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COVID-19 Vaccine: What Side Effects — or No Side Effects — Mean

Everyone's body responds to the COVID-19 vaccines differently.

By Leslie Nemo
Mar 23, 2021 11:00 PMMar 24, 2021 9:01 PM
man has a headache after his covid-19 vaccine - shutterstock
(Credit: lakshmiprasada S/Shutterstock)


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If you’re gearing up to get your COVID-19 vaccine, maybe keep your schedule light for the day after your second shot — or only shot, if you’re getting the Johnson and Johnson variety.

Some people feel muscle aches, pains or just flat-out exhaustion afterward, side effects that other vaccines prompt in people, too. “These are not unexpected,” says Carlos Malvestutto, an infectious disease physician at Ohio State University Wexner Medical Center. “We see them to a higher or lesser extent with pretty much every vaccine.” And while who feels what symptoms and when might surprise some people, the way the side effects are playing out now that millions are getting vaccinated lines up with what a lot of medical professionals expected to see.

How You Know the COVID-19 Vaccine Is Working

First things first: “That reaction is not having COVID-19,” Malvestutto says. The vaccines can’t bring you down with the infection — they only supply snippets of the coronavirus itself, and you’d need the entire virus to make you sick. Instead, any aches, arm tenderness or fatigue you might feel a day or two after the shot comes from your immune system doing what it’s supposed to: Learn how to fight off a real infection. 

In the Moderna and Pfizer vaccines, for example, the shot introduces into your cells pieces of mRNA, the instructions for your body to make a protein from the surface of a coronavirus. Once your cells start churning out the virus protein, your immune system recognizes the particles as something that doesn’t belong to you. Flagging the proteins down as undesirable kicks off a campaign to eliminate them, which can come with all the symptoms of an activated immune system — aches, sore muscles, and feeling rundown.

Why You Got Bad Side Effects and Grandma Didn't

Not everyone might feel the same side effects, or any at all. Malvestutto said that he, for example, felt some chills after getting his vaccine, while his wife, another physician, felt under-the-weather for longer. In some cases, older individuals have fewer side effects after the vaccine. This may sound counterintuitive — these same people are more likely to get seriously ill or die from the virus — but the lack of aches and pains proves why seniors were a priority for a vaccine, Malvestutto says. Feeling as if nothing happened after the injections could mean your immune system isn’t quite as easy to activate. You might not respond to the real virus with as much vigor, a situation that is more likely to be true for immune systems that have been working hard for, say, 70 years. That being said, someone doesn’t have to feel like they have the flu after the shot in order to develop good resistance to the actual virus. 

People receiving the two-shot vaccine might also notice that they only feel run down after the second one — another expected pattern, Malvestutto says. If the first injection tees the immune system up, the second shot is the follow-through swing that sends the ball flying, as the follow-up dose is when the body gets exposed to much more of the protein it now knows to attack. “The second shot comes into a primed body that recognizes [the protein] and goes to work,” Malvestutto says. The two-part act also explains why people who are exposed to the virus between injections — or within a couple weeks of the second dose — might get sick. It takes time for your immune system to build up enough defense tools after the second shot.

If someone feels the weight of the symptoms after the first dose, that could be a sign they were already infected with the virus, Malvestutto says. Studies have found that people who were previously sick with the coronavirus developed side effects and larger immune responses one dose in, meaning their immune system was already in the “teed up” phase. “They already have some antibodies, so half the work is already done,” Malvestutto says. The force of a single dose for those who already had COVID-19 has persuaded a few nations to consider dropping the second shot for previous patients.

The potential day or two spent wanting to stay in bed after the vaccine could be a small price to pay compared to coming down with the virus itself, Malvestutto says. Sure, a large percentage of people infected with the coronavirus never get symptoms. But some patients who had to get hospitalized for the virus —  for a long time, that number hovered around 3.5 percent of all cases — as well as those who had more mild infections, are still reporting shortness of breath, chest tightness and other symptoms months after first falling ill. Estimates suggest up to one in 10 people who get sick with COVID-19 become “long-haulers,” or people dealing with symptoms for weeks to months.

And if you still have questions about what the vaccines do inside your body, that's okay. "Everyone has a right to get these vaccines," Malvestutto says. "Don't be afraid to voice concerns, and don’t keep concerns to yourself and quietly decide the vaccine isn't for you."

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