How Do COVID Vaccine Dosages Work for Kids?

Researchers used mathematical calculations and current adult data on COVID-19 vaccine dosages to determine safe doses to test in children. Based on that research, scientists have now determined an appropriate effective dose of the COVID-19 vaccine for children ages 6 months and up.

At this time, the Pfizer-BioNTech COVID-19 vaccine, the Moderna COVID-19 vaccine, and the Novavax COVID-19 vaccine are approved or authorized for children and teens between the ages of 6 months and 17. Children under 5 years old receive three doses of the Pfizer vaccine and two doses of the Moderna vaccine for their primary series. As of December 8, 2022, these children are eligible for boosters as follows:

• Children who completed the Moderna primary series (two shots) at least two months ago can get a Moderna bivalent booster.
• Children who are still in the process of getting their Pfizer primary series shots will get a bivalent booster for their third shot.
• Children who have completed the three-shot Pfizer series do not need a booster yet.
  

Those 5 years old and up receive two doses of the Pfizer, Moderna, or Novavax (ages 12 to 17) vaccines as the primary series, and are also eligible for a bivalent booster: Pfizer only for 5-year-olds, Pfizer or Moderna for ages 6 and up.

Pfizer-BioNTech, Moderna, Johnson & Johnson/Janssen, and Novavax COVID-19 vaccines are available in the U.S. The CDC recommends that all individuals 6 months and older receive the vaccine to protect against severe illness and death from complications of COVID-19.

How Doses for Children Were Determined

During clinical trials in children, a variety of test doses were evaluated to determine the most effective dose with the least amount of side effects.

Sharon Nachman, MD, PhD, chief of the division of pediatric infectious diseases at Stony Brook Children's Hospital in New York, explains that researchers used detailed mathematical formulations to determine two to three test doses for children, based on the known effective dose in adults.

“For example, if an adult got 500 milligrams [of a particular vaccine], you would probably in a child test 125 and 250 and 750 milligrams, because you don't know if the kid is going to metabolize it faster or slower," she explains.

During these trials, researchers draw blood samples to assess how the child's immune system has responded to each dose, how effective that dose is, and the side effects experienced by the participant.

Amina Ahmed, MD, professor of pediatric infectious disease and immunology at Atrium Health Levine Children's Hospital, explains that there is a fine balance when finding an effective vaccine dose with minimal side effects.

“You're sort of balancing the smaller dosage in terms of side effects versus how much immune response you build as well... So you have to tinker with that dosage a little bit," she says. "You have to methodically try different doses.”

Recommended Dosage for Children Based on Age

Based on clinical trials similar to those described above, it was determined that a smaller dose of the Pfizer-BioNTech vaccine was 90.7% effective at preventing symptomatic COVID-19 and producing a strong immune response in participants.

The COVID-19 Pfizer-BioNTech dosage for children under age 5 is three micrograms. The dose for those between the ages of 5 and 11 is 10 micrograms, one-third of the adult dosage of 30 micrograms. Smaller doses are effective in younger children because they have especially robust immune systems. As we age, our immune systems begin to decline. Children 12 and above receive the adult dosage of the Pfizer vaccine.

Moderna's COVID-19 vaccine is also available for children and teens and the dosage changes based on age. Those under age 6 get 25 microgram doses. Children between the ages of 6 and 12 receive 50 microgram doses. Those ages 12 and up are given 100 microgram doses, which is the same as adults.

As noted above, the Pfizer, Moderna, Novavax, and J&J/Janssen COVID vaccines are all available in the U.S. for people ages 18 and older.

How Many Doses Children Need

Children under 5 years old receive a three-dose series of the Pfizer vaccine and a two-dose series of the Moderna vaccine. Children over the age of 5 and teens mimic the adult vaccination schedule. This means that two doses of the vaccine are required, given three weeks apart.

Similar to other vaccines such as whooping cough, a booster shot of the COVID-19 vaccine is needed to maintain immunity. The CDC recommends that like adults, children who had the initial series also should receive a bivalent booster. (The exception is kids under 5 who have completed the Pfizer primary series.)

In addition, certain immunocompromised children between the ages of 5 and 11 should get a booster 28 days after their second shot. Recommendations for child boosters may change as researchers learn more about how the vaccine works in the pediatric population.

If the Dose Is Too High, Will My Child Get Sick?

The COVID-19 vaccine is not a live vaccine. So if a test dose is higher than necessary, it will not cause a case of COVID-19.

The body cannot get infected with COVID-19 without the full virus cell and all of its components. The mRNA vaccines (Pfizer and Moderna) only contain a portion of the virus.

Dr. Ahmed explains that the spike protein is the portion of the virus that our bodies recognize as an invading pathogen, prompting it to subsequently create antibodies. Therefore, only this portion of the virus cell is used in the vaccine.

According to the CDC, it takes two weeks after the second vaccine dose to build a full immune response. After this time, if the body faces a true case of the COVID-19 virus, it will recognize the spike protein on the outside of the virus and start fighting off the invading cells.

Dosage Safety

Although vaccine development for COVID-19 is moving quickly due to the urgency of the situation, safety is still paramount. This is precisely why it took more time to make the vaccine available to children.

According to Dr. Ahmed, it’s realistic to expect some complications in such a new development, “Do things happen? Do people have reactions? Absolutely," she says. "That happens and we're going to hear about it. But I do think that everyone is being very careful about this."

Dr. Nachman also reassures parents that the risk of incorrect dosing is extremely low. She reports that the child’s weight, liver function, kidney function, blood volume, and overall health are determined prior to dosing. These details, along with the data obtained from adults, are used to calculate safe doses to trial in children.

“This happens all the time in medicine. Every medication that has to go into [a] kid, they practice on the math models before they ever get to the kid,” Dr. Nachman says. “And by the time they get to the kid, because these [math] models are so sophisticated, they're almost never wrong.”

Safety data for vaccine doses in children must be proven before the vaccine can be released for use in the pediatric population. Dr. Ahmed reassures parents that although the vaccines were developed quickly, safety hasn't been compromised. She says, “I know people are concerned that these vaccines came out really quickly. I will...reiterate that the degree of vigilance here is max.”

As always, consult with your healthcare provider to be sure the vaccine is safe for your child, based on their individual health history and concerns.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.