Overshadowed by a global pandemic, the 2020-2021 flu season was little more than a blip on the radar. For a virus that killed about 22,000 people during the 2019-2020 flu season, and 34,000 the season before, the influenza virus infected slightly more than 2,000 and killed only 600 during the 2020-2021 season, according to the Centers for Disease Control and Prevention (CDC).
Meanwhile, the coronavirus slogged on, infecting more than 33 million people in the United States and killing at least 598,000 since March 25, 2020, according to Johns Hopkins University.
What caused this dramatic drop in flu cases while COVID-19 cases continued to grow, and does this mean we could eradicate the influenza virus in the future?
Why Are There So Few Flu Cases This Year?
The short answer can be chalked up to the measures taken to stop the spread of COVID-19 — social distancing, handwashing, mask wearing, wiping down surfaces and following "safe at home" orders, says Dr. Ellen Eaton, an assistant professor of medicine in the University of Alabama at Birmingham (UAB)'s Division of Infectious Diseases, and author of the Facebook page Dr. Ellen Eaton Coronavirus Updates.
Those efforts worked because there are some commonalities between the seasonal flu and COVID-19. Both are contagious respiratory viruses, meaning they affect the lungs and breathing. They also most often spread from person-to-person, such as when someone inhales or swallows droplets that fly into the air when an infected person coughs, sneezes or talks. With both viruses, infected individuals can spread the virus before symptoms present.
But why did those efforts have more of an impact on the number of flu cases compared to COVID-19 cases?
Why Are There More COVID-19 Cases Than Flu Cases?
While there are similarities between COVID-19 and the flu, one major difference is how they are spread. Both viruses can also be spread through contact with surfaces that have been contaminated with droplets from infected individuals. However, we've learned this is much less likely with COVID-19, Eaton says.
"We know that a lot of schools and daycares and grocery stores have put a lot more time and energy into hand hygiene stations, into wiping down countertops, wiping down bathrooms, so it's likely some of that cleaning of surfaces could also help to have reduced the flu and coronavirus," she says.
One reason COVID-19 cases have eclipsed flu cases is that the coronavirus can also spread through the dissemination of small droplets — or aerosols — that can remain in the air over long distances and for long periods of time. This airborne transmission is to blame for so-called super spreader events, such as large gatherings where several people in close proximity become infected.
"Super spreader events are not something that we see with influenza, so that's an important distinction," Eaton points out.
When researchers began to realize that COVID-19 was associated with airborne transmission, extra precautions were taken in hospitals and health care settings to reduce the risk of spread, such as recommending N95 respirators for health care workers and using negative pressure rooms for patients with COVID-19 that prevent contaminated air inside the room from flowing outside to non-contaminated areas. Instead, contaminated air is sucked out of the room.
Because influenza isn't spread through airborne transmission, these extra precautions are not usually taken with someone diagnosed with the flu.
Could Some Flu Cases Have Been Mistaken for COVID-19 Cases?
The disparity between the number of coronavirus cases compared to influenza cases may have some people wondering if some flu cases were inadvertently reported as COVID-19 cases or vice versa. "Perhaps," Eaton says. "Early on [in the pandemic], when we did not have access to testing, when we did not have enough materials to meet demand."
Bureaucratic red tape and contaminated test components made tests hard to come by in the United States, and even months after the first cases of COVID-19 were reported, testing still lagged far behind demand.
"However, by the time we got to the traditional cold and flu season — which is really into the fall and winter months — we had improved our supply chain in terms of testing supplies," she says. "So, I don't think you can blame the reduction in reported influenza cases in the fall and winter months on testing shortage or lack of supplies."
Can You Have COVID-19 and the Flu at the Same Time?
It is possible for someone to become infected with two different viruses at the same time. For example, some children can develop both flu and respiratory syncytial virus (RSV) at the same time. And as the coronavirus pandemic lumbered on into the fall, health professionals feared a "twindemic" of COVID-19 and seasonal flu could happen and, if so, would overwhelm hospitals.
Dual infections turned out to be rare. That was great news because according to an analysis of cases from January to April 2020, people who developed both viruses at the same time were more than twice as likely to die than those with COVID-19 alone.
As far as reporting these cases, "It is my understanding that these dual infections with COVID and flu have been very, very infrequent and that they would be recorded, statistically, as both coronavirus and influenza," Eaton says.
Cases of dual infections were rare for a few reasons. First, by fall, health care professionals were getting more adept at diagnosing and preventing COVID-19. Secondly, there was an uptick of flu vaccinations during the pandemic.
"That is a huge win for us," she says. "It shows that people were concerned and adjusted their behaviors and wore their masks and got their flu shots."
Do Fewer Flu Cases This Season Mean Fewer Cases in the Future?
With public health measures to stop the spread of COVID-19 easing up, Eaton says she think next season's flu won't be as bad as ones we've seen in the past. "But I imagine it will be more significant than it was this year," she says.
That's because fewer transmissions of influenza means the virus has fewer opportunities to mutate, which in turn means the strain circulating in the community next season will likely be similar to those covered by vaccines administered in previous years.
But, if low rates of influenza circulating in kids continues, it could spell trouble for younger generations.
"What we know about kids who make it to adulthood who don't have antibodies and have not seen influenza over several flu seasons is that they don't have much immunity and when they do get infected, they could have very, very severe influenza," Eaton says. "And in that case, an entire generation of adults who are not exposed to the flu as young people can have more severe outcomes as adults because their immune systems won't have been prepared to respond accordingly."