Pfizer has begun testing its COVID-19 vaccine in children younger than 12, a significant step in turning back the pandemic. The trial’s first participants, a pair of 9-year-old twin girls, were immunized at Duke University in North Carolina on Wednesday.

Results from the trial are expected in the second half of the year, and the company hopes to vaccinate younger children early next year, said Sharon Castillo, a spokeswoman for the pharmaceutical company.

Moderna also is beginning a trial of its vaccine in children 6 months to 12 years of age. Both companies have been testing their vaccines in children 12 and older, and expect those results in the next few weeks.

AstraZeneca last month began testing its vaccine in children six months and older, and Johnson & Johnson has said it plans to extend trials of its vaccine to young children after assessing its performance in older children.

Immunizing children will help schools to reopen as well as help to end the pandemic, said Dr. Emily Erbelding, an infectious diseases physician at the National Institutes of Health who oversees testing of COVID-19 vaccines in special populations.

An estimated 80% of the population may need to be vaccinated for the United States to reach herd immunity, the threshold at which the coronavirus runs out of people to infect. Some adults may refuse to be vaccinated, and others may not produce a robust immune response.

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Children younger than 18 account for about 23% of the population in the United States, so even if a vast majority of adults opt for vaccines, “herd immunity might be hard to achieve without children being vaccinated,” Erbelding said.

Pfizer had initially said it would wait for data from older children before starting trials of its vaccine in children younger than 12. But “we were encouraged by the data from the 12 to 15 group,” said Castillo, who did not elaborate on the results so far.

Children represent 13% of all reported cases in the United States. More than 3.3 million children have tested positive for the virus, at least 13,000 have been hospitalized and at least 260 have died, noted Dr. Yvonne Maldonado, who represents the American Academy of Pediatrics on the federal Advisory Committee on Immunization Practices.