COVID-19 Vaccine FAQ: Your Questions Answered

arm swelling covid 19 vaccine
Emma Yasinski
Emma Yasinski Staff Writer
Last updated:

The FDA has now authorized several COVID-19 vaccines. Both of the first two authorized vaccines rely on an immunity-building strategy that hasn’t been used in any other vaccine. Here are answers to some of the most frequently asked questions.

What is mRNA? Why does it matter?

All previous vaccines have been based on weakened virus molecules or proteins from the disease (pathogen) that our immune systems can easily overcome while learning to recognize it. The new vaccines from Moderna and Pfizer — the first two to make it to market so far in the U.S. — contain mRNA (messenger RNA), which is a genetic template that instructs our cells to build the viral proteins that our immune systems can then recognize. Its main perk is that by allowing our bodies to produce the proteins (rather than growing them in a lab like in traditional vaccines), it slashes the time it takes to produce the vaccine (as done in the past) in the lab. For nearly 20 years, researchers have been interested in using mRNA in vaccines. Some were even tested in early clinical trials for rabies, influenza and Zika. However, the vaccines for COVID-19 are the first mRNA vaccines ever authorized by the FDA. This long history of studying the mRNA concept is why the vaccine could be developed so quickly. mRNA is new to you and me, but it is well-known to scientists.

Will the COVID-19 vaccine change my DNA?

mRNA is a messenger in your body. It’s the way your DNA tells the rest of your body what proteins to create for various functions. In your body, mRNA is created in the nucleus of your cells, based on the DNA that resides there. Then the mRNA exits the nucleus to be used as a template for protein-making in the rest of the cell, and it has no way back in. Soon, it degrades without ever re-entering the nucleus or coming in contact with DNA. mRNA lives in the body between a few hours and a couple of days.

Like naturally occurring mRNA that has already left the nucleus of your cells, the mRNA vaccine isn’t able to enter your cell’s nucleus. Because of that process, it can’t come in contact with or change your DNA. Click here to see  a video that explains the process more fully.

The Johnson & Johnson vaccine uses a viral vector — a harmless virus-turned-delivery system that’s been used in vaccines before — to introduce the gene that encodes a protein found on the surface of the COVID-19 virus. Our cells then create just this protein (not the whole COVID-19 virus) so our immune systems learn to recognize it. It also will not change your DNA.

Why do I need multiple shots of the Pfizer or Moderna vaccines?

Multiple doses of the vaccine expose our immune systems to more of the viral proteins, giving it the chance to develop more antibodies and protection. The immunity takes time to develop. Researchers believe that 10 days after the first dose, patients developed immunity that is about 52% effective. About a month after receiving their second dose, immunity reached 94% against the original strain of SARS-CoV-2. A similar strategy is used for other vaccines, like Shingrix, developed to prevent shingles.

Since the initial assessment, new analysis of the research suggests that two weeks after the injection, a single dose of Pfizer or Moderna’s vaccines is more protective than previously thought, potentially up to 92% against the original strain of SARS-CoV-2. However, over time and as the virus mutated, that immunity has begun to wane.

Third shots were first authorized for the subset of patients who are immunocompromised due to organ transplants or high doses of corticosteroids. That’s because many of these patients didn’t mount a full-fledged response to the first two shots to begin with, but some studies have shown they respond well to a third. Now, however, as researchers are finding evidence of immunity waning over time, they are recommending third shots all of the adult population about six months after the second shot of Moderna or Pfizer, and two months after Johnson and Johnson. For older adults, the FDA recommended a fourth shot, and at the end of August 2022, the agency issued an emergency use authorization for new booster shots targeted specifically toward the omicron variants.

April 2023 update: The FDA authorized the omicron-specific (bivalent) booster shots to be used for all shots. The original Moderna and Pfizer vaccines are no longer authorized. People who have not received the bivalent shot yet, adults over 65 who received the bivalent booster more than four months ago, and those who are immunocompromised and received a bivalent shot more than two months ago are eligible for a second bivalent shot.

How does the FDA decide if a side effect is severe?

