US biotech firm Moderna said Thursday it had submitted a request for an emergency use authorization in the United States for its Covid vaccine for children aged six months to under six years.

Very young children are the only group that are yet to be eligible for the Covid-19 vaccine in the United States and in most countries, a source of concern for many parents.

"We believe (this vaccine) will be able to safely protect these children against SARS-CoV-2, which is so important in our continued fight against Covid-19 and will be especially welcomed by parents and caregivers," the company's CEO Stephane Bancel said in a statement.

In March, the company announced results from a trial that showed the two-shot regimen was found to be safe and produced a strong immune response.

Specifically, two doses of 25 micrograms given to babies, toddlers and preschoolers generated similar levels of antibodies as two doses of 100 micrograms given to young people aged 18-25, indicating there would be similar levels of protection against serious cases of the virus.

The trial included 4,200 children aged two to six years and 2,500 babies aged six months to two years.

Side effects were generally mild and consistent with those seen in older age groups.

The company did, however, find estimate relatively low efficacy, with its trial taking place during the Omicron variant wave.

The current generation of vaccines were designed against the original strain of the virus.

Vaccine efficacy in children six months up to age two was 51 percent, and efficacy was 37 percent in the two to five years age group, when limiting the analysis to only cases confirmed positive on a positive PCR test.

Moderna said these were similar to vaccine efficacy estimates in adults during Omicron, and it is also currently studying booster doses for all pediatric cohorts.

Vials of the Moderna COVID-19 vaccine against Covid-19, are being prepared for local residents in a tray at a mass vaccination center operated by Japanese Self-Defense Force in Tokyo
Vials of the Moderna COVID-19 vaccine against Covid-19, are being prepared for local residents in a tray at a mass vaccination center operated by Japanese Self-Defense Force in Tokyo POOL via AFP / Eugene Hoshiko

Back in February, the Food and Drug Administration (FDA) postponed a meeting of a panel to consider the Pfizer-BioNTech Covid vaccine for children younger than five, saying it wanted to see data on how three doses performed before considering the matter.

Pfizer's vaccine, when given as two doses of three micrograms to children aged six months to two years elicited a similar level of antibodies to the full 30 micrograms given to people aged 16-25, but the same was not true for children between two and four.

This week, Pfizer CEO Albert Bourla said in an interview his company was aiming to get the vaccines out by June if regulators agree.

The picture is further complicated by the fact that Moderna's vaccine is currently authorized only for adults aged 18 and up in the US, while Pfizer's is available to those five and up. Moderna is separately seeking authorization for older children.

In an interview with CNN+ this week, Anthony Fauci, the White House's chief medical advisor, suggested the FDA may review data from both companies for young children simultaneously in order to not "confuse people."

Scientists evaluating a vaccine for infants must closely consider the risk-benefit balance.

Even when they are unvaccinated, children under five are at very low risk for severe disease. There have been only 476 deaths in the United States this age group since the start of the pandemic, according to official data.

Among all US children, there have also been almost 8,000 cases of MIS-C, a post-viral inflammatory condition, that caused 66 deaths.

Emergency physician Jeremy Faust of Brigham and Women's Hospital tweeted that Moderna's results were very positive news, despite the mediocre-sounding efficacy estimates.

"What these vaccines have been shown to really do, over and over, is protect against severe disease and hospitalization, long-term consequences," he said, adding the immune response outcomes predicted success on these measures.

"I'll be quite comfortable getting my 4-year-old vaccinated with Moderna."