The United States is in the midst of a “tripledemic” of three highly contagious respiratory illnesses: COVID-19, respiratory syncytial virus (RSV), and seasonal flu. One theory circulating on social media blames so-called “immunity debt” for this wave of illness, arguing that COVID-19 mitigation efforts like lockdowns and masking weakened our immune systems and robbed us of our natural ability to fight off infections.

However, “there’s very little basis for assuming what’s happening so far has anything to do with immunity debt,” says Michael Osterholm, PhD, a professor and director of the Center for Disease Research and Policy at the University of Minnesota in Twin Cities.

Here’s a closer look at the immunity-debt claim and other possible explanations for the spike in respiratory infections this winter.

What Is ‘Immunity Debt’?

The notion of immunity debt as the cause of this surge in respiratory illnesses appears to have originated from a position paper published in August 2021 in Infectious Diseases Now, in which French researchers suggested many infectious diseases like RSV that receded during the pandemic quickly rebounded once people stopped masking and isolating themselves.

The researchers argued that a lack of exposure to infectious diseases during lockdowns left us with compromised immune systems. All the way at the end of this paper, however, they concede that their idea is merely a hypothesis, not a fact backed up by rigorous scientific evidence.

Immunity Isn’t a Muscle

To the layperson, it might make sense to think of immunity as a muscle that gets weaker when you don’t use it. But scientists say that isn’t how the immune system works.

They tend to argue that respiratory viruses and bacteria are infecting more people now than during the earlier years of the pandemic because more people are emerging from social isolation and engaging with the world mask-free — and encountering germs like RSV.

Historically, two-thirds of babies typically catch RSV before their first birthday, according to the pharmaceutical company Sanofi, perhaps from other infants at daycare or from older siblings bringing the virus home from school. Many young children become infected more than once, the Centers for Disease Control and Prevention (CDC) notes. Parents who kept their babies and older children home during the pandemic succeeded in limiting exposure to all kinds of respiratory germs, including RSV. But now RSV is spreading freely.

Beyond this, it’s also quite possible that influenza and RSV cases will ultimately return to normal pre-pandemic levels, Dr. Osterholm says. Some years flu or RSV cases spike earlier in the winter without resulting in more cases, hospitalizations, or deaths over the course of the entire season. Flu and RSV cases have declined sharply in recent weeks, according to CDC data.

Neglected Preventive Care and Out-of-Date Vaccinations May Be Lowering Our Immunity Now

One reason for the tripledemic may be that people have been neglecting routine healthcare for chronic health problems like asthma, diabetes, or heart disease. This can make them more vulnerable to all kinds of infectious diseases, says Peter Hotez, MD, PhD, a professor of pediatrics and molecular virology at Baylor College of Medicine and co-director of the Texas Children’s Hospital Center for Vaccine Development in Houston.

Another issue is that many people simply aren’t up to date on vaccinations — not just for COVID-19 but also for other infectious diseases like the flu. “We need to max out vaccines in order to take as many respiratory pathogens as possible off the table,” Dr. Hotez says.

Many of those who got COVID-19 vaccines earlier in the pandemic may not realize that in fall 2022 the CDC began recommending new bivalent booster shots to protect against the omicron variant in children and adults. Plus people may not be aware that vaccine protection wanes over time, so they may mistakenly believe that the shots they got a year or two ago are still offering significant protection from severe disease.

And COVID-19 vaccines are only part of the picture. Everyone 6 months and older needs an annual flu vaccine, the CDC advises. Young children should get several vaccines to prevent infectious diseases like measles, rubella, whooping cough, and pneumococcal disease, the CDC recommends. Some adults may also require vaccines to prevent several infectious diseases including shingles, measles, meningococcal disease, and pneumococcal disease, per the CDC.

Prior COVID-19 Infections Could Be Taking a Toll on Immunity

Beyond a lack of vaccination, it’s also possible that COVID-19 infection left some people with weakened immune systems, suppressing their ability to fight off other viral infections, Hotez says.

He notes that two mainstays of preventing infections — masking and hand-washing — are great ways to lower your risk of getting sick with a wide variety of viruses circulating right now.

As for the role of so-called immunity debt in all of this, Hotez adds, “I’m not sure it’s a useful concept.”