The FDA has a toxicity grading scale to rate adverse events related to preventive vaccines. It ranks both local events (like injection site pain) and systemic events (like fever and nausea) as mild (Grade 1), moderate (Grade 2), severe (Grade 3) or life- threatening (Grade 4.) Several effects, like pain at the injection site, headache, fatigue and nausea, are considered Grade 1 if they do not interfere with your activity at all. If side effects do prevent you from doing ordinary tasks, they may be designated Grades 2 or 3. If the symptoms cause you to go to the emergency room for treatment, they are Grade 4.

Is the COVID-19 vaccine safe?

The most common side effects for all three vaccines — headache, fatigue, fever, chills and muscle and joint pain — are more common and intense than those for the flu shot, for example. And the side effects caused by the COVID-19 vaccine tend to be worse after the second shot than the first.

The FDA reported that up to 4.6% of Pfizer vaccine patients, 9.6% of Moderna’s patients, and 2% of Johnson & Johnson’s patients experienced severe (Grade 3) adverse effects. Grade 3 effects are those serious enough to interrupt a person’s life — for example, by causing them to take a day off of work — but not so severe that the person is hospitalized. Younger patients were more likely than older ones to experience severe adverse effects. No life-threatening effects were linked to the Pfizer vaccine and less than 0.1% of patients experience Grade 4 adverse effects to Moderna’s vaccine. Swelling, pain, rashes and redness at the injection site are common.

While some patients have experienced anaphylaxis, all have recovered.

The vaccine will need to be tested on millions of more people for scientists to understand how the vaccine may interact with certain chronic illnesses or other drugs.

Check out MedShadow’s vaccine side-effects tracker for more details and frequent updates on side effects. You can also check out MedShadow’s COVID-19 topic page to find more in-depth reporting on specific findings, like what doctors say about the risk of myocarditis for kids.

How long were test patients followed and why?

The FDA required that patients in the Phase 3 trials be followed for a median of two months after the second shot before the data would be reviewed for its approval.

When it comes to side effects, the CDC considers vaccinations different from other medicines, because they aren’t taken continuously. In general, the CDC designates adverse events that happen within six weeks of administration to be potentially linked to vaccines. In the case of the Pfizer and Moderna vaccines, the mRNA is broken down within days of the injection, so it doesn’t remain in your body long term.

The CDC is tracking possible adverse reactions in early recipients through its V-safe app. The agency initially announced that it would follow these patients for six weeks but has since said it will check in on them at three months, six months and one year after receiving the vaccine. Individuals can also report adverse effects through VAERS, the government’s vaccine adverse event reporting system.

Who has been tested for the vaccine?

The Pfizer and Moderna vaccines have been authorized for adults and children six months and older.

Originally, Moderna’s and Johnson & Johnson’s vaccines had the okay for patients 18 and older. Because Pfizer’s original trial included a small number (163) of 16- and 17-year-olds it was recommended for patients 16 and older. On March 31, 2021 Pfizer announced that the vaccine was safe and effective for patients 12-15 years old, and it received emergency use authorization for these patients shortly after. In October 2021, the FDA authorized the Pfizer vaccine for use in children age 5-11. In younger patients, Moderna’s vaccine had been associated with a heightened risk for myocarditis, and regulators took extra time to review that data. Both Pfizer and Moderna’s vaccines were authorized for all ages above 6 months in June 2022.

As of April 2023, the bivalent shots were also authorized for everyone over the age of six months who hasn’t had a bivalent booster yet. Second bivalent shots were authorized for people who are immunocompromised or over the age of 65.

What do we know about how the vaccines work in kids and teens?

Pfizer reported that common adverse effects like chills and fever were more common in 12-15 year olds group than adults. Adverse reactions in adolescents 12 through 15 years of age included pain at the injection site (90.5%), fatigue (77.5%), headache (75.5%), chills (49.2%), muscle pain (42.2%), fever (24.3%), joint pain (20.2%), injection site swelling (9.2%), injection site redness (8.6%), lymphadenopathy (0.8%), and nausea (0.4%). These effects generally subsided within 1-2 days. While data on syncope (fainting) was not included, the announcement mentioned that fainting after vaccination is more common in teens than adults, and that safety measures should be in place to reduce this risk.

Both Pfizer and Moderna’s shots have been authorized for all patients over the age of 6 months. However, each company took a different approach to dosing the youngest patients (those under 5 years old). Pfizer’s early trials showed that the little ones experienced a high number of side effects like fevers at a 10ul dose, so the company tested a smaller, 3ul dose. Unfortunately, two shots at that dose didn’t elicit a strong enough immune response, so the company ended up adding a third shot to the regimen. That seems to have paid off and the 3-shot course was authorized. Moderna used a higher dose and was able to inoculate the children with two 25ul doses.

