COVID-19 precautions shut down flu this season, but what will next winter bring?

As its name suggests, the Seattle Flu Study was designed to monitor the virus that rolls in like clockwork every fall, trailing misery in its wake. But this season — for the first time — there hasn’t been much to track.

Out of the more than 17,000 specimens the team has analyzed, fewer than a dozen tested positive for influenza. Dr. Helen Chu, a study leader and UW Medicine virologist, had to cancel a study on flu treatment in homeless shelters because there was no flu.

“In the last two decades, I’ve never seen anything like this,” she said.

Nationwide, the Centers for Disease Control and Prevention says this has been the lowest flu season on record. Washington, which averages about 160 confirmed flu deaths a year, has recorded none.

And it’s not just flu that’s absent.  

Chu and her colleagues test for 27 respiratory pathogens, and nearly all have been close to nonexistent.

Scientists mainly credit social distancing, masking and other steps to control the novel coronavirus for also shutting down flu and its microbial kin. The phenomenon demonstrates the power of simple measures to rein in a wide variety of dangerous viruses, and Chu and others think it will lead to permanent changes in behavior, like seasonal masking.

But there’s also a chance this winter’s lull could leave people more vulnerable to a resurgence next season. And in a perverse twist, the lack of circulating flu strains is making it tougher to craft next season’s iteration of the vaccine.

“There are a lot of questions about what next year is going to look like,” said Dr. Steven Pergam, an infectious disease specialist at the Fred Hutchinson Cancer Research Center and director of infection control for Seattle Cancer Care Alliance. “These viruses are still out there.”

After concerns that COVID-19 and flu could collide in an overwhelming “twindemic,” the reprieve has been welcome — particularly for the vulnerable cancer and transplant patients Pergam works with. About 40 percent normally develop respiratory infections. This season he hasn’t written a single prescription for the antiviral drug Tamiflu.

During winter months, Seattle Children’s normally treats 100 to 200 kids a week with RSV, or respiratory syncytial virus, which can be very dangerous, said infectious disease specialist Dr. Janet Englund. The total this winter is under 10.

She’s convinced that the most important factor is school closures. “Children are well known to be the main vector of spread for influenza.”

 Hong Kong saw a huge boom in coughs, sniffles and colds when schools reopened, and Washington can probably expect the same, Englund said.

The main culprit is likely to be rhinoviruses, the most common cause of the common cold. Levels have been lower than normal this winter, according to testing by the Seattle Flu Study. But unlike other respiratory bugs, rhinoviruses have continued to circulate and will be poised to rebound quickly when kids start mingling again.

The reason for rhinoviruses’ persistence lies in their hardiness, which means they can survive for long periods on surfaces and are readily transmitted by touching the eyes and nose, Englund said. People infected with rhinoviruses can also shed virus for weeks.

The individual characteristics of different viruses also help explain why flu was obliterated this winter even as COVID-19 reached its highest levels, Chu said.

The novel coronavirus spreads much more readily than flu, she pointed out. In the absence of social distancing, each infected person passes the virus on to two to three others. For flu, the number is about 1.2.

People infected with flu generally start feeling crummy within a day and stay home, which reduces transmission. But people with the novel coronavirus can be infectious for several days before they get sick, and many never have symptoms at all.

Most people also have some residual level of immunity to flu, while no one had ever encountered the novel coronavirus when the pandemic started. Factor in mask-wearing, social distancing and a huge slowdown in international travel — a major route for flu transmission early in the season — and the combination was enough to push flu and other viruses off the map, Chu said.

Flu vaccination almost certainly played a role as well, with rates for adults higher than ever before, Pergam said. The CDC reported a record 193 million doses distributed, about 25 percent more than during the past several seasons.

Some researchers are also exploring the intriguing possibility that getting infected with one virus can protect against others. There were some hints during the 2009 swine flu epidemic, when people in France with common colds seemed to be shielded. Since then, lab experiments found that immune signaling chemicals released in response to rhinovirus were able to fend off flu.

“The idea is that your immune system is ramped up and responding, which makes you less susceptible,” Chu explained.

It’s not clear whether that type of viral interference played a role in flu’s pandemic disappearing act, but some experts worry lack of exposure this season may set up conditions for a nasty round in 2021-22.

“There is concern it will be worse next year, not just because of lack of immunity, but also because people are tired of washing their hands and social distancing,” Englund said.

Young children who have never been exposed to flu could be among the most vulnerable, she added. “I think our big concern is the fall.”

If flu resurges later this year, vaccines might not be as effective as in previous years. The vaccine is tweaked every year to cover the dominant, circulating strains of virus. Scientists start collecting samples from the Southern Hemisphere during July and August, their winter season, to see what’s likely headed toward the rest of the planet.  

But the Southern Hemisphere flu season was the first to fizzle out. Now, so few strains are circulating that anticipating next year’s mix is a guessing game.

“It’s going to be hard to make those predictions and design a vaccine that’s protective,” Chu said.

After seeing the dramatic reduction of flu and other respiratory viruses this year, Pegram believes it will be hard to go back to pre-pandemic habits.

He’s convinced people won’t entirely abandon masks but will continue to wear them around vulnerable people or in risky situations, like visiting hospitalized friends or family or in nursing homes. Hand-washing habits nurtured during the pandemic could persist, along with a greater awareness of the importance of staying away from others when you’re sick.

“This season shows that some good public health practices and social distancing can make a huge difference,” Pergam said. “I think you’re going to see real changes in how we live our lives.”