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COVID-19 Pandemic Tied to Historically Low Influenza Cases, Leading to Uncertainty about Next Winter’s Flu Vaccine

Clinical laboratories may see increase in flu and COVID-19 specimen processing as people return to pre-pandemic social behaviors, experts predict

While SARS-CoV-2 infections continue to ravage many parts of the world, influenza (flu) cases in North America have hit a historic low. As winter approached last year, infectious disease experts warned of a “twindemic” in which the COVID-19 outbreak would combine with seasonal influenza to overwhelm the healthcare system. But this did not happen, and many doctors and medical laboratory scientists are now investigating this unexpected, but welcomed, side-effect of the pandemic.

“Nobody has seen a flu season this low, ever,” said William Schaffner, MD, Professor of Preventive Medicine in the Department of Health Policy and Professor of Medicine in the Division of Infectious Diseases at Vanderbilt University School of Medicine in Nashville, in a report from WebMD, titled, “What Happened to Flu Season?

From the start of the current flu season in September 2020, clinical laboratories in the US reported that 1,766 specimens tested positive for flu out of 931,726—just 0.2%—according to the CDC’s Weekly US Influenza Surveillance Report. That compares with about 250,000 positive specimens out of 1.5 million tested in the 2019-2020 flu season, the CDC reported. Public health laboratories reported 243 positive specimens out of 438,098 tested.

The graphic above taken from the CDC’s Weekly Influenza Surveillance Report for the week ending April 17, 2021, illustrates how “Nationwide during week 15, 1.1% of patient visits reported through ILINet were due to ILI [Influenza-like Illness].” This percentage, according to the CDC, is below the national baseline of 2.6%. “Seasonal influenza activity in the United States remains lower than usual for this time of year.” (Graphic copyright: Federal Centers for Disease Control and Prevention.)

Fear of COVID-19 Linked to Fewer Flu Deaths in Children

WebMD reported that just one child in the US has died from the flu this year, compared with 195 in 2020. Why the low numbers?

Speaking to WebMD, Isaac I. Bogoch, MD, Clinical Investigator, Toronto General Hospital Research Institute (TGHRI) and Infectious Diseases Consultant and General Internist at the Toronto General Hospital, cited three likely reasons:

  • Precautions people take to avoid COVID-19 transmission, including masking, social distancing, and handwashing.
  • Reduced human mobility, including less international travel.
  • Higher-than-usual flu vaccination rates. As of February 26, the CDC reported that nearly 194 million doses of flu vaccine had been distributed in the US.

WebMD noted this could be a record, but that the CDC data doesn’t indicate how many doses were actually administered.

However, Schaffner told WebMD that efforts to keep kids home from school and away from social gatherings were likely a bigger factor. “Children are the great distributors of the influenza virus in our society,” he said. But due to fears about COVID-19 transmission, kids “weren’t even playing together, because mothers were keeping them off the playground and not having play dates.”

Repercussions for Fighting Flu Next Year

Public health experts welcomed the low flu levels, however, Politico reported that limited data about flu circulation this year could hamper efforts to develop an effective vaccine for next season’s flu strains.

“We may have a combination of low public health measures at the population level with a low effectiveness vaccine,” Lawrence Gostin, JD, University Professor at Georgetown University, told Politico. “And then, so you might have a raging flu season next year.” Gostin leads the O’Neill Institute for National and Global Health Law and serves as Director of the World Health Organization’s Collaborating Center on National and Global Health Law.

Each February, Politico explained, experts convened by the World Health Organization (WHO) look at data from the current and previous flu seasons to predict which strains are likely to predominate in the Northern Hemisphere next winter. That includes data about which strains are currently circulating in the Southern Hemisphere. The WHO uses these predictions to recommend the composition of flu vaccines. In the US, the final decision is made by an FDA advisory committee.

A similar WHO meeting in September guides vaccine development in the Southern Hemisphere.

The WHO issued this year’s Northern Hemisphere recommendations on Feb. 26. The advisory includes recommendations for egg-based and cell- or recombinant-based vaccines, and for quadrivalent (four-strain) or trivalent (three-strain) vaccines.

In a document accompanying the recommendations, the WHO acknowledged concerns about this year’s limited pool of data.

“The volume of data available from recently circulating influenza viruses, and the geographic representation, have been significantly lower for this northern hemisphere vaccine recommendation meeting than is typical,” the document stated. “The reduced number of viruses available for characterization raises uncertainties regarding the full extent of the genetic and antigenic diversity of circulating influenza viruses and those likely to pose a threat in forthcoming seasons.”

The report notes that experts identified changes in circulating Influenza A(H3N2) viruses this year, and that the changes are reflected in the new vaccine recommendation.

But Paul A. Offit, MD, who serves on the FDA’s vaccine advisory panel, downplayed worries about the vaccine. “The belief is that there was enough circulating virus to be able to pick what is likely to be the strains that are associated with next year’s flu outbreak,” he told Politico. Offit is a Professor of Vaccinology and Pediatrics at the Perelman School of Medicine at the University of Pennsylvania and Director of the Vaccine Education Center at the Children’s Hospital of Philadelphia.