As of April 2023, the FDA has authorized the bivalent booster for everyone over the age of six months, rather than the original shots described above.

Is it safe for pregnant women? Will it protect the baby?

None of the initial trials included pregnant or lactating women, but since then, several studies have found positive results. Simultaneously, researchers have found that pregnant patients who contract COVID-19 are at high risk for severe complications, preterm birth, and mortality.

The CDC published an analysis of data on from the first few months of voluntary V-safe surveys on April 22 that suggested that the Moderna and Pfizer vaccines are safe for pregnant women. Pregnant patients self-reported slightly fewer side effects like fever, headache and chills than the general population, but more said they experienced nausea and injection site pain. Rates of adverse pregnancy outcomes such as pre-term birth were similar to rates seen before the pandemic. While promising, officials noted that more studies will be required.

Two more studies published in May found that the vaccine does not damage the placenta and that pregnant women developed sufficient immune responses. Antibodies were passed to their babies through blood and breast milk.

An August 2021 study found the vaccines led to antibodies being transferred to the fetus through cord blood.

A study published in October 2021 in the New England Journal of Medicine found that vaccination was not associated with an increased risk of spontaneous abortion.

Is the vaccine protective for everyone?

Recent studies have suggested that in some patients who take strong immunosuppressants for organ transplants, the vaccine may offer little to no protection. After the second shot, only about half of patients who had received an organ transplant developed sufficient antibody responses. It’s unclear how effective the vaccine will be for other groups who are immunocompromised. Catherine Liu, MD, a vaccine and infectious disease researcher at the Fred Hutchinson Cancer Research Center, suggested this is one reason it’s especially important for healthy people who spend time with patients who have cancer or take immunosuppressants (family members and caregivers, for example) to be vaccinated themselves.

Will the COVID-19 vaccine protect me from mutated versions of the virus?

Researchers have identified several mutated forms of the virus spreading in the UK, South Africa, Brazil, India and elsewhere. While some variants have been associated with a higher portion of breakthrough infections–infections in people who have been fully vaccinated–these patients usually did not end up seriously ill or hospitalized. Early on, the vaccines seemed to provide substantial protection from these variants, even if that protection was slightly lower than it is for the original SARS-CoV-2 virus. Still, over time and as the virus mutatesd the protection we got from the vaccines waned, and companies developed bivalent (or omicron-specific booster shots) that offered greater protection against more variants.

April 2023 update: The FDA authorized the omicron-specific (bivalent) booster shots to be used for all shots. The original Moderna and Pfizer vaccines are no longer authorized. People who have not received the bivalent shot yet, adults over 65 who have received one more than four months ago, and those who are immunocompromised and received a bivalent shot more than two months ago are eligible for a second bivalent shot.

Ok, but what about the Delta and Delta Plus Mutations?

Update: Delta and Delta plus are no longer widely circulating. The majority of disease is currently being caused by Omicron variants.

The Delta variant is believed to be far more contagious than previous forms of the virus that causes COVID-19. Researchers have suggested it may be more likely to send you to the hospital as well. So far, scientists say that the Pfizer, Moderna and Astrazeneca vaccines are still likely to protect you from the variant—provided you’ve received both doses and two weeks has passed since that second dose.

While a single dose provided pretty high protection from previous variants, its only generates about 33% efficacy against the Delta variant. After both doses, however, a recent Lancet study suggested that Pfizer’s vaccine is 79% effective against Delta, and Astrazeneca’s is 60%. Protection against hospitalization with Delta was higher, at 96% and 92% for Pfizer and Astrazeneca’s vaccines, respectively. Many studies report different numbers because real-life studies lack the controls of clinical trials, but scientists emphasize that the vaccines do seem to provide protection, especially from hospitalization.

“Their overall implications are consistent: that protection against severe disease remains very high,” said Naor Bar-Zeev, an associate professor at the Johns Hopkins Bloomberg School of Public Health told the New York Times.

Moderna released results of a small lab study that suggests its vaccine is still effective against the variant. Johnson and Johnson also released data from small studies suggesting its vaccine is effective against Delta.

