Children in the US will “hopefully” start to be vaccinated against Covid-19 by late spring or early summer, Dr Anthony Fauci said on Friday.

Joe Biden’s chief medical adviser, the head of the National Institute for Allergies and Infectious Diseases, was speaking at a White House coronavirus briefing, an event reinstituted by the new president after falling away during the last months of the Trump administration.

“Hopefully by the time we get to the late spring and early summer we will have children being able to be vaccinated,” Fauci said.

Covid-19 vaccines are not yet approved for children. Supplies for adults and the logistics of providing shots are proving steep challenges for manufacturers, the administration and state governments. As of Thursday, only about 1.3% of Americans had received the required two doses of currently available vaccines.

But vaccination of children is a key step in the pursuit of widespread immunity to a virus that has infected nearly 26 million in the US and killed more than 433,000. For the US to reach “herd immunity”, or widespread resistance, about 70% to 85% of the population must be vaccinated. Children make up about 25% of the population.

“Children tend to not become as severely ill as adults [from Covid-19] but they can still become ill and some have tragically died,” Dr Leana Wen, a public health expert, told the Associated Press. “Children can also be vectors of transmission, and getting children vaccinated is important.”

The federal Food and Drug Administration has not yet approved vaccines for children, due to insufficient testing data. Fauci said data was being gathered through a process called “age de-escalation testing”. The Pfizer/BioNTech vaccine received emergency approval for use in people 16 and older. The next step, Fauci said, involves testing in children down to 12. If successful it will be followed by another round of testing, down to nine years old.

Since initial tests to validate the safety and effectiveness of the Pfizer and Moderna vaccines involved tens of thousands of people, age-related testing on children can be done using smaller groups.

“You don’t want to have to … go through an efficacy trial, where you’re involving tens of thousands of children,” said Fauci. “What you can do, is in a much smaller trial, measured in hundreds to a couple of thousands … what we call safety and … immunogenicity.”

That is a term for whether the vaccine successfully triggers an immune system response.

After a frustratingly slow start, the US is administering about 1m shots a day to adults, although the pace is still seen as insufficient. Biden’s administration has set a goal of 100m shots in its first 100 days and the president has talked about achieving 1.5m shots a day.

“We’re going to make sure everybody has enough,” Biden said on Friday, visiting veterans in hospital near Washington. “We’re going to get the supply up, nationwide.”

Two more vaccines from US companies are nearing FDA approval. One from Johnson & Johnson requires a single shot.

Biden has also set a goal of reopening most schools by the summer, and directed government agencies to work with communities to advance it.

His American Rescue Plan legislation in Congress calls for $50bn to finance a major expansion of testing, which is seen as necessary for the safe reopening of schools and businesses. Robust testing can detect outbreaks before they spread and trigger shutdowns. Under Donald Trump, testing in the US had a chaotic start. Experts say in many parts of the country it is still subpar.