- Coronavirus pandemic
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The US Food and Drug Administration (FDA) has approved Moderna and Johnson & Johnson vaccines to be used as booster jabs, and said Americans can now receive doses of vaccines that are different from the one they initially received.
The news that mixing-and-matching will soon be allowed comes one month after the FDA authorised Pfizer booster jabs for some Americans, including those over 65 or at higher risk of severe illness and who work in frontline jobs.
However, the FDA's ruling still requires final approval from the Centers for Disease Control and Prevention (CDC) before jabs can begin being delivered.
Less than 5% of Americans have so far received a booster jab, according to US health officials.
But with tens of millions of US residents already eligible for a third jab, Americans across the country remain confused about boosters, who needs them and how they help.
Here's what we know so far.
What's the status of each vaccine?Pfizer
Numbers: To date, more than 103 million US residents have been fully vaccinated with two Pfizer doses, while approximately 7 million have received boosters.
Efficacy: Data shows that a full dosage of the Pfizer vaccine is 88% effective in preventing hospital admission. CDC data released in mid-September shows that the vaccine's effectiveness falls to 77% after 120 days.
Company Claim About Booster: Pfizer has been supportive of the need for boosters, with CEO Albert Bourla telling reporters that studies have shown that the vaccine's effectiveness steadily declines to about 84% for vaccinated people four to six months after receiving their second dose.
FDA Ruling: Pfizer boosters have been approved for older adults and 50 to 64 year olds with medical conditions, as well as adults with underlying medical conditions or those who live and work in high-risk settings.
Numbers: To date, more than 69 million people have been fully vaccinated with the Moderna vaccine, with about 1.5 million people having received Moderna booster jabs.
Efficacy: New data shows that Moderna's vaccine was about 93% effective at reducing the risk of being admitted to hospital with Covid-19. It stays about 92% effective after 120 days.
Company Claim About Booster: Moderna has said that a half-dose booster jab would boost antibodies to a higher point than the initial two shots and believes a booster will be necessary "prior to the winter season". Currently, Moderna boosters have only been approved for certain people with weakened immune systems, such as cancer patients or transplant recipients.
FDA Ruling: The FDA approved people 65 and older and those with weakened immune systems to receive a half-dose of the vaccine, six months after their last jab.
The CDC must now make a final decision on whether to authorise Moderna boosters.
Johnson & Johnson
Numbers: Nearly 15 million US residents have received a Johnson & Johnson (J&J) vaccine, which is administered in one dose. CDC data shows that only about 9,800 people have so far received J&J boosters.
Efficacy: Research shows that the J&J vaccine is 71% effective in preventing the need for hospital care. After just 28 days, the vaccine's effectiveness falls to 68%.
Company Claim About Booster: Like Moderna, J&J had submitted a request for emergency use authorisation for its booster jab. In late September, the company said that research shows that a booster provides a nine-fold increase in antibodies. Four weeks later, it had climbed to a 12-fold increase.
FDA Ruling: On Thursday, the FDA recommended that boosters be given to anyone over 18, at least two months after the initial dose.
A number of panel members said that J&J should ultimately be considered a two-dose vaccine like Pfizer's and Moderna's.
Preliminary data from a federal clinical trial released earlier this month suggested that those who received a J&J vaccine may benefit more from having a Moderna or Pfizer booster.
The research showed that those with a J&J vaccine followed by Moderna or Pfizer boosters saw significantly higher antibody responses than with an extra dose of J&J.
The researchers noted that the data suggests "that if a vaccine is approved or authorised as a booster, an immune response will be generated regardless of the primary Covid-19 vaccination regimen."
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What Americans are sayingImage source, Getty ImagesImage caption, Tens of millions of Americans are eligible for a booster shot
A recent Reuters/Ipsos poll shows that a vast majority – 76% – of Americans that have been partially or fully vaccinated want a booster jab.
