What are the side effects of the Moderna COVID-19 vaccine in young kids?

Families/Kids Vaccines

A: After the Moderna COVID-19 vaccine, your child may experience mild, short-lived, side effects similar to those commonly caused by flu shots. Notably, many of these symptoms are NOT due to the vaccine’s active ingredients – they are just as likely after a fake vaccine (placebo)!

The Moderna COVID-19 vaccine (Spikevax) has been tested in roughly 5,000 young kids, including 3,040 kids 2 to 5 years old and 1,762 kids 6 to 23 months old. The Phase 2/3 clinical trial also included roughly 1,500 kids who got a placebo, or fake vaccine. The results are very reassuring.Below, we’ll take a closer look at these side effects and other adverse reactions. By comparing the real shot and the fake shot, we can suss out in whether symptoms are due to the vaccine’s active ingredient or other causes. It turns out that many vaccine “side effects” are simply due to getting poked and prodded, missing a nap, fighting a cold, or feeling run down for other reasons.


Solicited side effects are a predefined list of common vaccine side effects (like pain at the injection site or fatigue) that subjects are asked to report for 7 days after vaccination.Kids under 5 who got the Moderna vaccine may experience one or more of the following symptoms: pain or redness at the injection site, fatigue, irritability, headache, myalgia (muscle soreness), loss of appetite, nausea, chills and arthralgia (joint pain). These symptoms were typically mild and short-lived, and were more likely after the second dose.

After the first vaccine dose, these side effects were just as common in the placebo group and the vaccine group, which suggests that they are not directly due to the vaccine’s active ingredients. Here are the solicited systemic side effects after the first dose (vaccine group vs placebo group):

▶️ 37 months to 5 years: Fatigue (40% vs 36%), headache (12% vs 12%), myalgia (10% vs 9%), nausea (7% vs 8%), chills (6% vs 6%), arthralgia (6% vs 5%)
▶️ 24 to 36 months: Irritability/crying (55% vs 51%), sleepiness (30% vs 29%), loss of appetite (24% vs 22%)
▶️ 6 to 23 months: Irritability/crying (68% vs 62%), sleepiness (37% vs 37%), loss of appetite (30% vs 26%)

After the second dose, side effects were more common in the vaccine group than the placebo group, suggesting that the body’s response to vaccination was at play. Here are the solicited systemic symptoms after the second dose (vaccine group vs placebo group):

▶️ 37 months to 5 years: Fatigue (48% vs 29%), headache (16% vs 8%), myalgia (16% vs 8%), chills (12% vs 5%), nausea/vomiting (10% vs 5%), arthralgia (9% vs 5%)
▶️ 24 to 36 months: Irritability/crying (54% vs 45%), sleepiness (36% vs 27%), loss of appetite (31% vs 21%)
▶️ 6 to 23 months: Irritability/crying (64% vs 59%), sleepiness (35% vs 33%), loss of appetite (32% vs 25%)

May be an image of text that says 'Solicited systemic side effects after Moderna COVID-19 vaccine or placebo grade 100% ungCh Arthralgia Nausea/ Vomiting Chills Grade1 PD2 PD2 Solicited Systemic Reactions within Toddler: (6-23 Months) 100% Days After Dose Months), Infant/Toddler Irritability Crying 75% Sleepiness 21% 25% PD1 PD2 Post Dose 1; PD2 Post Dose Source: FDA presentation from Moderna: mRNA-1273 (Moderna COVID-19 Vaccine) Û Request Emergency Use Authorization Individuals 6 Months Years of Age'

Fevers were relatively common after vaccination, but were typically low to moderate and resolved after a day or so. Fevers were more common after the real shot compared to the placebo, especially after the second shot. This suggests a direct link between the vaccine and this immune response. Here are the numbers:

Fever rates, dose 1 (vaccine vs placebo):
▶️ 2 years to 5 years: Low (5.3% vs 3.0%), Moderate (1.9% vs 2%), High (1.6% vs 1%)
▶️ 6 to 23 months: Low (5.5% vs 4.6%), Moderate (3.5% vs 1.7%), High (1.9% vs 2.1%)

Fever rates, dose 2 (vaccine vs placebo):
▶️ 2 years to 5 years: Low (8.4% vs 3.6%), Moderate (4.7% vs 2.1%), High (3.8% vs 0.9%)
▶️ 6 to 23 months: Low (7.7% vs 3.6%), Moderate (4.3% vs 1.3%), High (2.6% vs 3.4%)

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The most serious vaccine-related side effect was a “febrile seizure” due to a very high fever in one of the 5,000 children who got the real shot. This child recovered quickly with no medical consequences.


