Moderna mRNA Vaccine and Reported Side Effects

Last week I wrote up a post on Pfizer’s mRNA vaccine safety data, and this week we were gifted with Moderna’s information as well.  The vaccine goes up for Emergency Use Approval on 12/17/2020.  Below is a quick synopsis of the safety data provided.  If you are still unsure about WHAT these vaccines are all about, you can read my write up on what the mRNA vaccines are and what they are NOT.

Disclaimer: Although I am a board-certified physician, the information provided below should not be treated as medical advice and is for your education only.  Discuss with your personal physician on whether or not any vaccine is right for you based on your health. 

As seen in most vaccine trials, there were more reported adverse events in the vaccine group when compared to the placebo group.  Remember from my last post that “adverse event” is the term used to describe any side effects after a vaccine, including fever, muscle soreness, fatigue, etc.  In most cases these side effects are YOUR immune system responding to the vaccine and creating the desired immune response.  It is NOT the vaccine making you contract a COVID infection or making you sick. 

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One interesting thing about this study is that they did include participants who were positive for a previous COVID infection.  The Pfizer study did not include many of these participants.  When looking at the adverse event data, those individuals with a POSITIVE seropositive test prior to receiving the vaccine actually had LESS side effects reported than the seronegative group.  In other words, if you have had a SARS-CoV-2 infection in the past and then get the vaccine, you may experience less side effects after vaccination.  This makes sense.  We see that in other infections that you have the worst symptoms the first time you contract the disease.  This is due to your body having to do the full leg work in terms of producing a defense against the infection while also keeping your body healthy.  The next time you get exposed to the pathogen your symptoms are less since your body is already prepared to mount an immune response.  However, like the Pfizer trial, participants had an increase in side effects after the 2nd dose of vaccine compared to the 1st.  So maybe my theory is completely wrong.  

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Adverse Events 30 Minutes After Injection

With the recent news of two health care workers having a significant allergic reaction after their initial Pfizer vaccine in the UK, many people have been concerned about risks for allergic reaction after other inoculations as well.  When we reviewed the Pfizer data last week, no reported allergic reactions were noted.  The Moderna data is similar, where both the vaccine group and placebo group reported a 0.6% frequency of symptoms within 30 minutes of injection, none of these being serious life-threatening events.  

Local Adverse Reactions (Reactions at the Injection Site) Within 7 Days of Injection

When comparing the vaccine group to the placebo group, more individuals in the vaccine group reported symptoms at the injection site than the placebo group.  Again, we expect this since the body is mounting an immune response to the vaccine, which leads to localized symptoms.  After the first dose, 87.4% of the vaccine group reported localized symptoms versus 21.3% in the placebo group.  After dose 2, the difference was 90.5% vs 20.7%.  These symptoms are similar to other vaccine trials, including pain, swelling, redness, and localized lymphadenopathy (again, lymph nodes swell during an immune response to create cells to fight infection, so this can be expected).  To note, NONE of these reactions led to an emergency room visit.  

Pain was the most common complaint, with 83.7% of the vaccine group reporting pain after dose 1 and 88.4% after dose 2.  In the placebo group, pain was reported in 19.8% after dose 1 and 17% after dose 2.  Just like having pain after your flu shot or tetanus shot, this is not an unexpected event.  

The symptom I find interesting is the enlarged lymph nodes in the axilla (arm pit) of the arm that received the vaccine.  I do not remember seeing this mentioned as much in the Pfizer vaccine data, but in the Moderna data it is the 2nd most common complaint within 7 days after vaccination.  Frequency of complaints was 10.2% vs 4.8% after dose 1 in vaccine vs placebo group, and 14% vs 3.9% after dose 2.  These symptoms were more common in the 18-64 age range when compared to the >65-year-old group.  

Again, NO localized adverse events after either the vaccine dose or the placebo dose led to emergency room visits or other emergent intervention. 

Systemic Adverse Events Within 7 Days of Injection

Just like in the Pfizer data, Moderna vaccine recipients complained of systemic side effects more commonly than the placebo group, and the frequency always seemed to be higher after the 2nd dose compared to the 1st dose.  

When comparing vaccine group vs placebo group, the frequency of systemic symptoms after dose 1 was 54.9% vs 42.2%.  After dose 2 the reported frequency was 79.3% vs 36.5%.  I always find it interesting when I see the value of the placebo group being as high as it is, and I sometimes wonder if there is some psychosomatic involvement in symptoms after getting a shot (i.e. I think I was vaccinated so I expect to have symptoms, thus I do have symptoms).  

