Joe Biden has said that he and first lady Jill Biden plan on getting a booster shot and is comfortable doing so while millions around the world remain unvaccinated – because America has “provided more to the rest of the world than all the rest of the world combined”.

He added that the US will also provide an additional half a billion shots around the world within the first half of next year. Critics say it remains a drop in the ocean in the context of what is needed.

Biden made the remarks during an ABC News interview that aired on Thursday, a day after the administration announced Americans who received the Pfizer or Moderna vaccines eight months ago could start getting booster shots by next month. But it’s an announcement that was not received well by all health experts.

The initial doses of the vaccine have still proven highly effective against severe disease from Covid-19. The vast majority of hospitalizations and deaths in the US in recent weeks have been among the unvaccinated.

But US health officials said plans to dispense Covid-19 booster shots to all Americans would shore up their protection amid the surging Delta variant and early signs that the vaccines’ effectiveness in preventing infection – rather than severe disease – could be slipping.

“Our plan is to protect the American people, to stay ahead of this virus,” the CDC director, Dr Rochelle Walensky, said.

People who received the single-dose Johnson & Johnson vaccine will also probably need extra shots, health officials said. But they said they are waiting for more data.

Officials said that before any booster program starts up, the Food and Drug Administration (FDA) and a CDC advisory panel would need to evaluate the safety and effectiveness of an extra dose.

The World Health Organization (WHO) condemned the announcement on booster shots, claiming that administering a third dose to all Americans was unethical while millions around the world are yet to be vaccinated.

David Dowdy, a professor at Johns Hopkins Bloomberg School of Public Health, shared a CDC study co-authored by David R Holtgrave of Suny, and pointed out that the “waning effect” of the vaccine that many are worried could be influenced by behavior.

“Before jumping on “waning immunity” bandwagon, look at age breakdown,” Dr Dowdy wrote, referring to tables from the study, pointing out that the waning effect was not recorded for those who are 65 and older, but for those in the age bracket of 18 and 49.

In a follow-up tweet, he said it could be that people in that age bracket are more readily exposed because they interact with each other more.

Others expressed concerns over vaccine inequity.

Boghuma Kabisen Titanji, a fellow at the Emory University School of Medicine, tweeted on Wednesday: “A modest decline in protection against mildly symptomatic infections doesn’t justify a policy of #boosters for all vaxxed starting September.”

She went on to encourage that first doses for the unvaccinated should be prioritized over booster shots.

Muge Cevik, an expert on infectious diseases and medical virology at the University of St Andrews, added in response that it’s likely that everyone will get Covid at some point – and some might get it multiple times, but the booster shot still isn’t necessary.

“Two doses reduce the risk of worst outcome pretty well. I understand boosting elderly and immunocompromised, but there is nothing to justify boosters for all,” she wrote.