As the U.S. enters its regular flu season, there's still a lot unknown about coinfection of flu and SARS-CoV-2 viruses.
According to The Atlantic, an early study from China in January 2020 found zero cases of coinfection among 99 COVID-19 patients, but a follow-up a month later at a COVID-19 hospital found about one in eight patients had both illnesses at the same time.
While flu numbers last year were significantly lower than normal, pandemic restrictions have relaxed and some experts are warning about a possible "twindemic." Though researchers still don't know a lot about the coinfection of flu and COVID-19, new studies suggest that coinfections of many pathogens may be much more common than previously thought.
Below are seven things to know about coinfections, per The Atlantic:
1. New molecular diagnostics make coinfections easier to identify now, with recent studies finding 14 to 70 percent of patients hospitalized with flu-like illness test positive for more than one viral pathogen.
2. For COVID-19 patients, acquiring multiple infections appears to be linked to worsened outcomes. People with severe COVID-19 who ended up in intensive care units were prone to developing additional illnesses while hospitalized, such as ventilator-associated pneumonias and sepsis. According to an analysis from last spring of more than 100 studies, people who tested positive for both SARS-CoV-2 and a second pathogen had triple the odds of dying compared to patients who had only COVID-19.
3. That's not to mention chronic coinfections, such as herpes viruses; the varicella-zoster virus, which causes chicken pox and shingles; and the Epstein-Barr virus, which can cause mononucleosis (mono). These lifelong infections are usually kept in check by the immune system but can be reawakened by a new disease that temporarily weakens defenses. Some small studies cited by The Atlantic have found severe COVID-19 is associated with a quickening of latent cytomegalovirus, or herpes simplex virus.
4. Chronic infections can also make people more vulnerable to acute infection, such as COVID-19.
5. Coinfections aren't always bad — researchers believe a viral infection may sometimes offer protection by activating the body's immune response, priming it to fight a second infection.
6. It's possible that the collision of COVID-19 and flu might actually have benefits for some. In 2018, scientists in Idaho squirted a rhinovirus or coronavirus up the noses of mice. Two days later, with a squirt of a mouse-adapted flu virus, these mice fared better — living longer and showing fewer symptoms — than mice that were never given colds. The researchers suggested that the rhinovirus or coronavirus set off an "early but controlled" inflammatory response in the lungs, which then helped the mice defeat the flu virus.
7. Children sometimes turn out to have a mix of rhinovirus, parainfluenza and respiratory syncytial virus, Aubrey Cunnington, PhD, head of the pediatric infectious diseases section at Imperial College London, told The Atlantic. "We often see two or even three different viruses come up positive. Coexisting infections with different organisms, particularly viruses, are the rule rather than the exception."
"We have rather limited understanding of how they interact with one another, with bacteria, and other pathogens, to result in the illness affecting each patient," Dr. Cunnington said.
Ultimately, the implications of the varying results remain unclear, though scientists are learning more about COVID-19 and flu coinfection everyday.