Immunologist here. It's a fairly old idea that immunity is built only in childhood - if we reacted to everything that our immune system didn't "see" in our childhood as adults, then any new food we tried, new tissue developed during puberty, and new diseases such as this pandemic would kill us straight away. But people still survive.

In actual fact, our immune system is continually learning. Any adult has a small population of "memory" immune cells that remember past infections and are ready to react should they reappear, and a much larger population of "naïve" immune cells that are effectively blank that react to new diseases. When they do react, they multiply exponentially to deal with the infection, then most die off while a small number will become new memory cells for that infection.

An infant that has been in quarantine will still encounter pathogens from their parents, pets, food, etc, as well as have antibodies handed down through breast milk. Their immune systems are generally weaker than adults anyway, but there is no reason that an average infant who is fully reintroduced to our dirty world later in their childhood won't go on to develop a full and strong immune system.

Edit: Thank you so much for all the awards and comments, I'm so glad that this has interested so many people and reassured so many new parents.

That said, there are still a lot of comments coming in and I can't keep up with them all, so I thought I'd pop back on here to address a couple of FAQs.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4707740/ Here is a good review on the development of immunity over the course of a lifetime. Immunity in pregnancy is a huge and still developing field, but this review summarises what I've said in a few different comments. Immune maturity is achieved by accumulation of immune memory to infections and encounters with pathogens, whether that results in illness or not - your body can still obtain immune memory from infections and encounters that aren't symptomatic. There isn't a time limit or timeline for this.

A lot of people are asking me about allergy. My specialty is in vaccines and innate/adaptive crosstalk, not allergy, so I can't answer any detailed questions on this. However, here is a review on early allergen exposure and food allergy development. https://www.frontiersin.org/articles/10.3389/fimmu.2019.01933/full Exposure to food allergens isn't dependent on infants or children leaving the house, and maternal exposure to environmental allergens will have been beneficial to the child in utero.

I've also seen a few comments asking about the causes of childhood leukemia. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5080868/ Overactivation of the immune system in response to certain infections may lead to eventual leukemia, but as with all cancers, this isn't enough on its own. Cancer is a complex disease that usually requires a lifetime of accumulating multiple risk factors from both environmental and genetic risk factors, hence why a lot of cancers are suffered by the elderly. Childhood leukemia is also thought to be caused by multiple factors, including exposure to pollution, tobacco, paint/solvents, pesticides, and unbalanced maternal diet. But I would like to reiterate that a "perfect storm" is needed with several of these to occur, and immune dysfunction plays only one role. Cancer is also slightly off topic from this discussion so I'll leave this landmark paper for anyone who is more interested in what causes it. https://www.cell.com/fulltext/S0092-8674(11)00127-9

To all new pandemic parents - you're doing amazingly, you're giving them the best shot that you can, and statistically they're going to be fine.

I read this article from Science Mag that you may find curious. It is not answering your question directly, but, discussing how long term outlooks for children will be as we move forward.

https://science.sciencemag.org/content/371/6530/741

The abstract suggests that as Coronavirus SARS COV-2 moves through its initial pass through the population, the primary populations infected will be children. Since it appears to be much less fatal in children than it does in older adults, SARS COV2 may become common but mild for them. The long term effects could be a large amount of the population protected against the disease due to exposure as children.

Pediatrician here. Will echo the excellent comment from the immunologist. Will also add that there are decent cohort studies and case-control studies that support the idea that you acquire the infections when you get exposed (no kidding, right?!) and that there really aren’t any effects on the efficacy of the immune system if you acquire a little later. Rather, the big things they found is that kids who went to daycare before entering school had more infections (about 8-12 colds/year) when they were in daycare and fewer infections (and subsequently, less school absenteeism) when they entered school. Kids who hadn’t gone to daycare before entering school had more infections and more absenteeism. Effects weren’t huge, and there were a lot of confounders and caveats.

So what does that have to do with Covid? Looking at my hospital’s regional infectious epidemiology weekly reports, we have seen almost no other viruses circulating this season. Usually we are awash in influenza, RSV, and others like rhinovirus. Like, hundreds of positive tests a week. Last week, apart from Covid, we had four cases of rhinovirus. That was it. No flu. No RSV. Nothing else. We aren’t unique, but I don’t want to comment on data from other regions that I haven’t personally reviewed. So what will happen to these “quarantine kids?” Unless we keep up with masking and social distancing post-Covid, the other viruses will start circulating again and those kids will catch those infections then and their immune systems should be fine.

As a side note, deep down, I’d like to hope that every doctor would like nothing more than for them and their entire profession to go out of business because everyone is healthy. You could make an argument that at least for viral respiratory pathogen-mediated diseases and their sequelae, you really could make a big difference by continuing to mask and distance, at least during the viral season.

The hygiene hypothesis is mostly supported by data, but with the giant caveat that this applies mostly to environmental triggers, allergens, and inert substances, not germs that cause acute human diseases.

Some specific conditions aside (chicken pox perhaps) there has never been any overall benefit proven to children who are exposed to more diseases as a child. The more colds you get doesn't make you more resistant to colds later. But getting sick a lot can negatively affect a child's development in other ways. Fevers, lack of sleep, weight loss, nutritional interruption, and other side effects of being sick (besides the specific symptoms of the disease itself) are not good for growing bodies and minds. The more of those things you experience as a child the less likely you are to reach the full potential of growth.

It's still a developing scientific picture, but the idea that exposure to disease causing germs is good for kids has been disproven. As that applies to the pandemic: if the lockdown meant you stayed inside in a sterile environment instead of going outside, then yes it has affected your health to some degree. But if the lockdown just meant you didn't go to a classroom fully of snotty kids, and thus avoided being exposed to dozens of viruses and bacteria, and you still played outside in the dirt every once in a while, then no it probably won't. It might even help.

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