Masking, social distancing, and other measures aimed at preventing the spread of Covid-19 had the "unintended but welcome effect" of stopping other respiratory viruses, Tara Parker-Pope writes for the New York Times. But now that those prevention measures are being rolled back, common respiratory viruses have returned with a vengeance—and people's immune systems are struggling to keep up.

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A rude awakening for immune systems

According to infectious disease experts, measures taken to reduce the spread of Covid-19 reduced people's exposure to different microbes. Without regular exposure to microbes, people's immune systems, while not weakened, may take longer to fight back against infections, Parker-Pope reports.

"Frequent exposure to various pathogens primes or jazzes up the immune system to be ready to respond to that pathogen," said Paul Skolnik, an immunovirologist and chair of internal medicine at the Virginia Tech Carilion School of Medicine. "If you've not had those exposures, your immune system may be a little slower to respond or doesn't respond as fully, leading to greater susceptibility to some respiratory infections and sometimes longer or more protracted symptoms."

Isolation isn't the only factor at play. Satya Dandekar, chair of the department of medical microbiology and immunology at the University of California-Davis School of Medicine, added that other factors, "including stress, poor sleep habits, and increased alcohol consumption," could have also affected how people's immune systems respond to spreading respiratory viruses, the Times reports.

“When a person gets exposed to a pathogen, there has to be a rapid ramp up of the response from the immune system and immune cells," Dandekar said. "With stress and other factors, the army of immune cells is a little hampered and slows down and may not be able to react fast enough to attack, giving enough time for the pathogen to get a hold on the host."

Surge in respiratory viruses

According to CDC, cases of common respiratory viruses, such as respiratory syncytial virus (RSV) and human parainfluenza viruses, are currently on the rise.

According to Parker-Pope, CDC officials have said this increase in RSV, which can be dangerous to the very young and very old, is "particularly unusual" during this time of year. RSV cases have primarily been traced to young children, the Times reports, and some have been hospitalized with severe symptoms.

The RSV surge is not limited to the United States, the Times reports. Increased rates of RSV have been reported in Europe, South Africa, Australia, and New Zealand.

According to Parker-Pope, the surge is likely due to global pandemic restrictions, which led to a larger population of susceptible children. Infants, who are now toddlers, were protected from RSV during the pandemic because few people were going outside. As a result, now both they and new infants are vulnerable to infection—essentially enabling the virus to infect about twice as many people as it typically does. And these infected infants and toddlers can spread the virus to older children and adults.

Sue Huang, director of the World Health Organization's National Influenza Centre at the Institute of Environmental Science and Research-New Zealand, and her colleagues in February published a study in Nature that found New Zealand's strict restrictions prevented not only the spread of Covid-19 but also the spread of RSV and influenza. But when New Zealand opened up to Australia, RSV cases spiked in just a few weeks.

"I haven't seen anything like this in 20 years of working as a virologist," Huang said. "There's usually a degree of pre-existing immunity due to the previous winter. When you don't have that kind of protection, it's a bit like a wildfire. The fire can just continue, and the chain of transmission keeps going."

'Where is the flu?'

Experts are not surprised by the current spread of RSV and other respiratory viruses, the Times reports. In fact, researchers published a study last winter in the Proceedings of the National Academy of Sciences (PNAS) predicting that there would be an increased rate of infections from respiratory viruses this summer.

However, experts are not sure about when the flu virus specifically will resurface—or how it will affect people. Rachel Baker, the lead author of the PNAS study and an epidemiologist at Princeton University, said a potential concern is that the flu, RSV, and Covid-19 might circulate in the population at the same time.

"The big puzzle is where is the flu?" Baker said. "I think it's a very uncertain flu season. It's not necessarily going to be worse, but when is it going to come back? And what is it going to look like?"

In the meantime, however, Allison Agwu, an infectious disease specialist at Johns Hopkins Children's Center said people should take precautions to prevent the spread of respiratory illnesses, which includes getting vaccinated against Covid-19.

"Do the things we tell fifth graders: Wash your hands, cover your sneeze, get rest, all those things," Agwu said. "And do your best to get vaccinated against the things you can. Get your Covid vaccine so you’re less paranoid when you get a cold." (Parker-Pope, New York Times, 7/22)