In the late spring and early summer of this year, it appeared as though the COVID-19 pandemic was finally receding and the world could actually begin to consider a return to normalcy. Infection rates were dropping, thanks in part to effective measures like masking and social distancing mitigating the spread of illness, and more importantly, thanks to the roll out of effective vaccines that dramatically reduced the likelihood of infection and, when infections did occur, the risk of serious disease and death.

That was then; this is now. Just as things were getting better, the emergence of the highly contagious and more severe Delta variant spread rapidly, affecting the unvaccinated disproportionately and once again leading to dramatic increases in case numbers, deaths, and fear of overwhelming the health care systems.

In the two years since COVID-19 first began to impact the world’s population, a lot has been learned about how the virus causes illness, but a lot has also been learned about how people’s behaviors impact whether or not they become ill and how ill they get.

For example, during periods of heavy restriction on activity, research showed that those who were able to maintain exercise and healthy nutrition were less likely to gain weight or suffer from emotional distress. And now we are learning that those same habits may confer important benefits in boosting immune function to prevent infections in the first place and to augment the vaccination response, which could improve the efficacy of COVID-19 vaccination and further diminish the likelihood of breakthrough infections among the vaccinated.

A recent paper in the journal Physical Activity and Infectious Disease synthesized the results of dozens of studies encompassing hundreds of thousands of individuals in order to determine whether or not regular exercise confers benefits by strengthening the immune response.

I have written previously on the effect of training and racing on the immune system. In that article, I presented data that suggested that regular exercise was felt to boost overall immune system health, but prolonged high-intensity events temporarily depress immune function and can lead to a brief window of infection susceptibility.

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In this new study, the authors found additional compelling evidence to augment the previously reported effects of training on immune health. And, they did this with a renewed sense of urgency, given the current worldwide pandemic. The authors argued that people who exercise regularly are more likely to be resilient to COVID-19. “First, it has been hypothesized that physically active people are likely to be more resilient to infection through better immunosurveillance against pathogens. Secondly, given that severe infections are more likely in individuals with poorer cardiovascular and metabolic health and who might have pre-existing chronic conditions, we might hypothesize that physical activity also has an indirect protective effect against infectious disease by improving cardiovascular and metabolic health and lowering the risk of chronic diseases.”

The findings were very compelling. In studies with a total population of nearly 560,00 people, the likelihood of contracting a serious respiratory illness (pneumonia) was reduced by 31% and the risk of death from pneumonia or other serious infectious disease was decreased by 37%.

Additional findings included higher concentrations of protective antibodies and immune helper cells necessary for preventing infection, and lower numbers of immune cells typically involved in chronic inflammatory processes—though no data was presented on the actual risk for chronic inflammatory diseases.

Still, resiliency to infection is not the same as preventing infection, even for the supremely fit, and if we are ever to successfully exit this pandemic we have to embrace as an entire population the need to prevent infection at every turn. With the newly FDA-approved vaccines, this goal is now very much in reach if the vast majority get vaccinated with these safe and effective shots, particularly those who are healthy and are best positioned to eliminate spread of the virus.

For a real life look at how vaccines can change the state of play for the pandemic, compare the state of Massachusetts, with the highest proportion of vaccination in the country, to any state in the southeast, where rates are less than 50%. Since the emergence of the Delta variant, even as infection numbers have risen in Massachusetts, the total number of hospitalizations in the state is less than six hundred. In the heavily unvaccinated areas in the south, by comparison, hospitals are packed to capacity and COVID deaths are skyrocketing again.

This leads to possibly one of the most interesting findings in this paper and the most relevant to this moment. In six studies with a total of 600 study subjects there was “…a statistically significant effect of physical activity compared to control with higher antibody titers” after vaccines. That is to say: People who exercised regularly had a more robust immunologic response to vaccination, suggesting an improved protective effect compared to those who did not exercise regularly. This means that while there have been documented cases of athletes experiencing long-term effects from COVID, athletes are now well positioned to minimize the odds of severe illness if they get vaccinated.

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To be clear, none of these studies were on any of the now available COVID vaccines. Rather, they tested responses to influenza, pneumococcal (pneumonia), and shingles vaccines—but there is no reason to think that the findings would be different for the COVID vaccines.

These results led the authors to conclude that: “Regular physical activity should be promoted in the general population to decrease the risk of community-acquired infection and infectious disease mortality, strengthen the potency of immunization programs, and help lessen the impact of pandemics, such as the recent COVID-19.”

This is an important conclusion and results like this can be used to justify allowing and encouraging continued safe outdoor exercise, even during heavy restrictions. While restrictions around gatherings and indoor events confer protection by enforcing social distancing and breaking the chain of transmission, regular exercise—whether solo or done in a safe socially distanced or masked manner—has the ability to strengthen people’s immune response. And most importantly, regular exercise combined with vaccination has the ability to take the effectiveness to the next level, which is vital in the fight against the ongoing scourge of this virus.

More than ever: Train hard, train healthy.