Some health experts are warning of increased potential for Covid-19 and flu coinfections this winter—but several measures can be taken to protect against this "double whammy."

'How do we know this gentleman isn't also infected with the pandemic virus?'

In February 2020, a man visited a ProHealth Urgent Care facility in Queens, N.Y., with a cough and fever. At that point, Covid-19 cases were rising in certain areas of the country, but New York City did not yet have any confirmed cases, The Atlantic reports.  

Hoping the man's symptoms were just a "grim coincidence," an ED physician swabbed the patient's nose for a flu test. Much to the health care team's relief, his test came back as positive for influenza.

However, Daniel Griffin, a researcher at Columbia University and the chief of the infectious-disease division at ProHealth, assumed the worst until they were able to run some more tests. "Wait a second," he said, "How do we know this gentleman isn't also infected with the pandemic virus?"

In early March, the man's test results confirmed Griffin's suspicions—the patient had also tested positive for the coronavirus. In fact, his whole family tested positive for both Covid-19 and the flu.

What are coinfections—and how prevalent are they?

Leading up to the 2020-2021 flu season, health care experts warned of a potential "twindemic" of influenza and Covid-19—but it never happened. Many believed the historically low number of 2020 flu infections occurred because of Covid-19 safety measures, such as masking and social distancing.

Currently, experts don't fully understand how Covid-19 and influenza combine within the same body—or how often coinfection really occurs, The Atlantic reports.

However, people hospitalized with severe Covid-19 were prone to developing additional illnesses while in the ICU, experiencing what is known as a "superinfection." Some researchers have estimated that as many as half of all Covid-19 deaths can be attributed to mixed infections, The Atlantic reports, though this statistic is disputed.

Between relaxed Covid-19 restrictions and fewer people getting their flu shots, Griffin said he "actually think[s] [coinfections are] more of a risk this year." In fact, Griffin argued that coinfection cases like the one he saw at the start of the pandemic could become even more prevalent.  

New research also suggests that various coinfections could be more common than previously thought, The Atlantic reports.

According to Aubrey Cunnington, the head of the pediatric-infectious-diseases section at Imperial College London, throat and nasal swabs on children often indicate several coinfections—for example, a combination of rhinovirus, parainfluenza, and respiratory syncytial virus. 

"We often see two or even three different viruses come up positive," Cunnington told The Atlantic. "Coexisting infections with different organisms, particularly viruses, are the rule rather than the exception." Cunningham also noted that children with more than one pathogen don't necessarily seem sicker than others, indicating that a coinfection may not always lead to worse outcomes.

And while new molecular testing can be highly sensitive, often detecting coinfections from lingering fragments of a dormant infection, Griffin argued that this type of screening should still be used for patients hospitalized with Covid-19 and other diseases with high mortality rates. This way, physicians can determine the best treatment plans for their patients, such as administering Tamiflu to a patient who has a coinfection of influenza and Covid-19.

Ultimately, Griffin warned that if molecular testing has identified multiple infections, the results need to be validated with additional testing. "You still want to do the cultures," Griffin said.

The best way to avoid Covid-19 and influenza this season

According to the Washington Post, vaccines should be considered the "first line of defense" against both Covid-19 and the flu.

In addition, a British clinical trial suggested it is safe to get both a flu shot and the second shot of a Covid-19 vaccine at the same time—results that echo the advice of U.S. health experts.

For the study, doctors gathered 679 volunteers from April to June who had already received their first dose of the AstraZeneca or Pfizer-BioNTech Covid-19 vaccines.  As participants returned for their second Covid-19 vaccine dose, researchers gave half of them a flu shot and half of them a placebo. According to the New York Times, participants were then monitored for side effects, such as aches and fevers.

The researchers' preliminary report determined that "[t]here [were] no safety concerns raised in this trial." However, the study did not look at the immune responses of people who received a flu shot at the same time as a third Covid-19 shot—a scenario that could become more common as more countries authorize Covid-19 vaccine booster shots.

In addition to vaccination, the Post suggested other safety and prevention measures to help individuals avoid infection, including masking, frequent hand washing, and isolating when sick. (Khamsi, The Atlantic, 11/17; Zimmer, New York Times, 10/9; Roberts, Washington Post, 11/15)