Onslaught of Viral Respiratory Tract Infections: Human Metapneumovirus Joins the Fray

“Reports of human metapneumovirus (HMPV), a common respiratory illness that causes flu- and cold-like symptoms, are growing across the Northern Hemisphere, including in China, though neither the World Health Organization (WHO) nor the CDC have expressed alarm at this time.”            

Shannon Firth, MedPage Today, January 8, 2025

“‘Sowing seeds for next pandemic’: Trump order for US to exit WHO prompts alarm.”

The Guardian, January 21, 2025

Odds are that you and/or at least one family member or friend has been sickened by an upper respiratory infection this year. In mid-January, the Centers for Disease Control and Prevention (CDC) reported a rise in cases and hospitalizations due to influenza A and respiratory syncytial virus (RSV). At the same time, cases of SARS-CoV-2 infection, the cause of COVID-19, were increasing in at least nine states. (I have featured these viruses multiple times in past Germ Gems posts.) In addition, the CDC reported recently an increase in cases of human metapneumovirus (HMPV)—a respiratory tract viral infection that I haven’t even mentioned in a post. It seemed like a good time therefore to provide Germ Gems readers with an overview of HMPV. I end this post, however, on another note—with some alarming news about the CDC and the World Health Organization (WHO). I feel I would be remiss not to do so.

What to know about HMPV? HMPV was isolated for the first time in 2001. It is a single-stranded RNA virus of the family Pneumoviridae and is closely related to RSV.

HMPV causes symptoms similar to the common cold, including cough, runny/stuffy nose, fever, headache, sore throat, fatigue, wheezing and shortness of breath. Most HMPV cases involve mild  symptoms that resolve after a few days. But in immunocompromised patients, HMPV infections can lead not only to hospitalization but can even be fatal.

HMPV spreads in the same way as most other respiratory viruses, that is, from an infected person or a contaminated surface. Hence, patients should cover their coughs and wash their hands often with soap and water—the same precautions one should take if one has a common cold.

The virus is distributed worldwide. In temperate regions, HMPV has a seasonal distribution generally following that of RSV and influenza viruses during late winter and spring. Serologic studies have shown that by the age of five, virtually all children worldwide have been exposed to HMPV. Despite near universal infection during early life, reinfections are nonetheless common in older children and adults.

From 2010-2012 in the U.S., HMPV was responsible for 12% of cases of acute respiratory tract illness (outpatient) in otherwise-healthy children and 15% and 8% of cases (respectively) of community-acquired pneumonia requiring hospitalization in children. It is now the second most common cause—after RSV—of acute respiratory tract illness in otherwise-healthy children under the age of 5. (HMPV has held this ranking since 2016.)

Who’s at risk?. HMPV usually causes a mild upper respiratory tract infection (e.g., the common cold). Those at risk for developing more severe disease and even hospitalization include premature infants, children younger than 6 months, people with a weakened immune system, people with asthma or chronic obstructive pulmonary disease and adults 65 years or older. In some studies of hospitalizations and emergency room visits, HMPV is nearly as common and severe as influenza in older adults. And numerous outbreaks of HMPV causing fatalities have been reported in long-term care facilities for children and adults.

Co-infection of HMPV with other viruses is also possible. These include RSV, influenza virus, and SARS-CoV-2. Because of overlapping of symptoms of these viral respiratory tract infections, clinical diagnosis of HMPV is difficult, if not impossible. In the case of more severe illness, testing of respiratory tract secretions with molecular assays is indicated.

There is no antiviral treatment of HPMV and vaccines against this virus don’t yet exist. Therefore, it’s all the more important to get vaccinated against influenza, RSV, and SARS-CoV-2.

Updates on other viral respiratory tract infections. In comparison to the viral pathogens of the respiratory tract for which we have effective vaccines (influenza virus, RSV, and SARS-CoV-2), HMPV is of much less concern in terms of severe illness and mortality. It is highly unlikely that HMPV will precipitate the next human pandemic.

Influenza A H5N1 (the cause of bird flu), on the other hand, is waiting in the wings to trigger the next human pandemic. The mutation allowing influenza A H5N1 to jump from person-to-person has not developed, at least not yet. It is reassuring to me that the situation is currently being closely monitored.

Recently, the CDC issued an advisory to clinicians and laboratories to subtype all influenza A isolates from hospitalized patients with the flu, thereby increasing surveillance for H5N1. In addition, the US Department of Human Services awarded $590 million to Moderna to develop vaccines against pandemic flu threats, including H5N1 bird flu.

Bad news and when is no news not good news. The WHO is a specialized agency of the United Nations responsible for global public health. (See “Why Who?”, Germ Gems, April 7, 2021.) In 1948, the U.S. played a pivotal role in the founding of the WHO and has provided vital support to the organization since then.

You may recall that on July 8, 2020, during his first term in office, President Trump ordered the U.S. to withdraw from the WHO because of his dissatisfaction with the WHO’s handling of the COVID-19 crisis. President Biden reversed this decision on his first day in office in January 2021.

On his second inauguration day and at about the same time that the WHO was conferring with the CDC about the increase in HPMV cases worldwide, President Donald Trump again ordered the withdrawal of the U.S. from WHO. This act came as an enormous disappointment and shock to those involved in global health and to some was seen as a return of the politicalization of public health.

Another worrisome order coming from the White House on January 22, 2025 was a freeze on communications of federal agencies with the public. This included the stoppage of publication of the CDC’s Morbidity and Mortality Weekly Report (MMWR).

The average person is probably not aware of MMWR, a report that the CDC has published without interruption since 1960. But, physicians and other health care professionals have depended and still depend on MMWR for timely alerts regarding health threats.

For the moment, we can only hope that these steps in the wrong direction for public health are quickly resolved. If not, we will soon discover what’s most toxic: the pathogens or the misinformation, disinformation, and information blackouts that are besieging us.