What will the 2020-2021 flu season look like?

As COVID-19 rages on, what will the 2020-2021 flu season look like? Could the flu create another public health crisis on top of the COVID-19 pandemic, or could COVID-19 somehow cancel out the flu this year? The answer is that no one really knows—which is a cause for concern.  

“I think we need to be honest and say upfront that we’re really not sure what’s going to happen,” said Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, in an interview with Cardiology magazine.

Health experts’ biggest fear for the 2020-2021 flu season is a “twindemic,” where cases of influenza pile on top of the already burdensome number of COVID-19 cases—or where patients get infected by flu and COVID-19 at the same time.

“The worst-case scenario is we have a very active flu season that overlaps with the respiratory infection of COVID-19,” Dr. Fauci said. “Worst-case because that would really complicate matters from a diagnostic standpoint, from a therapeutic standpoint, and the standpoint of putting a lot of stress on the healthcare system.”

He added: “We haven’t had a situation with this kind of potential, where we might have two illnesses, one that is of epidemic proportions and the other pandemic, co-circulating at the same time. It’s going to be a unique experience, and, as I mentioned before, we’re really not sure how it will actually play out.”

The first scenario

What we do know is that influenza testing, and subsequent reports of the flu, had a precipitous drop just about the time that the United States began to take active measures to prevent COVID-19.

A median of 49,696 specimens per week were sent to US clinical laboratories for influenza testing from October through February, but that number dropped to 19,537 per week from March to May—a 61% decrease. Specimens that tested positive fell from 19.34% to 0.33% during the same period—a 98% decrease in influenza activity, according to a recent Morbidity and Mortality Weekly Report (MMWR) from the CDC.

Summertime flu in the United States also hit historic lows, with a positive testing rate of 0.20% in 2020 compared with a rate of 2.35% in summer 2019.

The authors of the MMWR report attributed the major drop in the flu due to two reasons. First, fewer people went out to get tested for the flu after COVID-19 was declared a national emergency on March 1, 2020. Second, the prevention measures put in place at that time—stay-at-home orders, bans on mass gatherings, and school closures, as well as staying home when sick, hand washing, wearing masks, and social distancing—reduced transmission not only of the SARS-CoV-2 virus but also of the influenza virus.

“If extensive community mitigation measures continue throughout the fall, influenza activity in the United States might remain low and the season might be blunted or delayed,” the MMWR authors wrote.

“However,” they added, “in light of the novelty of the COVID-19 pandemic and the uncertainty of continued community mitigation measures, it is important to plan for seasonal influenza circulation this fall and winter. Influenza vaccination for all persons aged ≥ 6 months remains the best method for influenza prevention and is especially important this season when SARS-CoV-2 and influenza virus might co-circulate.”

The CDC confirmed that an individual can have the flu (as well as other respiratory illnesses) and COVID-19 concurrently. “While it’s not possible to say with certainty what will happen in the fall and winter [of 2020], CDC believes it’s likely that flu viruses and the virus that causes COVID-19 will both be spreading. In this context, getting a flu vaccine will be more important than ever,” the agency advised.

To that end, the CDC and flu vaccine manufacturers are stepping up their efforts to increase influenza vaccination this year. Manufacturers aim to supply as many as 194-198 million doses of flu vaccine for the 2020-2021 season. This beats the record number of 175 million doses supplied during the 2019-2020 flu season. Furthermore, the CDC is purchasing an extra 2 million doses of pediatric flu vaccine as well as an extra 9.3 million doses of adult flu vaccine. It’s also conducting a targeted PR campaign to encourage high-risk groups to get the flu vaccine.

In terms of detection, the flu and COVID-19 share some similar symptoms—such as cough, fever, shortness of breath, fatigue, sore throat, runny nose, muscle pain, headache, etc—which make it difficult to diagnose the two infections by symptoms alone. So, the CDC developed an assay that detects both influenza A and B viruses as well as SARS-CoV-2 virus, which is currently available.

The second scenario

Besides the “twindemic,” there’s another possible outcome—one in which the flu has very little effect this season because the COVID-19 virus will effectively shove it aside.

Said Dr. Fauci: “In this other possible scenario, where there is an ongoing rampant respiratory illness such as COVID-19, if another respiratory infection comes along such as seasonal influenza, there’s only enough room in the niche of respiratory infections for one or the other. Thus, the rampant illness dominates, takes the niche and bumps out the other one.”

He added that this may potentially cause another effect. “If there is one type of virus that hits [the niche] before another one (for example, SARS-CoV-2 vs influenza), it could induce a degree of innate immunity that would be beneficial in fighting off the second virus.”

Dr. Fauci cautioned that he was merely speculating on what might happen, not making any concrete predictions.

But perhaps one or both of these effects were responsible for what was a nearly non-existent flu season in the Southern Hemisphere. From April to July 2020, three sentinel countries in the Southern Hemisphere—Australia, Chile, and South Africa—had a positive testing rate of only 0.06%, an unprecedentedly low number when compared with a 13.7% rate during the same period in 2019, according to the MMWR report.

Dr. Fauci added that preventive measures in the Southern Hemisphere, such as better handwashing, masking, and physical distancing, may also have helped decrease the spread of flu, COVID-19, and other viral illnesses.

But, again, no one knows for sure. That’s why Dr. Fauci and other authorities are urging more Americans than ever to get the flu shot this year.

“We need to use every tool we have. We have to get as many people vaccinated for influenza as we possibly can,” he said. “You don’t want to get COVID-19 and you don't want to get influenza either.”

This effort should include doctors, nurses, and allied personnel, too. Notably, nearly 1 in 5 healthcare workers (18.9%) did not get the flu shot in the 2018-2019 season—a similar rate to previous years.