Unsolicited adverse events are a mixed bag of anything medical that clinical trial subjects reported within 28 days of vaccination – everything from rashes to runny noses to broken limbs. External experts review every report to determine whether or not it was likely to be vaccine-related.Overall, unsolicited adverse events were equally common in the vaccination group and the placebo group (40% vs 38% for ages 2-5 years, and 49% vs 48% for ages 6 months to 2 years). In kids aged 2 to 5 years, the most commonly reported unsolicited adverse events were upper respiratory tract infection, rhinorrhea (runny nose), and cough. In babies 6 months to 2 years, the most commonly reported unsolicited adverse events were upper respiratory tract infection, irritability, fever, and teething.

Only a handful of unsolicited adverse events differed between the vaccine group and the placebo group (vaccine vs placebo):
▶️ 2 to 5 years: Injection site erythema /skin inflammation (1.3% vs 0.2%); COVID-19 (3.1% vs 5.5%). Vaccine for the win!
▶️ 6 months to 2 years: injection site lymphadenopathy / swollen lymph nodes (1.4% vs 0.2%), injection site erythema / skin inflammation (1.1% vs 0.2%).

Once again, we see that many adverse events are NOT related to the vaccine ingredients. Jab or no jab, babies and tots often face scrapes, sniffles, fevers, and more. This is an important reminder to be careful about assuming that something bad is due to whatever you just did (e.g. “Oh no! I just got a parking ticket. It must be because a black cat crossed my path.”).


No cases of myocarditis were reported, but this was not surprising as the trial was not big enough to detect rare adverse events. Vaccine safety continues to be actively monitored through surveillance and reporting systems. In children 5-11 years we have seen roughly 1 case of myocarditis per million doses (22 verified reports of myocarditis after 19,813,898 doses of Pfizer-BioNTech).

Most cases occurred within a week of the second dose. While it is encouraging that no safety problems have been reported thus far, we’ll have more confidence in estimates of rare events like myocarditis later in the summer when many more kids are fully vaccinated. As of July 7, 2022, the White House reported that roughly 300,000 kids under 5 have received at least one dose of a COVID-19 vaccine.

It bears mentioning that our global vaccine monitoring systems have been doing an amazing job of catching rare adverse events. For example, the rare Astrazeneca blood clotting issue was first flagged for regulatory review roughly ten weeks after roll-out in the UK.


In clinical trials, Pfizer’s vaccine caused fewer side effects in young kids than Moderna’s vaccine, likely due to the lower dose. Both vaccines have excellent safety profiles.In the Pfizer trial, the first two doses were very well tolerated. Side effects did occur, but the rates were strikingly similar in the vaccine and placebo groups. After the third dose, there were hints of slightly higher rates of a few side effects (like redness at injection site, drowsiness, fever), but not to the same extent as the second Moderna dose.

The Moderna and Pfizer vaccines also differ in their dosing and schedules. The pediatric Moderna vaccine delivers good protection after 2 doses a month apart, with full protection by 6 weeks after the first dose. By contrast, Pfizer’s pint size vaccine requires 3 doses and 13 weeks for full protection. The Moderna vaccine also uses a higher mRNA dose than Pfizer. Read more about Moderna vs Pfizer in our article here.


For parents who want to get their baby or young child (under 5) protected as soon as possible, the Moderna vaccine is a great choice.Side effects are no fun, but they are not reason for alarm – and many of them are not even due to the vaccine ingredients! The temporary and typically mild symptoms are outweighed by the upside of protecting your child and others. Ideally, time your kids’ shot so they can lay low for a day or two. You can use pain relievers for kids if desired, after vaccination (but not before).

Thanks to the thousands of kids in the Moderna vaccine trial, we can say with confidence that getting vaccinated is the safer choice for kids, as the *known* burden* of COVID-19 in young children is substantial. In fact, the health impacts of COVID-19 are greater than flu and other vaccine-preventable diseases in children.

As always, don’t hesitate to contact your local healthcare provider if your child experiences any medical concerns.

Additional Resources:

Health Impacts of COVID-19 in Young Kids (Dear Pandemic)