In the meantime, social distancing and face masks can still help protect you against Delta and any other variants.

Now what about Omicron?

Omicron has been the dominant viral strain for most of 2022 and 2023. There are several subvariants. Researchers have found that the omicron variant is much more contagious than previous variants, and is able to evade vaccine-induced immunity, though vaccinated people still had high protection from severe disease and hospitalization.

Not only has the variant been able to evade protection from vaccines, but it is also able to evade protection built in response to a prior infection. Scientists are reporting high levels of re-infections over the course of this year. Each re-infection raises the risk of severe outcomes.

April 2023 update: The FDA authorized the omicron-specific (bivalent) booster shots to be used for all shots. The original Moderna and Pfizer vaccines are no longer authorized. People who have not received the bivalent shot yet, adults over 65 who have received one more than four months ago, and those who are immunocompromised and received a bivalent shot more than two months ago are eligible for a second bivalent shot.

What is the Omicron-specific booster shot? Do I need it?

Moderna and Pfizer have developed bivalent vaccines that they hope will better protect against Omicron and future variants. A bivalent vaccine is made to elicit an immune response to both the original strain of SARS-CoV-2 and the newer Omicron strain. The FDA authorized the vaccines on August 31, 2022. Pfizer’s is authorized for people 12 and older, while Moderna’s is authorized for adults 18 and older. Either shot should be given at least two months after your most recent shot or infection.

The booster shot is a lower dose than previous shots and meant to be used only as a booster. Because they needed to be developed quickly and they’re based on the old vaccines, they weren’t tested in large human clinical trials like the previous shots. Instead, they were updated more like flu vaccines: the scientists determined the strain that’s circulating, and developed shots that would help neutralize that variant (Omicron BA.5, which is causing the vast majority of cases in the U.S.). They tested the shots in petri dishes and animal models. In those studies, the new vaccines seemed to elicit broad immune responses against all Omicron sub variants. The companies did test an earlier version of the shots, targeting Omicron BA.1 in some humans and found that they stimulated an antibody response that could neutralize Omicron.

April 2023 update: The FDA authorized the omicron-specific (bivalent) booster shots to be used for all shots. The original Moderna and Pfizer vaccines are no longer authorized. People who have not received the bivalent shot yet, adults over 65 who have received one more than four months ago, and those who are immunocompromised and received a bivalent shot more than two months ago are eligible for a second bivalent shot.

Does the Bivalent Omicron Booster Shot Cause Side Effects?

Because the shots weren’t tested in large clinical trials, it is hard to tell. However, in the data that’s available from smaller trials of the shot that targeted Omicron BA.1 (the shot that was authorized targets BA.5), side effects were similar to those seen with the original Moderna and Pfizer vaccines. Based on the little data we currently have, Andrew Pekosz, PhD, a virologist, and co-director of Johns Hopkins Center of Excellence for Influenza Research and Response, says, “the side effects from the bivalent booster were very similar to what we saw with the regular booster and even going back to the initial vaccination. So nothing different in terms of the side effects are being reported. Most often it’s redness at the site of inoculation, some soreness feeling tired for a day or two on after words. All the same side effects were seen at relatively the same rates with the bivalent booster.”

When Should I Get the Bivalent Booster Shot?

Regulators recommend waiting at least two months after your last shot or infection to get the bivalent booster shot. However, researchers say that there may be reasons to wait a few more months. “Boosters are most effective when there’s some time-space between your initial either vaccination or infection and the time that the boosters are given,” says Pekosz, the Johns Hopkins virologist. “So two to three months is a good starting point. Many immunologists would say that even extending that out to as much as six months would stimulate those.”

However, it’s also important to consider your own risk factors and schedule. In some cases, it may make sense for you to get the booster only two months after your last shot. For example, if you’re going to be traveling or spending time in high-risk environments. It may be helpful to get the shot in advance if you have plans to see vulnerable family members.

April 2023 update: The FDA authorized the omicron-specific (bivalent) booster shots to be used for all shots. The original Moderna and Pfizer vaccines are no longer authorized. People who have not received the bivalent shot yet, adults over 65 who have received one more than four months ago, and those who are immunocompromised and received a bivalent shot more than two months ago are eligible for a second bivalent shot.