When looking at symptoms that affected one’s daily routine (excessive fatigue, for instance), the vaccine group reported a frequency of 2.9% after dose 1 when compared to 2% of the placebo group.  HOWEVER, after dose 2 we see a big increase to 15.7% vs 2%.  We tend to see this second dose causing more systemic side effects in both mRNA type vaccines, and I do think it is important for people to plan for this.  

The most common systemic complaints during this trial were (in order) fatigue, headache, muscle aches, joint pain, chills, nausea/vomiting, and fever.  

Systemic Adverse Events After 7 Days

Although less common, some participants in both the younger age group (18-64-year-old) and older age group (>65-year-old) reported persisting systemic symptoms a week after receiving an inoculation.  In the younger age range, 9.8% of the vaccine group and 8.9% of the placebo group still had symptoms a week out.  The older group was similar at 9.4% vs 8.1%.  The most common complaints were fatigue, headache, muscle pain, and joint pain.  

In 7 of the vaccine participants and 4 of the placebo participants, fever lasted longer than 7 days (all of these were in the 18-64-year-old group).  Why the placebo group had a small number of participants with a fever lasting longer than 7 days is beyond me, and I suspect this was due to outside influences (like contracting a different illness from the community). However, I do not have any data to support this.   

Severe Adverse Events

As mentioned in the Pfizer side effect review, life happens, whether you are in a vaccine trial or not.  However, when a significant event occurs (like death) while in a trial, those get reported in the final data.  Just because these are mentioned below does NOT mean the vaccine or placebo led to the event. 

When looking at the number of deaths reported, 6 vaccine participants and 7 placebo participants died during the trial.  Per the trial data report, 2 deaths were in participants older than 75 with prior cardiac disease, having a cardiac event 21 days after dose 1 and 45 days after dose 2 in the other participant.  Other causes of death in the vaccine group were heart attack, head trauma, acute kidney failure secondary to obstructive nephrolithiasis in a patient with known Crohn’s disease 40 days after dose 2, and 1 patient died due to suicide. 

Placebo deaths reported in the report were due to heart attack, intra-abdominal perforation, systemic inflammatory response syndrome, and an unknown cause.  

When the rates of these deaths were reviewed and compared to the general population, they are similar to what we see in populations not in a vaccine trial.  It was determined that these deaths were NOT due to the vaccine (or placebo for that matter).   

When looking at the frequency of serious events NOT related to death, both the vaccine group and placebo group were at 1%.  The most common events reported in the vaccine group were heart attack, gallbladder disease, and kidney stones.  The placebo group most commonly reported pneumonia, pulmonary embolism, and COVID infection.  

Of all of the serious adverse events reported, 7 MAY have been related to the vaccine.  After review, the FDA considered that only 3 of these MAY have been due to the vaccine  These were intractable nausea and vomiting in 1 patient (meaning they couldn’t stop and had to get help via the emergency room), and 2 episodes of facial swelling (both in participants with previous fillers injected in the face).  Of note there was another patient who had lip swelling after the vaccine, but she was a 29-year-old with previous lip fillers as well.  I guess the moral of the story is if you have any fillers in your face, watch for swelling.

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Pregnant Participants

Individuals who were pregnant were not included in the study, and those that became pregnant were removed from the study once notified.  A total of 13 participants became pregnant while in the study, and no obvious adverse events have occurred in these patients so far secondary to the vaccine they received.  However, they are still monitoring these mothers.  There was 1 case of spontaneous abortion and another of elective abortion, but both of these were in the placebo group.  


Overall this data reads similar to the Pfizer data.  Vaccine groups had a higher incidence of side effects compared to placebo, which you expect.  Both vaccines have a higher frequency of side effects after the 2nd dose compared to the 1st, and it is important for the general population to know this prior to getting their vaccines. 

NO allergic reactions occurred leading to an anaphylactic response, and I am hopeful we will get more data from the UK follow up on the two individuals who had this happen after the Pfizer vaccine administration. 

As I mentioned in the Pfizer data paper, I am comfortable receiving this vaccine once it is available and if this is the vaccine that is provided to me as a physician.  There is nothing in this data that would make me lean one way or the other when comparing Pfizer to Moderna.  I expect to get post-vaccine side effects, knowing that it is my body’s response to the spike protein being produce from the mRNA. 

It is important to remember that these vaccines are currently being recommended for EMERGENCY use.  The full vaccine approval for this vaccine and Pfizer’s will not occur until at least April.  Although many people will be focusing on this and discussing it, it will be a while before the general population has access to it. 

It is also important to reiterate that MASKING and SOCIAL DISTANCING is still needed after getting vaccinated until we have more data on the ability to transmit disease without symptoms once we receive the vaccine.  

Stay healthy.  Stay Safe.

Imperfect Dad, MD


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