March 5, 2021 -- Queirra Fenderson, a first-time mom in Fort Washington, MD, gave birth to her daughter, Arya, at the end of December 2019. On the advice of their pediatrician, she and her husband spent the first month of their daughter’s life living in a “bubble” with only a few close family and friends seeing their baby to protect her from germs during cold and flu season.

Right about the time the new parents were ready to start exposing Arya to more people and germs, the pandemic hit. So a year later, their 14-month-old still remains in that bubble.

“There are still people to this day, some are my closest friends, who haven’t met her yet. It’s crazy,” Fenderson says.

She says she and her husband have often wondered how this isolation is impacting their daughter’s developing immune system. They’ve even considered day care to boost her immunity, but balancing that against the risk of COVID-19 has led them to still keep her home. “Arya hasn’t been sick her whole entire life, and I want her to get more exposure to germs to boost her immunity. But in my gut, I’m afraid of the exposure to COVID, too,” Fenderson says. “Pandemic parenting is hard.”

Simone Christensen of California agrees. She’s been very cautious with her 9-week-old daughter, Scout, who has seen only a few family members since she was born. Christensen has asthma, and their pediatrician has stressed the need to be cautious about COVID-19. But she and her husband also worry how isolation is impacting their daughter’s immune system.

“Scout hasn’t really been exposed to much at this point, which seems crazy because I feel like at this age, I was probably sucking on a table at McDonald’s,” she says with a laugh. “Anytime you talk to someone who raised their child pre-pandemic, they always tell you how important it is to expose babies to germs so their immune system starts to build up, and I always thought I’d be the kind of mom who does that. But everything is different because of COVID. The risks are so much greater, and trying to figure out what’s best in a pandemic is scary and hard.”

Developing an Immune System During a Pandemic

Leana Wen, MD, is an emergency room doctor and a public health expert. She’s also a mom whose second child -- a 10-month-old girl -- was born during the pandemic. She says there’s good reason to be cautious with babies and what you expose them to early in life.

“Newborns are highly susceptible to illnesses,” she says. “They don’t have much of an immune system of their own, so even pre-pandemic, we advised people to be very careful to germ exposure with newborns, understanding that they develop more robust immune systems over time.”

But many others, like B. Brett Finlay, PhD, a Canadian microbiologist and professor of microbiology at the University of British Columbia, also stress that early exposure to microbes has been shown to help a baby’s immune system develop well.

“We’ve learned over the last decade or so that the normal development of the immune system absolutely requires the presence of normal microbes,” says Finlay, co-author of Let Them Eat Dirt: Saving Your Child from an Oversanitized World. “It makes a difference in childhood development, and lack of exposure to these good microbes affects allergies, asthma, eczema, obesity, and more,”

We don’t know everything about how the immune system is built, “but we do know the end result is you need these early life microbes to get normal immune system development.”

This question, often referred to as the hygiene hypothesis, was first raised in 1989, and it has long caused controversy and debate. It argues that the developed world’s focus on clean environments through the use of disinfectants, sanitizers, and bleach has a negative impact on the immune system. The FDA points out it is one of many explanations for “for asthma being the most common chronic disease in the developed world.”

But plenty of others aren’t so sure. Researchers who published a study in 2016 found “no good evidence that hygiene, as the public understands, is responsible for the clinically relevant changes to microbial exposures.” The study, published in Perspectives in Public Health, says a combination of things, including “natural childbirth, breast feeding, increased social exposure through sport, other outdoor activities, less time spent indoors, diet and appropriate antibiotic use, may help restore the microbiome and perhaps reduce risks of allergic disease.”

The hygiene hypothesis theorizes that cleanliness and lack of exposure to microbes have led to increased allergies, asthma, and other conditions.But exposure to germs is only one piece of the complex puzzle of the human immune system. Genetics, environment, birth order, and diet are a few things that may affect the development of an infant's immunity, says Ruchi Singla, MD, a pediatric allergist and immunologist at University of Chicago Medicine. “The marvel of the human immune system is that it has both innate, from birth, and adaptive responses. It's equipped to both handle new threats and learn and develop memory to tackle attackers as it grows.”

Finlay says that as a result of the pandemic, we’re testing this hypothesis in a widespread way that is likely to prove interesting to researchers down the road. But it raises difficult questions for parents trying to navigate it in the moment.

“It's good to avoid COVID, but we know from a microbial point of view that it’s probably not good to decrease our microbial exposure in the long term,” Finlay says.

