How will isolation affect long-term immunity?

Every time you kiss another human being intimately for 10 seconds, more than 80m bacteria are transferred from mouth to mouth. If you’re at a party and double dip your tortilla chip into the salsa three times, around 10,000 bacteria will be transferred from your lips to the dip. Say “hi” to your co-workers as you sit down at your office desk and you’ll also be greeted by over 10m bacteria on its surface.

Disturbing as these figures may seem, many scientists believe that exposure to these microbes helps finetune our immune systems – the network of cells and molecules that protect us from diseases. In 1989, epidemiologist David Strachan first proposed the “hygiene hypothesis” – the idea that being too clean causes defects in the immune system, leading to a rise in inflammatory diseases, such as asthma and allergies. While Strachan’s theory is debated and hygiene saves countless lives, decades of data support the idea that exposure to microbes helps the immune system develop.

But hang on a minute. For most of the last year, many of us haven’t been kissing strangers, double dipping at parties or sitting down to work in a crowded office. Instead we’ve been locked up at home by ourselves, sanitising our hands every time we go to the shop and holding on to distant memories of restaurants and gyms. So what’s happened to our immune systems in lockdown? What’s going to happen now that the country is opening back up?

Graham Rook, a microbiologist at University College London, proposed an alternative to the hygiene hypothesis in 2003. Rook’s “old friends hypothesis” posits that as humans evolved, our immune systems learned to cope with the microbes around us in the natural world. Rook argues that we need to be exposed to these “old friend” microbes in order for our immune systems to develop properly. (Strachan’s hygiene hypothesis focused on infections, while Rook’s focuses on more harmless micro-organisms.)

“Our immune system is a learning system, just like the brain,” Rook says, explaining that the system has two branches. We are born with an “innate” immune system encoded in our genes, but this is “tuned” by our “adaptive” immune system, which collects data from the microbes around us to determine which are safe and which are dangerous. Without the right data, the immune system starts attacking things it shouldn’t, causing allergies, asthma and autoimmune diseases (when the immune system targets your body’s own tissues).

So first, the good news. By the time you’re an adult, you’ve already encountered a whole host of microbes. Your microbiota – the trillions of microbes living on and in you – is “fairly stable and established,” says Rook. A year of isolation, then, is unlikely to severely damage your immune system’s regulatory mechanisms. But when it comes to children, Rook and other scientists have concerns about the effects of lockdown measures. “A child on the 24th