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The holiday season has always been a battlefield, from the standpoint of your immune system. This year, I’m afraid that analogy is especially apt, as we are confronting an insidious trio of COVID-19, RSV, and flu all at the same time.
Given how terribly contagious these diseases can be, it can feel like catching the flu or another one of these bugs is practically an inevitability (particularly if you’ve got kiddos at home). But it’s worth remembering that not everyone who is exposed to infectious microbes actually gets sick.
For example, household transmission studies show that if someone in your home catches the flu, the risk of someone else getting it is around 38%. And even the omicron variant, which is thought to be one of the most infectious diseases known to man, does not always spread to other members of a household.
Some of this just comes down to luck, of course. But we also have known for some time that immunity varies between individuals, meaning some folks are more susceptible to getting sick than others. And this may be due, at least in part, to factors within our control.
So what can you do to optimize your own immunity?
Well, adopting a healthy lifestyle is obviously vital, and regular physical activity is a linchpin of that approach. But experts in the field have long suggested that intense exercise may compromise your immune system — potentially increasing your probability of getting sick!
Is this true? Should you rein in your workouts during the holiday season to preserve your immunity?
An "Open Window” to Infection?
The open window hypothesis argues that the immune system is suppressed for around 2-72 hours after vigorous exercise. During this time, purportedly, athletes are exposed to a transiently increased risk of getting sick.
This idea goes way back. Ernst Jokl, a pioneer in the field of sports medicine, wrote in 1977:
“Physical fitness established by athletic training does not enhance immunological resistance against infections. At times, it renders athletes more susceptible.”
Now, there’s actually a fairly sound physiological basis for this hypothesis. Countless studies analyzing the blood work of athletes has shown that exercise alters the immune system in a distinctive biphasic pattern.
Let’s start with what happens while you’re training. During vigorous exercise, blood levels of certain immune cells jump up profoundly. For instance, cytotoxic T cells rise by approximately 2.5-fold, and natural killer cells skyrocket all the up to 10-fold higher than baseline!
These immune cells are usually kind of hiding out in storage in the spleen and lymph nodes, but exercise pulls them out into circulation, effectively stimulating your immune system. Sounds pretty good so far.
However, shortly after you’re done exercising, something weird happens — the numbers of those immune cells in the blood drop even more dramatically, down to a lower level than where they were before you exercised!
So, this would indeed appear to represent a window in which your immune system is knocked down, leaving you vulnerable to opportunistic infections.
But, like so many things in biology, it’s more complicated than that.
Suppression…or Redistribution?
Historically, researchers have surmised that these natural killer cells and cytotoxic T cells must be undergoing programmed cell death in the post-workout window. Which would obviously not be ideal. But more recently, scientists have begun to question this assumption.
Some elegantly designed experiments were devised to figure out exactly what was happening to them, and the results are illuminating. Let’s take a look.
Researchers in Münster extracted immune cells from a bunch of mice, and labeled these cells with fluorescent trackers so that they could follow their movement inside the body.
Then, they re-implanted the modified immune cells back into the mice, and divided the rodents into groups: some exercised on a treadmill, and others just chilled in their cages (controls).
When the mice ran, T cells were mobilized in large numbers from the spleen and started to circulate in the blood. No surprise there.
But here is where things get super interesting. In the post-exercise window, the immune cells didn’t die off en masse, as previously suspected. Instead, they were redeployed from the blood to mucosal surfaces in the body, most notably the gut and the lungs.
This makes a ton of sense, if you stop and think about it. Both the gut and the lungs are regions where pathogens are most likely to be encountered in the context of intense exercise. You’re breathing hard, and intestinal barrier function tends to be compromised after vigorous activity.
And from your body’s standpoint, those are suitable areas to have heightened immune surveillance in general, since these tissues interface with the environment and are constantly under siege by would-be invaders.
In other words, just because those immune cells are no longer floating around in the bloodstream does not mean that they have been eliminated, or that your immune system has been suppressed in any way.
In fact, it’s exactly the opposite — they’re just off doing their job, in the places where they are needed most!
Naturally Boosting Your Immune System
But in reality, we don’t need mechanistic studies using mice to recognize this. There is a staggering amount of human evidence that exercise is just plain good for your immune system.
For instance, observational research at the height of the COVID-19 pandemic found that patients with the virus who were consistently inactive had more than 2 times greater odds of hospitalization, and odds of dying from COVID-19 were nearly 2.5 times higher, compared to patients who consistently met physical activity guidelines.
And a recent analysis of pooled data from nearly one million participants found that regular physical activity was associated with a 31% reduced risk of acquiring an infectious disease, and a 37% lower risk of dying from such an illness.
Research examining how exercise influences the immune response to vaccination lends further support.
When researchers had participants exercise for 90 minutes after receiving seasonal flu vaccines or the first dose of the Pfizer COVID-19 vaccine, serum antibody levels were consistently higher in the exercisers 4 weeks later. A group that exercised for 45 minutes didn’t observe the same benefit, so it looks like more is better in this case.
Vaccinated people who log high amounts of physical activity (measured via wearable activity trackers) are also nearly 3 times less likely to wind up in the hospital due to COVID-19, compared to inactive folks who also got the vaccine. And large analyses of trials have found that exercising regularly is associated with stronger immune responses to the flu vaccine, especially in older adults. Interestingly, exercising the same arm in which the vaccine is administered seems to be more effective.
Key Takeaway
The bottom line is that strenuous exercise is not likely to lead to increased risk of infection. To the contrary, it appears to be protective, especially if performed regularly.
If anything, most of us should probably be more worried about failing to get enough physical activity, rather than overdoing it. Of course, you still want to make sure that you’re getting plenty of rest and recovery to avoid overtraining.
Furthermore, there is encouraging evidence that exercise can improve your immune response to vaccines, so it might be smart to perform a workout either shortly before or right after you get your shots. Based on the studies described above, I would suggest doing a walk or jog for at least an hour, combined with some arm exercises if you can, to attract immune cells to the site of the vaccination.
