Nov. 15, 2022 -- Headlines over the past few weeks are ringing the alarm about earlier and more serious flu and RSV outbreaks compared to previous years. Add COVID-19 to the mix and you have a dangerous mash of viruses that have many experts calling for caution and searching for explanations. 

RSV and the flu "are certainly getting more attention, and they're getting more attention for two reasons,"  says William Schaffner, MD, professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine in Nashville. 

"The first is that they're both extraordinarily early. The second is that they're both out there spreading very, very rapidly," he says. 

RSV usually follows a seasonal pattern with cases peaking in January and February. Both viruses tend to hit different regions of the country at different times, and that's not the case in 2022. Schaffner says RSV doesn’t usually affect the entire country at once. 

CDC data shows RSV is causing more hospitalizations and that they are happening earlier than in any previously recorded season, says Kevin Messacar, MD, an associate professor at the University of Colorado School of Medicine and a pediatric infectious disease specialist at Children's Hospital Colorado in Aurora.Although there could be some increase in diagnoses because of increased awareness, the jump in RSV and flu cases "is a real phenomenon for multiple reasons," says Peter Chin-Hong, MD, professor in the University of California, San Francisco, Health Division of Infectious Diseases.

With fewer COVID-related restrictions, people are moving around more. Also, during fall and winter, people tend to gather indoors. Colder temperatures and lower humidity contribute as well, Chin-Hong says, because "the droplets are just simply lighter." 

"I think those are all factors," he says. 

Paul Auwaerter, MD, clinical director for the Division of Infectious Diseases at Johns Hopkins University School of Medicine in Baltimore,  agrees there are likely multiple causes for the unusual timing and severity of RSV and flu this year. 

"Change in behaviors is a leading cause," says Auwaerter. More people returning to the workplace and children going to school without masks are examples, he says.

Less exposure to these three viruses also means there was less immune boosting among existing populations, he said. This can lead to "larger susceptible populations, especially infants and younger children, due to the relative absence of circulating virus in past years."

A Leading Theory

Are we paying a price now for people following the edicts from officials to mask up, stand apart, and take other personal and public health precautions during the COVID-19 pandemic? 

It's possible, but that may not be the whole story.

"When it comes to RSV, I think that theory of isolation, social distancing, mask wearing, and not attending schools is a very valid one," Schaffner says. "That's everybody's favorite [reason]." 

He says he is confident the jump in RSV cases is being driven by previous COVID-19 public health protections. However, he's "a little more cautious about influenza, in part because influenza is so variable.

"Like people in influenza say, if you've seen one influenza season, you've seen one influenza season," Schaffner said.

"There's a lot of debate," he added. "Nobody can say definitively whether the immune deficit or debt is a consequence of not being stimulated and restimulated by the influenza virus over the past two seasons."

‘A Perfect Storm’

"Now you kind of have the perfect storm," Chin-Hong says. "It's not a good situation for COVID with the variants that are emerging. For influenza, not having seen a lot of influenza the last 2 years, we're probably more susceptible to getting infected."

RSV cases rose during summer 2021, but now the weather is colder, and people are interacting more closely. "And it's very, very transmissible," he says.

Chin-Hong also predicts that "even though we don't have a lot of COVID now, COVID will probably pick up."

The rise in RSV was unexpected by some experts. "This early influenza is also a bit of a surprise and may be influenced by the fact that lots of us are going back and seeing each other again close-to-close, face-to-face in many enclosed environments," Schaffner says. 

He estimates the 2022–2023 flu season started 4 to 6 weeks early "and it's taken off like a rocket. It started in the Southeast, quickly went to the Southwest and up the East Coast. Now it's moving dramatically through the Midwest and will continue. I'm quite sure to hit the West Coast if it isn't there already."

A Phenomenon by Any Other Name

Some are calling the situation an "immunity debt," while others dub it an "immunity pause" or an "immunity deficit." Many doctors and immunologists have taken to social media to push back on the term "immunity debt," saying it's a mischaracterization that is being used to vilify COVID-19 precautions. 

