Even as the novel coronavirus surges nationwide, influenza and other common viruses have become virtually nonexistent—thanks in large part to measures put in place to stem the spread of the coronavirus. However, experts warn that common viruses could make a fierce comeback once coronavirus levels drop and prevention measures ease.

Levels of common viruses at extraordinary lows

Through the 52nd week of the 2020-2021 flu season, CDC has recorded just five influenza -related deaths, representing a 40-fold drop compared with the same week in 2019, Axios reports.

According to USA Today, CDC reported more than 65,000 influenza cases from Sept. 29 to Dec. 28 during the 2019 flu season. In comparison, CDC reported just 1,016 influenza cases during the same time period in 2020—a drop that's persisted despite a six-fold increase in testing at public health labs, many of which have been testing for influenza A and B when conducting coronavirus tests.

That dip is also reflected in clinical lab samples: According to CDC data, in the third week of December 2019, 16.2% of clinical lab samples tested came back positive for influenza A. In comparison, that rate during the same week in 2020 was 0.3%.

Similarly, CDC's map of flu activity in the United States shows "minimal" levels of influenza in every state for the week ending in Dec. 19, a sharp departure from the same time last year, when most states reported "high" or "very high" levels of flu activity.

"It's crazy," Lynnette Brammer, leader of CDC's influenza surveillance team, said. "This is my 30th flu season. I never would have expected to see flu activity this low."

And it's not just the flu, data shows. In August, CDC warned the public to watch for a condition called acute flaccid myelitis in children, a disease that based on its pattern of peaking every other year over the past six years—with 120 cases in 2014, 153 in 2016, and 240 in 2018—was expected to hit hard again in 2020. But by the end of 2020, just 29 cases were reported, according to CDC.

In fact, the levels of virtually every common virus has dropped, according to data from BioFire, a company that provides diagnostic tests for viral infections. According to BioFire's data, almost 60% of samples taken from patients exhibiting flu-like symptoms at this time last year came back positive for common respiratory viruses—such as influenza, norovirus, respiratory syncytial virus (RSV), or human metapneumovirus—or a respiratory bacteria, such as the ones that cause whooping cough or pneumonia.

However, starting in March 2020, as coronavirus countermeasures were beginning to be implemented nationwide, that rate dropped to about 6%--and even now sits at about 18%.

Why rates of common viruses are dropping—even as the coronavirus rages

While common viruses are hitting near or record lows, rates of coronavirus infection continue to skyrocket. There are several reasons for the disparity, according to David Hooper, chief of Massachusetts General Hospital's infection control unit, including the fact that the coronavirus is "more contagious and … less forgiving of any lapse" in prevention efforts that other common viruses. In addition, Hooper said, unlike the flu virus, people infected with the novel coronavirus can shed the virus for days before the onset of symptoms—if they even experience symptoms.  

According to experts, immunity also plays a role. Common viruses are referred to as "endemic," the Washington Post reports, meaning they're always present in some way. As a result, since many people have been exposed to those viruses before and developed at least some immunity to them, the transmission of those viruses can be more easily cut off by all the measures we've taken to stem the spread of the coronavirus.

However, experts note that novel coronavirus was unknown until last year, making everyone very susceptible to the virus. "Covid is a novel infection caused by the SARS coronavirus, and no one has any innate immunity to it," Susan Rehm, vice chair of the Cleveland Clinic's department of infectious diseases, said. "So the population is probably overall more susceptible to it than maybe to influenza."

In addition, a large portion of the country has been vaccinated against the flu. CDC data show an estimated 53% to 54% of Americans have received the vaccine.

It's also possible that the body's response to being infected by the new coronavirus could prevent infection from other viruses. For instance, Ellen Foxman's research has shown that infection with a virus triggers the release of interferon in the body, which keeps other viruses from replicating.

"The interferon response is one of the body's best defenses against respiratory viruses," Foxman, a professor at Yale University, said. "As soon as it gets turned on by one virus, any other virus that comes along and tried to grow in the respiratory tract can't."

Experts warn of a viral comeback

However, although levels of common viruses are low now, experts warn there could be a potentially dangerous comeback once levels of the new coronavirus drop and prevention measures ease.

"The best analogy is to a forest fire," Bryan Grenfell, an epidemiologist and population biologist at Princeton University, said. "For the fire to spread, it needs to have unburned wood. For epidemics to spread, they require people who haven't previously been infected. So if people don't get infected this year by these viruses, they likely will at some point later on."

And according to Ben Lopman, an epidemiologist at Emory University's Rollins School of Public Health, it's also possible that some of the infections could be more severe than they typically would be because of decreased immunity.

In fact, this phenomenon may already be occurring in Australia, where officials reported extremely low levels of flu-like illnesses in the beginning of May, when flu season typically starts in that hemisphere. However, in recent months—after levels of the new coronavirus dropped precipitously and restrictions were rolled back—flu cases among children rose six-fold in December, when flu levels are typically low.

"That's an important cautionary tale for us," Kevin Messacar, a pediatric infectious-disease physician at Children's Hospital Colorado, said. "Just because we get through the winter and don't see much [respiratory syncytial virus (RSV)] or influenza doesn't mean we'll be out of the woods."

Experts say nothing is certain, however. "The decline in respiratory viruses has really been fascinating," Foxman said. "It's incredible. We've never had a natural experiment like this before. What the consequence will be for the short and long term remains to be seen" (Hurley, Washington Post, 1/12; Walsh, Axios, 1/14; Rodriguez, USA Today, 1/11; Castronuovo, The Hill, 1/12).