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UF Health pediatrics expert answers questions about COVID-19 vaccines for kids under age 5

With the U.S. Food and Drug Administration authorizing the Moderna and Pfizer-BioNTech vaccines for children as young as 6 months of age, UF Health pediatrics expert Sonja A. Rasmussen, M.D., discusses what parents should know about the shots. Rasmussen is a professor in the departments of pediatrics and epidemiology at the UF College of Medicine and the UF College of Public Health and Health Professions.

Question: What were the major criteria and data the Food and Drug Administration used in making its decision to approve the Pfizer-BioNTech and Moderna vaccines for kids under age 5?

Answer: The FDA and their independent advisors review data from the vaccine companies to be sure the vaccines are safe and effective. The good news is that the FDA review of these studies showed that that the Pfizer and Moderna vaccines are safe and effective for young kids 6 months to 5 years of age. These vaccines protect children against severe disease from COVID-19.

Q: How rigorous was the vaccine-testing process in children under age 5? What side effects were seen most often?

A: The studies were carefully done — safety data were available on more than 3,000 children who received the Pfizer vaccine and nearly 4,800 children who received the Moderna vaccine. Local side effects (pain or redness at the injection site) were common. Side effects such as fever were more common after the Moderna vaccine than after the Pfizer vaccine. Fever is a common side effect after other vaccines that are given to kids.

Q: Research has shown the vaccine is about 91% effective in preventing COVID-19 in children ages 5-11. What does the efficacy data show among kids under age 5?

A: The 91% vaccine effectiveness estimate indicates the vaccine’s ability to protect against some of the earlier COVID-19 virus variants. The virus has changed since it first was identified so vaccine effectiveness against getting COVID-19 is lower for the Omicron variant that is circulating now than it was earlier in the pandemic. However, the main advantage of these vaccines is their ability to prevent severe disease, such as needing to be hospitalized, or death. The vaccine effectiveness is higher for that.

For kids under 5 years old, vaccine effectiveness against getting COVID-19 for the two-dose Moderna vaccine is 41.5%. For the three-dose Pfizer vaccine, the vaccine effectiveness was 80.3%, but that number was based on a small number of children so it could be lower or higher than this. As with older kids, we expect that these vaccines will do a better job of preventing severe disease than they do preventing mild disease.

Q: If parents are offered a choice, which vaccine should their child get?

A: Both the Pfizer and Moderna vaccines are safe and effective so the important thing is for children to get vaccinated. However, there are some differences that parents need to consider. The Moderna vaccine is a two-dose series, with the two doses separated by at least 28 days. The Pfizer vaccine is a three-dose series, with the first two doses done at least 21 days apart and the third dose given at least eight weeks later. Both the Pfizer and Moderna vaccines might require additional booster doses later.

Although the studies showed higher vaccine effectiveness for the Pfizer vaccine than the Moderna vaccine, parents need to consider several key issues: The Pfizer vaccine requires three doses so children won’t get that higher level of protection until after the third vaccine dose, while the Moderna vaccine requires two doses to get the level of protection found in the studies. The studies were conducted at different times and different places. That means the variants circulating were different so the vaccine efficacy estimates cannot be directly compared. Finally, the number of children who got COVID-19 in the Pfizer study was small. For that reason, the vaccine effectiveness might be higher or lower than the number quoted.

Based on what we know, we expect that both vaccines will be highly protective against severe COVID-19. Talk with your doctor if you need help deciding which vaccine your child should get.

Q: If children don’t experience severe COVID-19 symptoms as often as adults, why do they need the vaccine?

A: While children are at lower risk of severe disease, hospitalization and death than adults, that doesn’t mean they are not at risk. Over 400 children less than 5 years of age have died of COVID-19. We also have seen many children who have needed to be hospitalized during the recent Omicron wave. Children under age 5 had the highest rate of hospitalizations compared with other children. Less than half of the young children who needed to be hospitalized had an underlying medical condition. The other children were previously healthy. Among children who were hospitalized, about one in four needed intensive care unit care.

Additionally, cases of long COVID in which symptoms last for months after getting infected with the virus, can occur in kids. Vaccines reduce the chances of getting long COVID.

Also, it’s important to note the disruption in child care and school of children due to COVID-19. We expect that these COVID-19 vaccines will reduce infections, bringing us closer to returning our lives to normal.

Q: What is the difference in vaccine dosing and side effects in kids younger than 5 years old compared with adults?

A: The dose of the Pfizer vaccine is 3 micrograms, one-tenth of what adults receive. The dose of the Moderna vaccine is 25 micrograms, one-fourth of what adults receive. During the clinical trials, the side effects were minimal. For the younger age group (6-23 months), irritability and drowsiness were the most common side effects. In the older age group (2-5 year olds), pain at the injection site and fatigue were the most common side effects. For those receiving Moderna vaccine, 1 in 4 had a fever. For those receiving Pfizer, 1 in 20 had a fever. This makes sense since the dose of Moderna vaccine is higher. In the Moderna study, one child had a fever, seizure, and a rash after the vaccine. That was the only serious adverse event that was believed to be attributable to the vaccine.

Q: Are there any children under age 5 who should not get the COVID-19 vaccine?

A: Children who have had a severe allergic reaction to a component of the COVID-19 vaccine should not get the vaccine. Ask your doctor if you think your child might be allergic to a component of the COVID-19 vaccine.

Q: How soon and where locally might I be able to get my child under age 5 vaccinated?

A: We are hopeful that the vaccines will begin to be available sometime this week at doctors’ offices, pharmacies and other places where vaccines are usually given.Media contact:

Doug Bennett at or 352-265-9400.

About the author

Doug Bennett
Science Writer, Editor

For the media

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Matt Walker
Media Relations Coordinator (352) 265-8395