Top 10 Things You Might Not Know about the Moderna COVID Vaccine

This week, I had the privilege of “jabbing” several hundred grateful Austinites’ arms with the second dose of Moderna’s COVID vaccine at one of Austin Public Health’s vaccination PODs, which gave me the unique opportunity to get extensive direct feedback about people’s experience with their first shot. Note that the folks I immunized had to fall into categories 1A or 1B to qualify for their first vaccine a month ago, which means that they were either older than 65 (many in their 70s and 80s), any age front-line healthcare workers, or people ages 16-65 who also have at least one significant risk factor such as obesity, diabetes, chronic kidney disease, heart or lung disease, cancer, or pregnancy. Although age was skewed to the higher end, our patients otherwise had tremendous diversity in size, shape, disease burden, race, culture and socioeconomic status, so it feels like a solid cross-sample. I’d like to share some practical information about what you can expect, while answering the top ten COVID vaccine questions.

1. Do I really think the vaccines are safe? YES, yes, I do! Please see my previous post for more details and myth-busting, but the short version is that these vaccines went through all the standard testing, included far more participants than normal, and frankly, look how every doctor you know rushed to get their shots as soon as they were available! Most of us have completed our second dose, and those that haven’t been able to get their shots yet are chomping at the bit.

2. Do the mRNA vaccines alter your DNA? Nope, absolutely NOT. The microscopic mRNA is delivered into the cytoplasm of our cells, but it CANNOT even enter the nucleus of the cell, which is where the DNA lives. Our ribosomes (cellular protein factories) read the mRNA, make the spike protein (which lets our immune system “recognize” COVID in the future) and then the few copies of the vaccine mRNA are destroyed, just as our bodies destroy the many thousands of naturally occurring mRNAs inside our cells.

3. What should I expect with my first dose of Moderna? GOOD NEWS: The vast majority of people have little more than a sore arm for a couple days after the first shot, which is what I’d estimate 90% of people told me this week. Looking at Moderna’s trials, they saw that roughly a third of people in the study also notice some fatigue, low grade fever or a headache, while 23% reported muscle aches, 16% had joint pain, and 9% had nausea. Some amount of arm pain is pretty universal- roughly 87%-90% of people reported local injection site pain after the 1st and 2nd doses respectively, but only 3% and 5% were severe enough to be “grade 3”, meaning pain was enough to interfere with their day or require prescription pain medicine. However, many people did take over-the-counter pain relievers like Tylenol or Advil for pain or fever: 23% after the first dose, and over double that number- 57%- after the second dose. (Note that roughly 20% of people who received a placebo injection also reported pain, though only 0.2% reported the more severe pain.)

4. What other local symptoms might I develop? There are two things to note:

a. SWOLLEN GLANDS: About 11-16% of people in the trials developed some degree of lymph node swelling or tenderness in their armpit on the side of the shot, though only 0.3-0.4% were the grade 3 level defined above. This is not surprising- think about swollen neck glands when you have strep throat- we expect some degree of reaction to any local activation of our immune system. However, in women, these swollen lymph nodes often create worry about a breast issue, and in fact, they can show up as a possible concern for cancer when seen on mammograms. Therefore, to avoid this confusion, we now recommend that women who are due for a preventative health screening mammogram should ideally schedule that exam BEFORE starting the COVID vaccine series, or alternatively, wait 4-6 weeks after completing the vaccines. Please note that if you or your physician feel a lump in your actual breast tissue (not the armpit), that problem should be promptly evaluated.

b. “COVID ARM”: 5-9 days AFTER the vaccine, a small number of people (more women than men) will develop a relatively large red skin lesion on their shoulder that lasts for a few days. This delayed immune reaction at the injection site is typically a bit tender to touch, slightly firm, and roughly the size of the palm of your hand. It may be extremely itchy or not at all. Rest assured that this reaction is NOT dangerous, nor is it a reason to cancel your second dose. In fact, most who develop this rash after the first injection do not have it after the second dose. Treatment is symptomatic, primarily using topical cool compresses if needed. This hypersensitivity skin finding was noted in about 1.5% of vaccine study participants, and interestingly, also in 1% of placebo recipients. I wonder, however, if it may actually be a bit more common than reported. I’d estimate that at least 5% of the patients I vaccinated this week asked me about these delayed “COVID arm” marks when I broadly inquired how they did with their first shot, though most people were more curious than concerned about it (because it didn’t bother them much and it went away rather quickly.) Additionally, many affected people noted that they had other female family members with the same reaction.

