Infectious Disease

In midst of COVID-19, US sees ‘unusually low’ flu exercise

February 23, 2021

9 min read


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Biddinger, Duchin, Schaffner and Uyeki report no relevant financial disclosures.


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According to the CDC, influenza activity this season has been “unusually low,” with rates of hospitalization as of late January that were similar to 2011-2012 — the least severe season in the past decade.

According to experts, preventive measures to control COVID-19 transmission are the reason why.

Paul Biddinger

Paul D. Biddinger, MD, director of the Center for Disaster Medicine at Massachusetts General Hospital, said flu almost went away and almost overnight after COVID-19 mitigation measures were put in place last year.

Source: MGH Photography Department

“COVID-19 prevention measures have had a profound impact in the United States and globally, interrupting transmission of influenza and many other respiratory viruses while COVID-19 continues to spread quite successfully,” Jeffrey S. Duchin, MD, health officer and chief of the communicable disease epidemiology and immunization section for the Seattle and King County, Washington, public health department, and a professor of medicine at the University of Washington, told Infectious Disease News.

The impact was apparent in the U.S. during the early months of the pandemic, when there was a sharp decline in respiratory specimens that tested positive for influenza, followed by “historically low interseasonal levels,” according to a report in MMWR by Sonja J. Olsen, PhD, an epidemiologist in the CDC’s Influenza Division, and colleagues, which was based on data from around 300 U.S. laboratories. They also analyzed data from Australia, Chile and South Africa and identified just 51 positive influenza results among more than 83,000 specimens, demonstrating a similar effect in the Southern Hemisphere.

Infectious Disease News compiled existing influenza data and spoke with experts to discuss the impact that COVID-19 has had on this influenza season and what it could mean for next season and beyond.

In a typical influenza season, the U.S. still sees a “substantial amount of flu” into March and April. However, during the 2019-2020 season, influenza was “essentially gone” by the end of March, said Paul D. Biddinger, MD, director of the Center for Disaster Medicine at Massachusetts General Hospital.

“What we saw was that there was moderate flu activity as COVID-19 started to present in the community and then relatively quickly, as people began masking and physical distancing and as there were restrictions on people’s interactions, flu almost went away — and almost overnight,” Biddinger told Infectious Disease News.

The trend has continued. In the week ending Jan. 23, for example, only one state — Oklahoma — reported influenza activity above what the CDC considers “minimal.” Just 1.3% of patient visits reported that week were for influenza-like illness — far below the national baseline of 2.6% — and the percentage had not climbed higher than 1.6% all season. Last year, it peaked at around 7%.

Additionally, of the more than 266,000 specimens tested by public health laboratories since September, only 179 have been positive for influenza.

“We normally start to see flu — at least a little bit — in November and December, and by January, we see significant flu. Here in New England, we have seen almost no flu,” explained Biddinger, who said his hospital is actively testing every patient admitted for both COVID-19 and influenza.

In October, experts warned about the potential for what Infectious Disease News Editorial Board Member William Schaffner, MD, called a “double-barreled respiratory virus season” — COVID-19 and influenza circulating at high levels at the same time.

It has not materialized.

William Schaffner, MD
William Schaffner

“The current influenza season is by far the mildest in recent memory,” Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center and medical director for the National Foundation for Infectious Diseases (NFID), told Infectious Disease News. “Thank goodness that the anticipated ‘twindemic’ has not yet appeared as COVID-19 alone is straining health care resources.”

Tim Uyeki, MD, chief medical officer for the CDC’s Influenza Division, told Infectious Disease News that it is too early to make conclusions “other than, to date, influenza activity has been very low throughout the U.S.”

“We have experienced late influenza seasons in the U.S. and we have seen over the past several months that when some countries reduced COVID-19 control measures, such as in some Southeast and South Asian countries, influenza activity increased.”

Still, experts said the as-yet low level of transmission is likely a combination of behavioral changes made as a result of the coronavirus pandemic and a successful influenza vaccine, although complete data on vaccine efficacy was not yet available.

Among the measures that are likely having an impact to reduce influenza transmission are social distancing and mask wearing, experts agreed. Schaffner added another factor — the switch from in-person to virtual schooling.

“My favorite theory relates to what is happening in schools,” he said. “Children are thought to be the most effective disseminators of influenza viruses in our population because they shed large amounts of virus for considerable periods. Since COVID-19, there has been much home-schooling, and even when children have attended school in person, social distancing and mask wearing are often the norm.”

There are indications that other respiratory viral infections have also declined significantly during the COVID-19 pandemic. During a 16-week period from April through July at Stamford Hospital in Connecticut, for example, Michael Parry, MD, chair of infectious diseases at Stamford Health and professor of clinical medicine at Columbia University Vagelos College of Physicians and Surgeons, and colleagues reported that only one case of influenza and three positive BioFire tests — which are used for diagnosing all other respiratory infections — were recorded in 2020, compared with an average of 138 combined cases per season over the past 5 years.

The study also demonstrated that the proportions of positive BioFire and influenza tests were significantly lower, with only 1.4% of BioFire tests positive in the 2020 season compared with 38.9% in previous seasons, despite an increase in the volume of testing. Only 1.3% of influenza tests were positive in 2020 compared with 10.8% of tests performed during the historical seasons.

“We think that the systematic use of masking, hand hygiene, social distancing and stay-at-home policies can dramatically reduce the risk of all viral respiratory tract infections, including influenza,” Parry told Infectious Disease News after the study was published.

Experts speculated that, given the drastic reduction in respiratory illnesses, some measures may be kept in place.

