The Moderna vaccine causes a strong immune response in children aged 6 to 11 years

Three-quarters of the children received the vaccine and the rest received placebo injections of salt water. Roughly helped, the children came from communities of color. To assess the potency of the vaccine, the researchers measured the antibodies produced after immunization. (Pfizer’s vaccine studies relied on this same approach, called immunobridging.)

Children who received the vaccine produced slightly higher levels of antibodies than those seen in young adults, a promising sign. The studies were not large enough to evaluate the vaccine’s ability to prevent serious illness or death.

But based on a small number of Delta variant infections among the participants, the researchers estimated the vaccine to be 88 percent effective against infections.

“Immunobridging is basically a plausible hypothesis we take – that the same level of immunity will be just as protective in a younger age group as it was in an older age group,” said Dr. Permar said. “So it’s nice when you can also follow it effectively.”

The strokes appeared to produce only minor side effects, including injection site pain, headache, and fatigue, and less often than in adults. Those who helped the children also had a fever for about a day.

This side effect can become a problem in children under the age of 5, because high fever in very young children requires invasive testing to rule out dangerous bacterial infections, said Dr. Permar said.

The study wasn’t large enough to detect rarer side effects, such as heart problems that were seen in other age groups. The Pfizer-BioNTech vaccine appears to cause fewer cases of so-called myocarditis among young children than adolescents or young adults.