5. What should I expect after the 2nd Dose of Moderna? Although certainly there are plenty of people who happily sail through the second vaccine dose with minimal or no symptoms (especially in some of our elderly patients), here’s the scoop:
a. You can likely count on a whopper sore arm, starting on day 1, the evening of your shot. Move it around as much as you can, but sleeping on that side will likely be a challenge, and the next day, you might be so sore that it’s hard to fully move your arm (like pulling a shirt over your head.)
b. Bonus features, IF included in your personal immune response, typically start the day after your shot (day 2) and include fever, chills, headache (may be wicked), body aches, joint pain and nausea or diarrhea. Happily, this typically lasts less than 24 hours, and many people wake up day 3 feeling completely back to normal.
c. But wait, there’s more! Once again, for a small percentage of people, there CAN be swelling in your local lymph nodes, so if you have tenderness or feel a lump develop in your armpit or just above your collar bone on the side of the shot, kudos to your robust immune system! Ditto if you develop COVID ARM.
d. Finally, we have the BEST side effect…that warm sensation bubbling up deep inside you is HOPE and JOY, knowing you are part of the solution and that these amazing vaccines are giving you the super power to stay OUT of the hospital, even if you are part of that small 5% who may still get infected with COVID.

6. Should I take Tylenol or Advil/Motrin/Aleve before I get my shot? What about afterwards? I’m sure you’ve heard mixed messages about this issue, because frankly the scientific evidence is not there yet for a full answer. Here is the THEORY though: Non-steroid Anti-Inflammatory Drugs (NSAIDs) like ibuprofen (Advil/Motrin) and Naprosyn (Aleve) do indeed decrease inflammation- that’s how they work. Acetaminophen (Tylenol) lowers fevers, and fevers are also part of the inflammatory response. What we actually WANT with a vaccine is to activate our immune system, producing an inflammatory response, so the concern is that if you pre-medicate before your vaccine with any of these medications, you could potentially lower the body’s response and subsequently end up with less immunity from the vaccine. However, there is not clear evidence that this is true, because the few studies we have from other vaccines- mostly with childhood immunizations and using or not using Tylenol- have conflicting evidence.

Here is my advice, based on what our very smart infectious disease and immunology experts are saying:

a. Take NOTHING “proactively” before your shot (meaning take no OTC pain relievers like Tylenol, Advil, Motrin, Aleve, etc.) Please DO take your prescription medications and eat/drink normally.
b. WAIT until you are having significant symptoms (if you even get them) before taking anything, then start with two extra-strength Tylenol (which reduces pain and fever, but not inflammation.) If, after an hour, the Tylenol doesn’t dull your symptoms to a tolerable level, then add in one or two tablets of your NSAID of choice (Advil, etc). Know that Tylenol and NSAIDs work better together than either separately, so please, there is no need to take 3 or 4 Advil if you are adding that into the mix.
c. Is it better to grin and bear it, not taking anything for fever, headache, etc.? Some physicians are still giving this advice, but I don’t believe that is essential. There is no evidence that long term immunity is adversely affected by taking over-the-counter medicines to ease pain or fever that develop AFTER a vaccine is given, and we know that the fact that you are HAVING symptoms like high fever or body aches indicates your immune system is already reacting to that shot. My advice is don’t take anything pro-actively, but there is no need to truly suffer if you end up with intense symptoms.
i. Full disclosure: Day 2 after my second Moderna vaccine, my always-overly robust immune system bestowed upon me a persistent 101* fever, chills, muscle aches, and fatigue, which were not fun but tolerable. Later in the day, however, after dealing with an incredibly intense migraine-style headache for many hours, I ultimately took two Tylenol and one Advil, which allowed me to rest.

7. Do I need to get the second shot in the same arm as the first? Which arm is better, anyway, my dominant arm or the other one?
a. It is NOT necessary to get the second dose in the same (or different) arm as the first one, in fact there is debate within the medical world about which one theoretically might be better. Consider this:
i. Which side do you sleep on? If possible, choose the OTHER side for your shot, to avoid a potentially sore arm that will wake you every time you roll on to it in your sleep.
ii. Do you have a “bad” shoulder that’s already hurt or had a recent surgery, or perhaps have you had a mastectomy on one side? If so, consider using the other side.
iii. Did you get swollen nodes in your armpit from the last shot or the “COVID arm” rash? If so, consider using the other side.
b. Dominant or non-dominant side? I swear I’ve seen medical people come to near blows debating this! Here are the arguments- both have merit:
i. Choose the DOMINANT side because you use it/move it more, and the more you move a recently immunized arm, the less it hurts.
ii. Choose your NON-dominant side because if it’s really sore and you need to get work done, you want your “good” arm free and clear.