Can I Throw Away My Masks After I Get the Vaccine?

Early on, the vaccines seemed to protect against most disease transmission, but now that the virus has mutated, vaccinated people are can contract and spread the infection. The CDC has repeatedly shifted its guidance on mask-wearing, but the bottom line is that while vaccines can prevent you from severe illness requiring hospitalization, a mask—especially a high quality N95—can help provide an extra layer of protection against infection and spreading the disease to others.

Can I Take Monoclonal Antibodies Instead of the Vaccine?

On December 8, 2021, the FDA authorized Astrazeneca’s monoclonal antibody treatment as a vaccine alternative for those 12 or older with compromised immune systems who might not respond to the vaccine or who have a history of severe vaccine reactions. Up until now, monoclonal antibodies have been given to patients who contract COVID-19 to prevent them from getting severely ill, but have not been used in uninfected patients for prevention.

The monoclonal antibodies are given in two injections (much like the vaccines) and are thought to decrease the chances of symptomatic COVID-10 by 82% for six months to a year.

The main difference between the treatment and the vaccine is that the vaccine teaches your body how to create antibodies, and leaves B and T cells behind that can ramp up defenses again if need be, whereas monoclonal antibodies will only provide your body with temporary antibodies, it does not teach your body how to make its own.

Unfortunately, monoclonal antibodies are no longer protective against current strains of COVID.


Got more questions you want MedShadow to answer? Contact us.

DISCLAIMER: MedShadow provides information and resources related to medications, their effects, and potential side effects. However, it is important to note that we are not a substitute for professional medical advice, diagnosis, or treatment. The content on our site is intended for educational and informational purposes only. Individuals dealing with medical conditions or symptoms should seek guidance from a licensed healthcare professional, such as a physician or pharmacist, who can provide personalized medical advice tailored to their specific circumstances.

While we strive to ensure the accuracy and reliability of the information presented on MedShadow, we cannot guarantee its completeness or suitability for any particular individual's medical needs. Therefore, we strongly encourage users to consult with qualified healthcare professionals regarding any health-related concerns or decisions. By accessing and using MedShadow, you acknowledge and agree that the information provided on the site is not a substitute for professional medical advice and that you should always consult with a qualified healthcare provider for any medical concerns.

Was This Article Helpful?

Staff Writer
Show Comments (24)
0 0 votes
Article Rating
Notify of
Most Voted
Newest Oldest
Inline Feedbacks
View all comments
Arturo Kelly Sr

Thank you for your undue diligence in making sure that we know as much as we can thanks!

Gary Johnson

What are the risks for people who have a history of ITP Blood disorders?


Should I receive second Modena vaccine if I had an immediate allergic reaction? I have received multiple answers from dr, pharmacists and Health District. I had a tingling in head, tongue and heaviness in entire body. I was given Benedryl and tingling stopped within 7 or so minutes. Heaviness continued and I slept about 4 hours upon returning home.


I received the first dose of Moderna. I had fever and chills that lasted a day or two but I was warned about those, no big deal. But my arm where I received the shot hurts so bad it feels like it’s about to fall off and it’s been almost two weeks. I’m not sure if this is something to be concerned about and if I should report this or who I should report this too. This isn’t a minor pain, it hurts like heck.


i heard the side effect is more severe when you receive the 2nd shot. I also heard advices like taking ibuprofen ahead of the 2nd shot may lessen the side effect. Any truth to that? any other advices?

Sallyann Wickham

Just wanting to report for statistics.
Dose 1
No pain at all at injection
2 hours post- very sore arm and fatigue
Overnight fatigue, painful muscles and joints. Restless sleep,
Next day – severe headache lessening joint and muscle pain
Previous conditions
Typ1 diabetic for 35 years cured by transplant
Kidney pancreas transplant Dec 21, 2021.
Anti-rejection meds and other post transplant drugs and supplements.

Anne avella

I received my second Phizer vaccine on February 17. About a week later I started to get these very very tiny red dots on my body. They last about 4to5 days and then they turn brown. Could this be a reaction from the Covid vaccine. I went to a dermatologist and they can’t give me an answer as to what it is. I hope you can answer my question and put my mind at ease. Today isMarch 11 and I am still getting the red ducks but less. Thank you and I hope I can get an answer

Karene W

Will physer vaccine cause mouth sores? Am experiencing something strange happening in my mouth.

