Headlines over the past few weeks are ringing the alarm about earlier and more serious influenza (flu) and respiratory syncytial virus (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," said 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 told Medscape Medical News.
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.
"This is particularly striking for RSV, which usually doesn't affect the entire country simultaneously," Schaffner said.
"Yes, RSV is causing many more hospitalizations and earlier than any previously recorded season in the US," according to figures from the Centers for Disease Control and Prevention (CDC) on RSV hospitalizations, said Kevin Messacar, MD, PhD, 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," said 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 said, because "the droplets are just simply lighter.
"I think those are all factors," he told Medscape Medical News.
Paul Auwaerter, MD, MBA, agreed that there are likely multiple causes for the unusual timing and severity of RSV and flu this year.
"Change in behaviors is a leading cause," said the clinical director for the Division of Infectious Diseases at the Johns Hopkins University School of Medicine in Baltimore. More people returning to the workplace and children going to school without masks are examples, he added.
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 said. "That's everybody's favorite [reason]."
He said he is confident that the jump in RSV cases is being driven by previous COVID 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 said. "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 said.
Chin-Hong also predicted 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 said.
He estimated 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 physicians 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 precautions, such as masking, social distancing, and other protective measures taken during the pandemic.
"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 said.
"To me, the immunity gap is a scientific observation, not a political argument," he added.
In a July 2022 publication in The Lancet, Messacar and his colleagues stated 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 noted 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 said.
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 and the flu was reported in people at some centers on the West Coast, Auwaerter said. Now, however, "the unpredictable nature of the Omicron subvariants and the potential for further change, along with the never-before-seen significant lessening of influenza over 2 years, leave little for predictability.
"I do think it is less likely, given the extent of immunity now to SARS-CoV-2 in the population," Auwaerter said.
"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 added.
Schaffner said 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 cases among people in the hospital. "Coinfections are, at least to date, very unusual."
There needs to be an asterisk next to that, Schaffner added. "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.
"So this year may be more revelatory as we go forward," he said.
The future is uncertain, Messacar and colleagues wrote in The Lancet: "Crucially, the patterns of these returning viral outbreaks have been heterogeneous across locations, populations, and pathogens, making predictions and preparations challenging."
Chin-Hong used a horse race analogy to illustrate the situation now and going forward. RSV is the front-running horse, and influenza 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 said.
"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."
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Cite this: Experts Explain the 'Perfect Storm' of Rampant RSV and Flu - Medscape - Nov 15, 2022.