The Pandemic-Induced Parenting Dilemma

Wen acknowledges the topic has gotten far more complicated for parents trying to understand when it’s safe to start exposing their babies to germs. She says she urges parents to be very cautious.

“The good news is that children tend to get much less sick from coronavirus. So prevention is best, especially because this is a time-limited issue,” she says. “My baby hasn’t seen a lot of people or even met her grandparents, but at some point, this will change. Things are going to be very different later this year and next year.”

For that reason, Wen says she doesn’t spend much time worrying about how quarantining and self-isolation affect her daughter’s immune system. “I do think the real risk of coronavirus exposure outweighs any theoretical risk of harm because of lack of exposure to other things.”

Children will catch up, says Mobeen H. Rathore, MD, chief of infectious diseases and immunology at Wolfson Children's Hospital and a spokesperson for the American Association of Pediatrics. “The human body is an amazing thing,” he says.

He points out that research dating back nearly 20 years shows that whether children build immunity early (through day care) or later (in elementary school and beyond), they all eventually have similar immunity levels.

“Those who get fewer infections earlier in life will get more later and catch up, and in fact, there may be some benefit to it Because if an infant gets an infection like RSV and high fever, it can be very serious and there can be added concerns or even hospitalizations with that, too,” Rathore says.

Ultimately, he agrees that the risk of COVID far outweighs what may or may not be happening with a baby’s immune system in this moment. “Five hundred thousand people have died in less than a year from COVID. This is a very different time we are living in, and you have to take all of the necessary precautions,” Rathore says. “Whatever safe activities you are allowing your children to do, continue doing that.”

But Finlay does have concerns about the long-term implications of pandemic isolation on children’s development. He co-authored a piece in the Proceedings of the National Academy of Sciences Journal warning that “current pandemic control measures and practices will have broad, uneven, and potentially long-term effects for the human microbiome across the planet, given the implementation of physical separation, extensive hygiene, travel barriers, and other measures that influence overall microbial loss and inability for reinoculation.”

He admits there aren’t easy answers or solutions but says it’s something researchers and scientists will be paying close attention to. “I think we have to acknowledge this is an issue, and I think the best we can do at this point is encourage environmental exposures that parents feel comfortable with,” he says.

Wen stresses that even when they’re home, children are exposed to whatever their caregivers are exposed to, so the activities of the adults in their lives can add exposure for children, too. Finlay says stress can impact a caregiver’s microbiome, which can be transmitted to infants, so he encourages parents to try to manage their stress as best they can.

Fenderson is still being cautious for her daughter. She still hasn’t taken a tour of a day-care center and is instead focusing on what she can control at home, like having her daughter still drink breast milk, which contains immune-boosting antibodies. “That was one way I was adamant about boosting her immunity,” she says. “Arya still drinks breast milk to this day, and I think we have enough left in the deep freezer that she’ll make it another month or two.”

She says her daughter also plays regularly with her uncle’s dog, and she admits they live by the 5-second rule: If food drops to their floor at home, they let their daughter pick it up and eat it if she wants. They also get as much time outside as they can. “We’re trying to find as many natural ways as possible to do what we can to expose her to some things,” Fenderson says. “The only thing she hasn’t had exposure to is other children and their germs. Honestly, I just don’t know when we’ll be comfortable with that, but we definitely look forward to the day when it feels safe again.”

Show Sources

B. Brett Finlay, PhD, professor, Department of Microbiology and Immunology, University of British Columbia, Vancouver, Canada.

Simone Christensen, Concord, CA.

Queirra Fenderson, Fort. Washington, MD.

Mobeen H. Rathore, MD, pediatric infectious disease specialist, Wolfson Children’s Hospital, Jacksonville, FL.

Ruchi Singla, MD, pediatric allergist and immunologist, University of Chicago Medicine, Chicago.

Leana Wen, MD, emergency room doctor, Baltimore.

PNAS Journal: “The hygiene hypothesis, the COVID pandemic, and consequences for the human microbiome.”

BMC Pediatrics: “The relationship of prenatal antibiotic exposure and infant antibiotic administration with childhood allergies: a systematic review.”

Clinical and Experimental Allergy: “Too clean, or not too clean: the Hygiene Hypothesis and home hygiene.”

Perspectives in Public Health: “Time to abandon the hygiene hypothesis: new perspectives on allergic disease, the human microbiome, infectious disease prevention and the role of targeted hygiene.”

JAMA Pediatrics: “Influence of Attendance at Day Care on the Common Cold From Birth Through 13 Years of Age.”

FDA: “The Hygiene Hypothesis.”

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