How to know if you can exercise after your COVID-19 vaccine or if you should just rest up

• No strong evidence shows how exercise right before or after the COVID-19 shot affects its efficacy.
• Staying active supports your immune system, but overtraining can deplete it.
• Listen to your body to gauge if you're feeling too low to work out.

The day after his second dose of the Pfizer and BioNTech COVID-19 vaccine, Dr. Kevin Bernstein did a long endurance ride on his Peloton - and achieved a personal best.

Hours after her second jab of the same vaccine, nurse Kristen Choi felt so chilled, nauseous, light-headed, and feverish she thought for a moment she had COVID-19.

As more Americans get both vaccinated and keep an eye toward their post-pandemic body goals, some are questioning if it's a good, bad, or neutral idea to work out right before or after the vaccine - if, unlike Choi, they feel up for it.

While there's no strong evidence showing that exercising around the time of your vaccine will increase or decrease its efficacy, Bernstein, the chief medical officer at Peaks Coaching Group, told Insider there are some things to keep in mind when planning your vaccines and fitness routines.

Don't force a workout if you're dragging, but try not to be too sedentary either

The available COVID-19 vaccines can come with side effects, which is simply a reflection of the immune system learning how to fight off the virus. Usually, symptoms are mild and last only a day or two.

Some of the the most commonly reported side effects include headaches, fatigue, chills, nausea, and dizziness, according to a January CDC report analyzing safety data on the nearly 13 million people who'd received a vaccine. In about 1% of people, the vaccine leads to a high (over 102 F) fever.

Women seem to be a lot more likely than men to report side effects, perhaps because estrogen elicits a stronger immune response. The second dose, too, tends to be more likely to elicit side effects, and some evidence suggests COVID-19 survivors may have more severe reactions.

"My advice to patients is to listen to your body," Bernstein said. "If you have significant fatigue and fever, then it is likely not the day to do a high-intensity interval training workout. If you're not feeling it, see how you feel again tomorrow."

However, try not to lie around the house all day either, he added. Even people who received a placebo vaccine in clinical trials reported side effects. "I would still recommend at least getting outside for a nice walk and continuing to stay hydrated while your immune system works its magic," Bernstein said.

Overtraining ahead of the vaccine isn't smart

The vaccine works by training your immune system to fight off invaders that look like the coronavirus, so it's in your best interest to receive it with a system that's in good shape. In other words, you want to put your best players in the game, not those on the injured list.

That's why some experts have advised against taking painkillers or drinking alcohol before your vaccine.

Exercise, however, is something that can both support and deplete your immune system. Regular physical activity keeps it healthy, but overdoing it can backfire, Bernstein - who rode his Peloton as usual the days leading up to both doses - pointed out.

He said if you are at risk for overtraining syndrome - when you out-train your body's ability to recover - or have significantly amped up your workouts, dial it back ahead of the vaccine. Worsened performance, being moody, feeling a compulsion toward exercise, sleep problems, and more can all signal overtraining syndrome, something that should be discussed with a general practitioner or sports medicine doc, Bernstein said.

If you're training hard because of an upcoming race, consider timing your vaccine accordingly, if possible. "I would not plan on getting the vaccine within a few days of a goal event, including planned training activities," Bernstein said.

Light arm exercises can help with injection site pain

The day of both doses, Bernstein did pullups, which alleviated his only symptom: a sore arm. While pullups may be too intense for people who don't already do them regularly, the Centers for Disease Control and Prevention does advise people keep their arms active to help deal with site pain. A cool, wet washcloth and over-the-counter painkiller may help too.

"I believe there is a mental health component to getting a vaccine or any type of medical intervention. Exercise is a potent positive mental health coping mechanism. I believe that continuing to keep moving before and after the vaccine is important both for physical and mental health benefits," Bernstein said.