"I prefer the term 'immunity gap'...which is more established in the epidemiology literature, especially given the politicization of the term 'immunity debt' by folks recently," Messacar says.

"To me, the immunity gap is a scientific observation, not a political argument," he says.

In a July 2022 publication in journal The Lancet, Messacar and his colleagues said that "decreased exposure to endemic viruses created an immunity gap — a group of susceptible individuals who avoided infection and therefore lack pathogen-specific immunity to protect against future infection. Decreases in childhood vaccinations with pandemic disruptions to health-care delivery contribute to this immunity gap for vaccine-preventable diseases, such as influenza, measles, and polio."

The researchers said that because of isolation during the pandemic, older children and newborns are being exposed to RSV for the first time. Returning to birthday parties, playing with friends, and going to school without masks means "children are being exposed to RSV, and that's likely the reason that RSV is moving early and very, very substantially through this now expanded pool of susceptible children," Schaffner says.

How Likely Are Coinfections?

With peaks in RSV, flu, and COVID-19 cases each predicted in the coming months, how likely is it that someone could get sick with more than one infection at the same time? 

Early in the pandemic, coinfection with COVID-19 and the flu was reported in people at some centers on the West Coast, Auwaerter says. Now, however, "the unpredictable nature of the Omicron subvariants and the potential for further change,” along with the significant drop in flu cases the past two years make any prediction difficult. 

"I do think it is less likely, given the extent of immunity now to SARS-CoV-2 in the population," Auwaerter says. 

"I most worry about viral coinfections...in people with suppressed immune systems if we have high community rates of the SARS-CoV-2 and influenza circulating this fall and winter," he says. 

Studies during the pandemic suggest that coinfection with the coronavirus and another respiratory virus were either rare or nonexistent

Schaffner says these findings align with his experience at Vanderbilt University, which is part of a CDC-sponsored network that tracks laboratory-confirmed RSV, flu, and COVID-19 cases among patients in the hospital. "Coinfections are, at least to date, very unusual."

There needs to be an asterisk next to that, Schaffner says. "Looking back over the last 2 years, we've had very little influenza, and we've had curtailed RSV seasons. So, there hasn't been a whole lot of opportunity for dual infections to occur.

Future Concerns

The future is uncertain, Messacar and colleagues wrote in The Lancet and 

Chin-Hong uses a horse race analogy to illustrate the situation now and going forward. 

RSV is the front-running horse, and flu is running behind but trying to catch up. 

"And then COVID is the dark horse. It's trailing the race right now ― but all these variants are giving the horse extra supplements.

"And the COVID horse is probably going to be very competitive with the front-runner," he says.

"We're just at the beginning of the race right now," Chin-Hong said, "so that's why we're worried that these three [viruses] will be even more pronounced come later in the year."

 

Show Sources

Sources: 

William Schaffner, MD, professor of preventive medicine and infectious diseases, Vanderbilt University School of Medicine, Nashville. 

Kevin Messacar, MD, associate professor, University of Colorado School of Medicine, pediatric infectious disease specialist, Children's Hospital Colorado, Aurora.

Peter Chin-Hong, MD, professor, University of California, San Francisco, Health Division of Infectious Diseases.

Paul Auwaerter, MD, MBA, clinical director, Division of Infectious Diseases, Johns Hopkins University School of Medicine, Baltimore.

Forbes: “Are ‘Immunity Debt’ Claims After Covid-19 Precautions Accurate Or Misinformation?”

The Lancet: “Preparing for uncertainty: endemic paediatric viral illnesses after COVID-19 pandemic disruption.”

Open Forum Infectious Diseases: “Circulation of Non-SARS-CoV-2 Respiratory Pathogens and Coinfection with SARS-CoV-2 Amid the COVID-19 Pandemic.”

Clinical Infectious Diseases: “Multiplex Detection of Antibody Landscapes to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)/Influenza/Common Human Coronaviruses Following Vaccination or Infection With SARS-CoV-2 and Influenza.”

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