WASHINGTON, United States (AFP) — The US government said Wednesday it will make COVID-19 booster shots available to all American adults beginning next month, as it warned that vaccines are showing a declining effectiveness against infection.

The move comes as scientists and health experts grapple with how to beat back the surging Delta variant of the coronavirus, and follows extensive debate over whether a third injection of the Pfizer-BioNTech and Moderna vaccines would be appropriate.

“Recent data makes clear that protection against mild and moderate disease has decreased over time… likely due to waning immunity and the strength of the widespread Delta variant,” US Surgeon General Vivek Murthy said in a video press conference of the White House COVID-19 response team.

“We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalisation and death.”

Murthy and other officials said that while vaccines remain remarkably effective, the best way to shore up protection is to begin offering booster shots to fully vaccinated Americans aged 18 and older.

The booster plan is still pending a final evaluation by the Food and Drug Administration.

The boosters would become available beginning the week of September 20. Americans will be eligible for a shot eight months after receiving a second dose of the Pfizer or Moderna vaccines.

The officials said they also anticipate booster shots will be needed eventually for people who received the Johnson & Johnson vaccine, which rolled out in March.

More than 620,000 people have died in the United States from COVID-19, with case numbers increasing sharply in recent months due to the spread of the Delta variant. 

Just over 50 per cent of Americans are fully vaccinated, as the country’s mass immunisation program faces resistance in politically conservative regions in the South and Midwest.

– Waning immunity –

Anthony Fauci, the country’s leading infectious disease expert, pointed to studies showing significant drops in vaccine protection over time.

For instance, figures released by the Mayo Clinic showed that the effectiveness against the Delta variant slid from 76 to 42 per cent with Pfizer’s vaccine, and from 86 to 76 per cent with Moderna’s.

But a booster dose would increase the antibody levels in COVID vaccines tenfold, according to Fauci.

“Higher levels of antibodies are associated with higher levels of vaccine efficacy,” he told reporters.

Jeff Zients, the White House COVID-19 response coordinator, said the administration was marshalling a “war-time effort” to fight the virus, including redoubling efforts to get more Americans vaccinated.

By the end of Wednesday, 200 million Americans will have received at least their first vaccine dose, Zients said, describing it as a “major milestone”.

When confronted with the argument that the United States may be boosting immunisations for Americans at the expense of billions of people worldwide who have yet to receive any coronavirus vaccine, the administration was adamant the country was producing enough doses for domestic and international use.

“We’re already proving that we can protect our own people here at home as we help others,” Zients said.

While the administration gears up to administer some 100 million booster shots in the United States, it will be donating more than 200 million additional doses to other countries. 

Overall, it plans to donate more than 600 million vaccines worldwide, Zients added.