Flu, RSV and COVID-19 coinfections: 4 updates

As flu, respiratory syncytial virus and COVID-19 collide this winter, questions linger about the potential for people to contract multiple infections at once.

Four latest updates on coinfections:

1. Coinfections were less common than expected this fall, even amid an early and severe respiratory virus season, according to Helix, a lab that assists the CDC with variant tracking, found. And last fall and winter — the first with substantial spread of flu and COVID-19 — just 6 percent of hospitalized children with flu also tested positive for COVID-19, a Dec. 16 CDC report found.

"It's not as common as we thought it might be," Michael Chang, MD, a pediatric infectious disease specialist at Memorial Hermann Health System in Houston, told The New York Times.

2. There are several possible reasons as to why clinicians have not seen a flood of patients with multiple infections. For one, RSV and flu are peaking in different parts of the country at different times, according to the Times. Patients who contract one infection may also be more likely to stay home, making them less likely to catch another virus, Helix said.  

Another theory focuses on a biological phenomenon known as viral interference. The theory is that exposure to one respiratory virus may put the body at high alert and amplify its ability to fight off other viruses. However, this defense mechanism is not always effective, according to John Moore, PhD, a professor of microbiology and immunology at Weill Cornell Medicine in New York City.

"If you are unlucky to be exposed to three different viruses simultaneously, there's no law that says you can't get all three," he told The Washington Post.

3. Scientists are still studying whether concurrent viral infections cause more severe disease or blunt each others' effects. In the CDC's pediatric flu report, children with flu and COVID-19 were more likely to require respiratory support and had higher mortality rates than children sick only with the flu. However, "whether coinfection increases disease severity compared with that associated with infection with one virus alone is unknown," CDC researchers said.

4. Some groups may be more susceptible to severe disease from coinfection due to genetics, past virus exposures and the types of medications they use. Groups who may be more susceptible include pregnant women, adults older than 65 and people who are immunocompromised, according to the Times.


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