Pediatrician and internationally recognized expert in the fields of virology and immunology, Paul A. Offit, MD (above), told Politico that the low level of flu circulation this year, along with the resulting uncertainty, “is unprecedented.” Clinical laboratories might not have noticed the severe decrease in influenza specimens sent for processing due to being hyper-focused on COVID-19 testing. But as the pandemic subsides, loss of flu testing revenues will likely become more apparent. (Photo copyright: University of Pennsylvania.)

Offit suggests that efforts to mitigate the COVID-19 outbreak could be useful to combat other infectious disease outbreaks. However, both Offit and Gostin expressed doubt about that prospect.

“I mean, could we reasonably in a winter month, wear masks just at least when we’re outside in large crowds? … Or are we comfortable having hundreds of 1000s of cases of hospitalizations for flu and 10s of 1000s [of] deaths? I suspect the answer is B. We’re comfortable with that, we’re willing to have that even though we just learned, there’s a way to prevent it,” Offit told Politico.

“Remember after the 1918 flu pandemic, most people don’t realize what happened when that was over. But what happened was the roaring ‘20s,” Gostin told Politico. “People started congregating, mingling, hugging, kissing. All the things they missed. They crowded into theaters and stadiums and went back to church. That’s what’s likely to happen this fall and that makes the influenza virus very happy.”

So, what should clinical laboratories expect in future flu and COVID-19 vaccines? That is not yet clear. One thing is certain, though. New lab test panels that test for influenza and the SARS-CoV-2 coronavirus will be arriving in the marketplace.

Stephen Beale

Related Information:

What Happened to Flu Season?

Fears of ‘Twindemic’ Recede as US Influenza Rates Stay Low

The Mystery of the Flu-COVID ‘Twindemic’ That Never Happened

The Pandemic Dramatically Reduced Flu Cases. That Could Backfire.

Recommended Composition of Influenza Virus Vaccines for Use in the 2021 – 2022 Northern Hemisphere Influenza Season

Weekly U.S. Influenza Surveillance Report

For mRNA Vaccines, COVID-19 Is Just the Beginning

How COVID Unlocked the Power of RNA Vaccines

Coronavirus Vaccine Technology Is Paving the Way for a Whole New Approach to Flu Shots

Duke Researchers Working on mRNA Flu Vaccine That Would Last Up to 5 Years

Anticipated Influenza/SARS-CoV-2 “Twindemic” Has Not Developed, Says CDC, Much to the Relief of the Nation’s Medical Laboratories

The remarkably low number of influenza diagnoses makes it possible for clinical laboratories to stay focused on COVID-19

One positive note for clinical laboratories this winter is the fact that the number of biological samples being submitted for influenza (flu) testing have dropped significantly. This has given medical laboratories more resources for processing COVID-19 tests.

Dark Daily’s sister publication The Dark Report covered this phenomenon in “Surprise! Many Fewer Cases of Flu in Us, Canada, Europe,” December 7, 2020.

According to a feature published in Nature, the number of samples being submitted to medical laboratories for flu testing has dropped by 61%. More surprisingly, the number of positives has dropped by 98%. The combined flu/COVID-19 “twindemic” that some medical experts feared could crush our healthcare system has not materialized—yet, the Washington Examiner reported.

“In any given winter, hospitals are taxed by the flu,” Brian Garibaldi, MD, a pulmonologist and critical care specialist and Medical Director of the Johns Hopkins Biocontainment Unit told the Washington Examiner. “There’s always a concern that our emergency departments will be overwhelmed, and ICU capacity will be strained [due to the concurrence of flu and COVID-19 outbreaks], particularly with people who have coexisting conditions that then get influenza.”

The 2019-2020 flu season ended earlier than usual, likely because of precautions put in place in the spring to combat the coronavirus pandemic. Most years, the seasonal flu in the US peaks in February and trails off by May, Nature reported in “How Coronavirus Lockdowns Stopped Flu in Its Tracks.”

“Seasonal flu cases in the northern hemisphere usually peak in February and tail off by the end of May,” Nature wrote. “This year, unusually, lab-confirmed cases of influenza dropped precipitously in early April, a few weeks after the coronavirus pandemic was declared on 11 March. The data comes from tests of more than 150,000 samples from national influenza laboratories in 71 countries that report data to FluNet, a global surveillance system.”

Government Leaders and Health Experts Remain Concerned

Despite that encouraging data point, public health experts and political leaders were still concerned. In September, Arizona Governor Doug Ducey said, “The overlap of COVID-19 and flu season presents a perfect storm, and we aren’t taking any chances. We are approaching this fall with a proactive mindset and plan of action to limit the impact of the flu and preserve hospital resources,” the Washington Examiner reported.