8. What the heck is “V-Safe” and why should I bother to do it? V-Safe is a digitally based feedback system to further assess symptoms after receiving the COVID vaccines. Patients (or their family members) need the ability to text or email, but it only takes a minute to enroll. You will receive a symptom checklist through a daily text or email (your preference) for one week, then once per week. Did you have fever? Headache? If you say yes, then it asks was it mild, moderate or severe? Did your symptoms interfere with your day? Did you have to go to the doctor? And that’s it. Takes literally less than 30 seconds per day for most people. I love that when I explained the CDC’s V-Safe program to my fellow Austinites, the vast majority of them looked at me and said, “Oh, it’s just DATA! Sure, I’ll do that.” So, THANK YOU in advance for participating! NEXT year, when people ask us “so how many people got fever or headache after their second shot?” we can answer with real DATA.

9. When will I be fully immune, and when can I stop wearing a mask and social distancing? It takes up to two weeks after your second dose of COVID vaccine to reach maximum protection levels, and at this point, the truth is we don’t know exactly how long the immunity will last. Educated guesses at this point range from a minimum of six months to two years. What we DO know for sure is that people who have received these vaccines are NOT getting severely ill, needing hospitalization or dying, and that’s the goal. The biggest problem with COVID is that it overwhelmed and shut down our hospital systems, so if we can reduce COVID to basically cold symptoms, that’s all we need. As for masks, much to say but let’s leave it at this- in my opinion, at least for the next few months, we all need to keep masking if for no other reason than to maintain the social norm for all those in the community who have not been immunized nor achieved immunity after infection. It’s just not that big of a deal, let’s keep masking for now.

10. Is the Pfizer shot better, or should I wait to get the new Astra-Zeneca vaccine since it only takes one shot? The BEST COVID vaccine is the one you can get inside your body first. Please, everyone, take the first vaccine that is available to you! They are ALL good.


  • 28 days after your first Moderna is NOT a strict “due” date, so please don’t panic! The target time window for dose #2 is 28-42 days after dose one, and based on other vaccines, we are likely to find it doesn’t affect long term immunity if that date is extended further.
  • If a drop or two of blood drip down your arm after the shot, that’s okay, it just means the shot hit a tiny capillary. There are no major blood vessels in your shoulder muscle where you get the shot, but the blood drops happen maybe one out of 20 shots- that’s what the bandaid is for!
  • If you do have a touch more bleeding (especially if you take aspirin or another blood thinner), expect a bruise to show up as part of the package (again, no worries.)
  • Be sure to TAKE A PIC of your immunization card. You will need it for your second shot, and possibly later for proof of immunization for travel.

    COVID Immunization Card

BOTTOM LINE: YAY, SCIENCE! So thankful we have these new vaccines to fight COVID and get us all moving TOGEHTER past this terrible pandemic.

On a personal note, thanks for the continuous hard work and endless hours being put in by every person at Austin Public Health, along with the many volunteers helping immunize our city. I especially loved working side by side with so many retired physicians, many of whom trained me during my residency. You all continue to inspire me! Lastly, thank you to each and every person we vaccinated. Despite frustrations with scheduling or time delays, virtually every one of you lavished us with gracious thanks through your kind words, smiles, fist bumps, happy dances, and even Girl Scout Cookies!

2 thoughts on “Top 10 Things You Might Not Know about the Moderna COVID Vaccine”

  1. If you got COVID arm after the first dose, can you expect to have moderate to severe side effects after second dose? How long do side effects last? 24 hours? 48 hours? Longer? I’m debating whether or not to get my second dose because I tend to have an over reactive immune system.

    • Sorry for delayed response- hope you’ve had your second dose of the vaccine & are fully immunized now! There does not seem to be any direct correlation between developing COVID arm with the first dose & subsequent immune response-related side effects to the second dose. For people who do experience the flu-like symptoms (fever, chills, headache, fatigue, muscle aches), it typically lasts about 24 hours, usually the day AFTER your second shot. It’s perfectly fine to take tylenol and ibuprofen to reduce those symptoms if you experience them. I feel you on the over-reactive immune system, but I try to think of mine as super powers in this case, prepared to do battle with COVID!


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