“I think this is going to make us rethink and consider the use of community mitigation measures — and particularly face mask wearing — to prevent influenza and other respiratory viruses in the fall and winter months, and maybe even early spring,” Uyeki said. “If most people with influenza who are transmissible are those who are symptomatic, you can actually control that by having them stay at home, wearing face masks and having other people around them wearing face masks as well, combined with annual influenza vaccination. These strategies might be especially important for persons at high risk for influenza complications.”

Schaffner said it “seems sensible” that future public health recommendations to protect against influenza could include mask wearing and social distancing, “particularly for those at increased risk of complications and severe disease.”

“I think it’s logical to assume that masking, physical distancing and other restrictions have played a big role in decreasing the flu season,” Biddinger said. “But as with everything in medicine, I think we probably will need more data to know for sure and see if there’s anything about COVID-19 itself that could have played a role.”

Record vaccine deliveries

According to Biddinger, fewer than half of eligible adults are vaccinated against influenza every year.

“Everyone is very appropriately focused on COVID-19 vaccination, but one thing I would love to see — something that is very simple and easy to do and doesn’t require anyone to change how they see their friends or how they live their lives — is flu vaccination,” he said.

vaccines manufactured using eggs
Most influenza vaccines are still manufactured using eggs, but there are other technologies that can produce vaccines faster, which would be a benefit if the U.S. experiences a late increase in influenza, said Tim Uyeki, MD.

Source: CDC/Robert Denty

As of Jan. 15, more than 193 million doses of the influenza vaccine had been shipped in the U.S., according to a new influenza vaccination tracker sourced by the CDC. This is the highest number of doses distributed during a single influenza season in history, according to the CDC.

The tracker showed that more adults have been vaccinated in 2020-2021 than last season, although vaccination rates among certain groups — including non-Hispanic Black adults and children — have appeared to drop off.

Overall, the CDC estimates that 53% to 54% of people were vaccinated against influenza by December — higher than the previous season’s coverage estimates for adults, which were 42% by the end of December 2019 and 48% by the end of May 2020.

“This year we saw record numbers of patients and residents getting vaccinated,” Biddinger said. “I think people really didn’t want to have any sort of febrile illness, especially in the time of COVID-19.”

In early October, the NFID and CDC hosted an annual influenza and pneumococcal news conference to discuss new data on attitudes toward influenza and vaccination. Data from a survey showed that despite increasing vaccination rates and particularly deadly influenza seasons in recent years, including the severe 2017-2018 season, only 59% of U.S. adults said they intended to get vaccinated, even though 68% said they believed vaccination is the best protection against influenza.

“I think we may be challenged a little bit, maybe as we are every year with getting people to take flu seriously. Every year, we have to remind people that 20,000 to 50,000 people die each year from influenza and that a substantial portion of that is vaccine preventable,” Biddinger said. “If you get your flu vaccine, you’re less likely to have to go to the hospital, you’re less likely to have symptoms that make you wonder if its COVID-19. These are really, really strong arguments we hope to make again this fall.”

According to the survey, the top reasons cited for not getting vaccinated were not believing influenza vaccines work (34%), concerns over adverse effects from the vaccine (29%) and concern about getting influenza from the vaccine (22%). Additionally, 32% cited never getting influenza as their reason.

“I would love to think that people might see this positive experience of high rates of influenza vaccination and low rates of influenza and better health, but I think that’s going to depend on a lot of us trying to emphasize that point next year when we head into flu season,” Biddinger said.

Next season

Decreased activity this season raises questions about what next season will look like. There could be a severe combined influenza and COVID-19 season “given the lack of influenza exposure this season and the likelihood of continued circulation of COVID-19,” Duchin said.

Jeffrey S. Duchin, MD
Jeffrey S. Duchin

“I think we understand so little about the seasonality of influenza and how it moves that it’s hard to predict how quickly flu will rebound,” Biddinger added. “We probably have a lot of reasons to expect it will, just because it has, over the decades and over the century, been such a prominent feature every winter in human health.”

One potential concern Uyeki mentioned is that there has hardly been any circulation of influenza A(H3N2) viruses in the U.S. during the 2019-2020 and current season, and therefore, waning immunity could result in a severe influenza season if A(H3N2) viruses were to circulate widely. He noted that seasons predominated by A(H3N2) viruses are typically more severe, particularly for elderly persons in whom influenza vaccine effectiveness against A(H3N2) viruses is suboptimal.

Another challenge created by the reduced activity is the selection of viruses for next season’s vaccines, Uyeki said.

“We’re identifying far fewer influ­enza viruses than a typical season, and so we just have a lower number of samples of viruses to be able to make predictions on what influenza virus strains might be circulating next season,” he said. “This really complicates the process, but I want to highlight that it is a global process” involving WHO reference laboratories all over the world, including the CDC’s Influenza Division, which is the WHO reference center for North America.

Flu vaccine fast facts

According to Uyeki, it is possible that influenza activity could still pick up over the coming weeks and months.

“One of the issues is, if we have a later influenza season, this will further complicate efforts to provide viruses to make recommendations for virus strains to manufacturers, so that they will be able to produce influenza vaccines for next season,” Uyeki said.

Most influenza vaccines are still manufactured using eggs, but there are other technologies that can produce vaccines in a much shorter time, including tissue cell culture-derived viruses and recombinant DNA vaccine technology, Uyeki said.

“Both of those … don’t take as long to produce influenza vaccine, so they might be able to respond better if we have a later season, but this will certainly complicate efforts to select the vaccine strains for next season’s vaccine,” he said.

Click here to read the At Issue, ‘How close is the world to having a universal flu vaccine?’


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