The caution was certainly warranted. A normal flu season strains resources, but a severe flu season coupled with a global pandemic could have been disastrous. Luckily, Ducey’s “perfect storm” did not materialize.

chart of flu-season during COVID-19 and years 2016 - 2020 with 4 years of flu data charted by FluNet Response System
Data from the World Health Organization’s FluNet Global Influenza Surveillance and Response System was used by Nature to develop the graphic above. It illustrates how the number of positive Influenza specimens in 2019-2020 declined compared to the previous two years. Some experts believe this is due to protocols implemented to combat the COVID-19 coronavirus by hospitals and clinical laboratories. (Graphic copyright: Nature.)

Why Is There Less Influenza?

So, why is there less flu and other respiratory infections?

Epidemiologist Lisa Lockerd Maragakis, MD, MPH, Associate Professor of Medicine and Senior Director of Infection Prevention at Johns Hopkins Health System, told U.S. News, widespread business and school closures provide fewer opportunities for influenza to spread. “We commonly see flu spread in communities, schools, businesses and through travel each year, so those changes are likely keeping the flu away.”

However, this may have a negative effect as well. Eili Klein, PhD, Associate Professor of Emergency Medicine at Johns Hopkins School of Medicine, warns that “Because of the current restrictions and precautions everyone is taking this season, far fewer people will be infected or exposed to the flu virus, and therefore won’t become immune to certain strains of the virus. So, the number of people who may have more severe infections next year is likely to be greater because immunity will be lower,” the Washington Examiner reported.

Other Viral Infections Also in Decline Due to COVID-19 Precautions, Vaccines

Masking, frequent handwashing, and social distancing certainly played a role in reducing the number of cases of flu reported this year. But influenza is not the only disease that saw reductions. “In Hong Kong, compared with previous years, the number of chickenpox cases dropped by about half to three-quarters,” Nature reported. “In April, cases of measles and rubella were their lowest, globally, since at least 2016, according to data available so far.”

Early in the COVID-19 pandemic, some public health officials were concerned that the decline in influenza cases was actually related to a lack of testing. “However, renewed efforts by public health officials and clinicians to test samples for influenza resulted in adequate numbers tested and detection of little to no influenza virus,” the Centers for Disease Control and Prevention (CDC) reported.

Another factor in the lower numbers of flu cases could be due to the fact that more people have gotten vaccinated this year. More than 188 million flu vaccines were distributed in 2020, an increase compared to the 169 million given in 2019.

“Flu vaccination in the community started earlier this year, as recommended by the CDC, and our community physicians report that vaccine uptake has been higher than usual,” Marie-Louise Landry, MD, Clinical Virologist, Professor of Laboratory Medicine and of Medicine (Infectious Diseases), and Director of the Clinical Virology Laboratory at Yale School of Medicine, told Healthline.

It may also be that influenza diagnoses are fewer because people are not seeking treatment. Hospitals at or beyond capacity due to the pandemic, or fear of contracting COVID-19, may have motivated people with flu-like symptoms to stay home rather than seek treatment. However, most healthcare experts agree that public health measures to fight COVID-19 are likely the larger reason there is less flu.

“Public health measures such as movement restrictions, social distancing, and increased personal hygiene likely had an effect on decreasing influenza and other respiratory virus transmissions,” the World Health Organization (WHO) told Nature.

What About the Next Flu Season?

Experts are more conflicted regarding what all of this means for coming flu seasons. Some experts think that because there’s less flu this year, there will be less immunity next year, and severe illness will result. Others are more optimistic and hope that some strains of flu will disappear, which could mean less flu in the immediate future. It’s not a simple prediction to make.

“A lot of different flus have been circulating in recent years. Are they all going to make it out of this or not? It’s possible that what this season will do is actually make the virological picture a lot simpler. That may be permanent, potentially,” Richard Webby, PhD, an infectious disease scientist at St. Jude Children’s Research Hospital and Director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds, told Nature in “How COVID-19 is Changing the Cold and Flu Season.”

Even if the low flu numbers this year mean some strains do not survive, it is unlikely that will remain the case. “I am sure that flu will come back with a vengeance at some stage in the future,” Robert Ware, PhD, a biostatistician, clinical epidemiologist, and Professor of Biostatistics with Griffith University in Queensland, Australia, told Nature.

Thus, clinical laboratories should remain vigilant for future influenza outbreaks. Hopefully by then the COVID-19 pandemic will have peaked and labs will be able to reallocate testing resources appropriately.

—Dava Stewart

Related Information:

How COVID-19 Is Changing the Cold and Flu Season

How Coronavirus Lockdowns Stopped Flu in Its Tracks

CDC Reports Record-Low Positive Flu Tests

What Happened to the Flu This Year?

Decreased Influenza Activity During the COVID-19 Pandemic—United States, Australia, Chile, and South Africa, 2020

Why Flu Cases Are Down During a Massive Pandemic