COVID-19 vaccines for children are moving closer to availability, but access remains limited.
Moderna announced late this week that it was submitting its vaccine to the U.S. Food and Drug Administration for emergency use authorization in children age 12 and older.
The Pfizer-BioNTech vaccine has already received the go-ahead for this age group.
The companies are testing their vaccines on younger age groups, down to 6 months. Results are expected in the fall.
The Pfizer-BioNTech and Moderna vaccines are moving into pediatric populations ahead of those from Johnson & Johnson and the University of Oxford-AstraZeneca partnership.
The AstraZeneca vaccine makes up the bulk of the doses delivered through COVAX, the WHO-backed program delivering shots to low- and middle-income countries.
Rare but serious blood clotting issues have slowed down the AstraZeneca and Johnson & Johnson vaccines. A study this week in the journal Nature Medicine found a rate of about one case per every 100,000 people who received the AstraZeneca shot.
The vaccine's benefits still far outweigh the risks, according to regulators in the United States and Europe and the World Health Organization. These kinds of blood clots can also be a complication of COVID-19, the Scottish study noted. They are also a rare side effect of other common vaccines, including those for influenza, hepatitis B and measles, mumps and rubella.
As to whether this side effect will affect children, "we don't yet know the answer to that," said William Moss, head of the Johns Hopkins University International Vaccine Access Center. "It's possible either way. The risk could be very similar to (that for) older adults, or the risk could be decreased."
Oxford said it has stopped enrolling children in its vaccine trial while it evaluates the blood clotting issues.
Though the delay will push back availability for children, the WHO says children are not the top priority for vaccination currently.
"Though they can get infected with COVID-19 and they can transmit the infection to others, they are at much lower risk of getting severe disease compared to older adults," WHO chief scientist Soumya Swaminathan told the organization's Science in 5 podcast.
"Except for (the) very few children who are at a high risk, (they are) not considered to be a high priority right now because we have limited doses of vaccines. We need to use them to protect the most vulnerable," including health care workers and the elderly, she said.
The G-7 group of industrialized countries this week pledged 1 billion doses to global vaccination efforts, including 500 million Pfizer-BioNTech shots donated by the United States.
That falls billions of doses short of what is needed, critics note, and most of the doses are not expected until next year.
While the Pfizer and Moderna shots have not had the same blood clotting issues as the AstraZeneca and Johnson & Johnson vaccines, officials have raised concerns about their possible link to swelling of the heart or the heart's lining.
A few more patients than expected developed this side effect after receiving these vaccines, according to reports from the United States and Israel, but regulators have not determined whether the vaccines are responsible.
Most recovered without incident.