respiratory syncytial virus lower respiratory tract disease" (archived link).

Infectious disease scientists told AFP the results of the study do not reflect the risks hMPV poses to most people.

"Infection with hMPV is most serious in patients who are notably immune-compromised, such as HCT recipients," William Schaffner, professor of preventive medicine at Vanderbilt University, told AFP on June 7. "But those concerns do not apply to the large general population."

Given hMPV infections are usually mild, widespread testing is not thought to be a wise use of medical resources, and is reserved for sicker patients or research projects, Schaffner said.

Larissa May, director of emergency department antibiotic stewardship at UC Davis Health, told AFP on June 7 that because there is no routine testing of the general population the data on hMPV infections in healthy individuals is "very limited".

The virus tends to start circulating in winter and lasts until or through spring, according to the CDC. The majority of infections are transmitted by close contact, similar to other respiratory viruses (archived link).

There is currently no treatment or vaccine for the disease, and the CDC advises people to prevent infection by washing their hands and avoiding close contact with anyone sick.