Doctors are cep purnugetting questions about the possibility of short-lived, though sometimes uncomfortable, side effects after the second Covid shot.

Mashable spoke with some of these doctors. They emphasize that the vaccines have continually proven safe, and any common side effects — like tiredness, a sore shoulder, fever, or aches — from the second doses of the Pfizer-BioNTech or Moderna vaccines are normal and relatively brief. The single-dose shot, from Johnson and Johnson, can have similar effects, too, though there's evidence the side effects are less common. Overall, these temporary symptoms are your immune system in action as it responds to the vaccine. (None of the FDA-authorized vaccines contain the actual coronavirus, just harmless genetic information about the virus that codes for only a small piece of the virus.)

"These symptoms are expected," emphasized Dr. Thomas Russo, the chief of infectious disease at the University of Buffalo's Jacobs School of Medicine and Biomedical Sciences. "It's a signal your immune system is working and you are responding to the vaccine."

Crucially, some people don't have symptoms after getting the second shot of the two-dose Pfizer-BioNTech or Moderna vaccines. But this certainly doesn't mean the vaccine is less effective in your body. "There's no evidence of that," explained Dr. Peter Gulick, a D.O. and professor of medicine at Michigan State University. Rather, different bodies respond to the vaccine differently, just as many of us respond to flu viruses, cold viruses, or bee stings in diverse ways. Some of us have these short-lived symptoms. Some don't. That's OK.

"We all react to everything differently," said Dr. Gulick. "That's just the way our immune systems are."

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The side effects

In addition to the possibility of soreness, swelling, or redness where you get the shot, the following potential side effects are common for some after receiving a Covid vaccine, according to the CDC:

• Tiredness

• Headache

• Muscle pain

• Chills

• Fever

• Nausea

Importantly, these side effects are temporary and go away within a day or 48 hours or so, said Dr. Russo. These are not the severe reactionsthat might require immediate medical treatment, like a serious allergic reaction (anaphylaxis). This sort of extreme reaction is quite rare, happening on the order of just one to five per a million people, said Dr. Gulick. That's expected. An extreme minority of people have a severe reaction to certain things, be it peanuts or penicillin or a vaccine.

The rare serious allergic reactions to a Covid vaccine are not unusual compared to other vaccines. "These Covid-19 vaccines have no red flags, so to speak," said Gulick. (To be safe, the health care professionals administering the vaccines ask you to wait 15 minutes post-vaccination to ensure you don't have this type of severe reaction, which they're prepared to treat.)

Yet, expect the possibility of milder side effects after the second Covid dose. Over half of people may experience some mild symptoms, explained Dr. Vin Gupta, a pulmonary and critical care physician. "We think it’s more likely than not after the second dose," explained Dr. Gupta, who is also an affiliate assistant professor of health metrics sciences at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington. 

Why are side effects common after the second dose?

The two-shot vaccines introduce a piece of harmless genetic code (called mRNA) into our bodies, which instructs our cells to make just the part of the coronavirus (the infamous spike protein) that binds with our cells and infects us. This allows our immune system to prepare defenses (called antibodies) against the virus so it can't attach, among building other cellular defenses. "You're getting parts of the virus to protect you from an infection," explained Dr. Russo.

By the second dose, our bodies have already seen this critical piece of the virus, and are now extra-ready to respond to the intruder. "The first dose gets that immune system primed," said Dr. Gulick. So when the second vaccine dose (which acts to turbo-charge our immune system against the coronavirus) comes, the immune system reacts strongly to the perceived threat, and produces many more antibodies. As a consequence, we can experience some common effects of the human body's typical immune response to an intruder, like a fever or aches. "It's part of the body's response to an infection," said Dr. Russo, though you're not actually getting an infection, he added.

So if you do have side effects, it's normal and temporary. "If you have those symptoms, it's reassuring your immune system has gone to work," explained Dr. Russo. But if you don't have symptoms, that doesn't mean your immune system isn't working. "Our immune response [to the Covid vaccines] is good whether you have these symptoms or not," Dr. Russo added.

Vaccine day

Some of us may be uncomfortable following the second dose with the likes of a headache or aches. That's why it's prudent to prepare for that possibility, and take the day off work if you need.

"If you can take the day off after the second dose, do so," recommended Dr. Gupta.

Think of it similar to a "snow day," said Dr. Russo. "You get a blanket and some Tylenol if you need, you do some binge-watching. But only a minority of people will actually need a vaccine day," he added.

[Ask your doctor about what over-the-counter medications you might consider taking if you have unpleasant post-vaccination symptoms.]

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The possibility of relatively mild, temporary symptoms are a small price to pay for the protection the vaccines provide. All three FDA-authorized Covid vaccines, including the single-dose Johnson & Johnson vaccine, are excellent vaccine options. They protect well against symptoms overall, and most importantly, have all proven in clinical trials (involving tens of thousands of people) to protect against severe Covid disease that results in hospitalization and death. "They keep you out of the hospital," said Dr. Gupta.

So don't be hesitant to get a Covid vaccine or the second dose, emphasized Dr. Russo. The vaccines are safe and build proven immunity.

"The vaccines are protecting us," said Dr. Gulick.

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