Barbara Hoberman

I know this sounds bizarre, but I had my second shot of Phizer vaccine today and I was injected well below the deltoid. Will I still be immunized effectively?

caramia visick

My resting heart rate has changed after the first vaccine of Pfizer. Usually 65-68, now it ranges 78-109. This is the third week since first dose. I get the second in a week. I feel my heart pounding louder at night when I first lay down. I wear a Fitbit, so it’s noticeable to track resting heart rate.


I had almost every possible side effect. about 8 hours after my second shot I had a terrible headache like someone hit me with a hammer. It got so bad it was like I had the drunk spins. Threw up a few times Had the chills so bad that I imagine what going thru drug withdrawls much feel like. Terrible body aches. Shortness of breath. Took my pulse / ox and my oxygen level was mid 80’s. It should be over 95. Felt the the same way all the next day. Got a fever, still had chills, dizzy and vomiting. Headache and slight fever also. Next day I got diarrhea for 2 days. Still achy, then 5 days after vaccine, I got “Covid arm” a hot red rash on the arm I h ad the shot on. Had that for a few days. Still hardly any energy. Luckily my husband took good care of me. Stayed on “the brat diet” all week. No appetite Took about 10 days to get back to about 90%. THEN, OVER 3 WEEKS PAST THE SECOND VACCINE, I had lymph nodes swell up in both my calves. Felt like I was very bruised! Went to the doctor and they said they should pass. The swollen nodes moved to both my arms, then my groin area and still there. I am a bit concerned about what this vaccine is doing to my body.


PFIZER . First shot no side effects at all. Second shot, fatigue, very sore arm-could not lift it. Pain came on gradually until it woke me up in the middle of the night. Loose bowels. Stiff legs. Lasted about 24 hrs. Starting to feel like I’m coming out of it but not sure yet on third day.


Received 1st dose Pfizer… nothing at all for side effects. Today received 2nd and my arm hurts soooooooo bad I’m in tears. It doesn’t hurt at injection site but bicep kills, near top side of arm/shoulder.. feels like my arm is in a vice grip!!!! OMG … God help me!


got me second shot and developed red dots on my back and pimples on my neck. Is this a normal reaction

J Slavin

The red dots may be petechiae. It may be a good idea to see a hematologist and/or PCP and get a complete blood count panel to test your platelet levels. Blessings and healing to you.

Heather Stewart

I’ve had my first Pfizer vaccine. No sore arm at all, but headache and one week after vaccine chills and fatigue. Feels like a cold or allergies. Seems a long time before I had any side effects.

M. Manning Mrs

I had what I thought was my second jab only to be told the needle was empty, but was told they put me down as having had it ? Its been a month now I’ve had blood tests and it says no I didn’t have it so I have to have jab again is this correct,


How long does the shedding occur after this shot????


I’m wondering if anyone has had this reaction to the Pfizer vaccine? Myself, husband & 12 year old daughter recieved our 1st Pfizer vaccine on Saturday 7/16/21. I had headaches the next day 7/17, daughter had tiredness & hubby had no symptoms until Monday 7/19. High fever, chills, feeling very tired. By Wednesday 7/21 hubby’s fever wouldn’t break (we were going to make Dr appt) but that afternoon out of no where within hours (late a.m. to 2:30pm) he developed major swelling & huge bright red rash on right leg from ankle up to mid leg to groin. In addition to the leg, he was lethargic, chest pain & felt like passing out.
I took him to ER & he was immediately admitted. He had elevated blood pressure, high fever. He was admitted & pumped with antibiotics, & fluids. He was there 5 days, discharged Sunday 7/25. ER did a slew of tests- blood work galore, CT & MRi of leg, Sonogram of leg. The Dr’s including infectious disease Dr said it was cellulitis that was leading to sepsis. Dr’s would not say it was vaccine related but they also wouldn’t rule it out. Hubby had no bites,scrapes or no trauma to that leg to explain this diagnosis. Hubby has never had skin problems nor ever had cellulitis. Coincidence? 4 days after vaccine the leg gets this out of no where? Has anyone heard of this or had this type of reaction? I’ve read that rash has been reported at injection area but not this. Plz reply if you have any info on this

Would love your thoughts, please comment.x