Many Americans, however, say they are confused about who can receive the boosters and what the benefits are.
"Of course, I'm confused. On one day the White House said that they'd give boosters to everyone. It turns out only some people can get them. I still don't know who decides," said Virginia resident David Williams. "It seems to me there's been a lot of contradictions."
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Others have reported being confused by the difference between the term "booster" and "third jab" and whether they mean the same thing or not.
Doctors typically use the term "booster" when referencing additional doses being given after the protection provided by the original vaccine begins to decrease. A third dose, on the other hand, typically refers to additional doses being given to immunocompromised people. Over the course of the pandemic, however, the terms have been used interchangeably in many instances.
"I wasn't confused until recently when I began seeing the language of 'third or booster'," said Nevada resident Doris Rueda. "I think so many people think they are one and the same, but I think knowing there is a difference is important, especially [if one has] immunocompromised relatives."
Greg Samuel, who lives in Washington DC, said that while he isn't confused about boosters, he doesn't expect a smooth roll-out process.
"The guidance I have received from my healthcare provider has been decent," he said. "I think most people will know how this game works after the first go-round, but since that system was a huge disaster…I expect another disaster rollout to follow."
Among unvaccinated Americans, a recent poll from the Kaiser Family Foundation found that 71% believe boosters are a sign that vaccines are not working.
"I do not see a need for boosters if the vaccine doesn't work like it's supposed to," said Jenson Bland, a 21-year-old unvaccinated resident of Georgia. "I only see it as a money-maker."
What scientists are saying Image source, Getty ImagesImage caption, Elderly and immunocompromised Americans have been recommended to get the booster shot
Dr Priscilla Hanudel, a Los Angeles-based emergency doctor, told the BBC she isn't surprised that people are confused.
"There's so many different steps in the process. I think it can be a little hard for people to understand until final approvals are in," she said.
Currently, Dr Hanudel recommends that immunocompromised people "definitely" receive an additional dose of the vaccine. She believes that it is likely that boosters will be authorised for the general public as immunity wanes.
"I think it's going to look similar to the flu shot once a year," she said. "Whether that's a booster or thought of as just another annual shot, I think it's going to happen forever for everyone eventually."
Julia Raifman, an assistant professor at Boston University's School of Public Health who tracks Covid-19 policies, said that the debate over boosters is a sign that the US needs to "reset" pandemic policymaking.
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"Strong, clear, well thought out and vetted messages from national leaders is key to communicating in a crisis," she said. "We didn't see a well-developed policy decision with boosters or with the May guidance that people remove masks. In both cases it really undercut public health."
Dr Monica Gandhi, an infectious diseases physician and professor at the University of California San Francisco, said that while she believes that immunocompromised people and at-risk frontline workers should get additional jabs, other vaccine doses should be sent abroad to countries with low vaccination rates.
Media caption, Ros Atkins looks at the ethics of Western countries rolling out Covid booster jabs while millions globally remain unvaccinated
"There's a moral and ethical obligation. We've had these vaccines for 10 months and we managed to only get 4% in the hands of low-income countries," she said.
The World Health Organization has called on wealthier nations to hold off on widespread rollouts of booster shots until vaccination rates go up in lesser developed countries. In September, WHO Director-General Tedros Adhanom Ghebreyesus said it was "really not right" to give boosters to "healthy populations".
Dr Gandhi added: "From a public health perspective, no one is safe from the emergence of other variants unless we get transmission down worldwide."
Dr Francis Collins, the director of the US National Institutes of Health, told BBC News on Thursday that the US can give boosters to its own citizens while also providing help to other nations at the same time.
"It think it's probably not correct to set these two against each other. It's not about either/or. We should be doing both/and," he said.
"The US is still the country with the highest number of deaths," he continued, adding that around one out of every four daily Covid deaths are still taking place in the US.
"We can't step away from those responsibilities" to vulnerable Americans, he said.