A doctor leading a trial of Moderna's COVID vaccine for young kids shares the 3 most common side effects he's seeing, and they're less severe than the ones seen in adults

• One of the pediatricians trialing Moderna's vaccine for kids 0-6 spoke with Insider about the symptoms he's seen.
• Kids in his trial haven't had arm pain or many other tell-tale side effects that were seen in adults.

Dr. Daniel Leonard is pretty sure he's vaccinated more than 50 kids under the age of 6 with Moderna's pediatric COVID-19 vaccine.

He doesn't know exactly how many he's actually vaccinated, because in order to properly conduct a clinical vaccine trial he's not allowed to know who got the real vaccine, versus who was administered a fake placebo jab.

But Leonard says he can't even try to venture a proper educated guess as to who's been vaccinated at his clinic so far, because Moderna's vaccine is being so well tolerated in this young age group, with such minimal side effects, that it's near impossible to tell who's been dosed.

"As a pediatrician, I'm used to kids getting shots all the time and having aches or pains or groans about them a day or two later, and I did not see that," he said. "I did not see any change in sleeping behavior or pattern, I did not see any changes in appetite, no changes in growth."

Moderna announced earlier this month that the company is making moves toward applying with the US Food and Drug Administration to authorize its baby-sized COVID-19 vaccine for kids 6 months to 6 years old this spring, after finding the company's miniature-dose vaccine generated a "robust" antibody response in kids, and had a "favorable" safety signal.

According to Leonard, that "favorable" safety signal has been so sound among his patients (with no cases of the heart-swelling condition myocarditis, and no vaccine-related hospitalizations) that it can be hard for parents to know whether their kids got the real deal.

Three noticeable side effects, including fatigue and crankiness

Leonard says in his clinic, there were just three short term vaccine-related side effects he noticed regularly in the 6 month to 6 year age group. They were:

1. Fatigue
2. Elevated temperatures (full-blown fevers were extremely rare in Moderna's trial, occurring in less than 0.2% of cases, but Leonard said thermometer readings around 99 or 100 degrees Fahrenheit did pop up.)
3. Irritability for about a day or two, usually between 48-72 hours after a shot.

That milder side effect profile, relative to older vaccine recipients, is not entirely surprising — given how baby and toddler immune systems work.

"The always dialed-up state of a child's immune system probably did lend itself to fewer noted side effects," Leonard said. "They should be able to mount immune responses and not have these overly dramatic, inconvenient side effects that many of the adult participants noticed."

One family enrolled all three of their children under 5 years old in the trial, and none of the kids had any noticeable symptoms.

For Leonard, that's an "awesome" result. "Parents of babies are not gonna have to freak out, have an additional sleepless night and a fussy baby," he added.

Once the FDA releases the data from Moderna's trial publicly, we'll be able to get a better look at whether Leonard's experience was mirrored widely at the other 80-plus vaccine trial sites across the US and Canada, where more than 11,700 kids under 12 years old (and their parents) have signed up to be the very last check in a rigorous, multi-step process leading up to an application with the FDA for Emergency Use Authorization (EUA) for Moderna's shot for young children.

Sore arms and aches were more common in older children

Tenderness and sore arms tended to be more common among older pre-teen patients at Leonard's trial site in Hastings, Nebraska. Among the babies and toddlers it "wasn't something I stumbled across at all," he said, whereas older kids "definitely complained more of sore arms, local swelling and tenderness feeling achy, feeling feverish and tired for a day or so."

The find was reassuring for Leonard.

"We care more about safety than anything else," he said.

But some parents, excited to have early access to a COVID vaccine for their kids, were clearly disappointed that they couldn't tell whether their young ones were vaccinated.

"These people wanted the product, they wanted the study vaccine," Leonard said. "They almost seemed disappointed if on the safety follow up phone calls or e-diaries, their children had not had symptoms, because they were hoping for that dose."

The vaccine was safe and well tolerated, but it wasn't great at stopping mild infections

According to Moderna, the vaccine was 43.7% efficacious for the youngest kids in the trial (6 months to 2 years old), while vaccine efficacy for the older kids (2-6 years) was slightly lower, around 37.5%. No severe COVID cases were recorded.

Dr. Leonard says he can "understand" these efficacy numbers may not sound that great to parents (though, all COVID-19 vaccines are performing worse against Omicron, especially when it comes to preventing such mild infections.) But, when considering vaccination, he also thinks about all the potential unintended or unrecognized consequences of unchecked viral infections.

"We can control the components of the vaccine," he said. "We can't control the variance of COVID."

The same is true of many other viral diseases. For example, doctors didn't discover until recently that pediatric RSV infections may be linked to asthma diagnoses later on in life. And it took many years to figure out that childhood mumps can lead to decreased sperm counts, in a small proportion of men who were infected as children.

Leonard says the COVID vaccine, like all other childhood vaccines, should be thought of as a tool for risk reduction, for controlling the impact of the virus as much as possible — both in the short term and the long run — even if that protection can't be absolute.

"If it is less efficacious for your child and they do end up acquiring natural COVID disease somehow, that stinks," he said. "But are you as sick as you otherwise would've been having not been vaccinated?"