Researchers in the United States and abroad are beginning to test children of different ages to make sure COVID-19 vaccines are safe and work for each age. The first shots are going to adults who are most at risk from the coronavirus, but ending the pandemic will require vaccinating children, too.

So far in the US, teen testing is furthest along: Pfizer and Moderna expect to release results soon showing how two doses of their vaccines performed in the 12 and older crowd. Pfizer is currently authorised for use starting at age 16; Moderna is for people 18 and older.

But younger children may need different doses than teens and adults. Moderna recently began a study similar to Pfizer’s new trial, as both companies hunt the right dosage of each shot for each age group as they work towards eventually vaccinating babies as young as six months.

Last month in the United Kingdom, AstraZeneca began a study of its vaccine among six- to 17-year-olds, Johnson & Johnson is planning its own paediatric studies. And in China, Sinovac recently announced it has submitted preliminary data to Chinese regulators showing its vaccine is safe in children as young as three.

Getting this data for all the vaccines being rolled out is critical because countries must vaccinate children to achieve herd immunity, noted Duke paediatric and vaccine specialist Dr Emmanuel “Chip” Walter, who is helping to lead the Pfizer study.

Most COVID-19 vaccines being used around the world were first studied in tens of thousands of adults. Studies in children will not need to be nearly as large: Researchers have safety information from those studies and subsequent vaccinations of millions of adults.

And because children’s infection rates are so low — they make up about 13 percent of COVID-19 cases documented in the US — the main focus of paediatric studies is not counting numbers of illnesses. Instead, researchers are measuring whether the vaccines rev up youngsters’ immune systems much like they do adults’ — suggesting they will offer similar protection.

Proving that is important because, while children are far less likely than adults to get seriously ill, at least 268 children have died from COVID-19 in the US alone and more than 13,500 have been hospitalised, according to a tally by the American Academy of Pediatrics. That is more than die from the flu in an average year. Additionally, a small number have developed a serious inflammatory condition linked to the coronavirus.

Apart from their own health risks, there still are questions about how easily children can spread the virus, something that has complicated efforts to reopen schools.

Earlier this month, Dr Anthony Fauci, the top US infectious disease expert, told Congress he expected that high school students likely would begin getting vaccinated towards the end of the year. The elementary students, he said, may not be eligible until early 2022.