US health officials may soon recommend Covid-19 booster shots for fully vaccinated Americans.
A look at what we know about boosters and how they could help fight the coronavirus:
Why might we need boosters?
It is common for protection from vaccines to decrease over time. A tetanus booster, for example, is recommended every 10 years.
Researchers and health officials have been monitoring the real-world performance of the Covid-19 vaccines to see how long protection lasts among vaccinated people. The vaccines authorised in the US continue to offer very strong protection against severe disease and death.
But laboratory blood tests have suggested that antibodies – one of the immune system's layers of protection – can wane over time. That does not mean protection disappears, but it could mean protection is not as strong or that it could take longer for the body to fight back against an infection.
The delta variant has complicated the question of when to give boosters because it is so much more contagious and much of the data gathered about vaccine performance is from before the delta variant was widely circulating. Delta is taking off at the same time that vaccine immunity might also be waning for the first people vaccinated.
France and Germany plan to offer boosters to some people in the fall. The European Medicines Agency said it too is reviewing data to see if booster shots are needed.
When would they be given?
It depends on when you got your initial shots. One possibility is that health officials will recommend people get a booster roughly eight months after getting their second shot of the Pfizer or Moderna vaccine.
Officials are continuing to collect information about the one-shot Johnson & Johnson vaccine, which was authorised for use in the US in late February, to determine when to recommend boosters.
Who would get them?
The first people vaccinated in the US would likely be first in line for boosters too. That means health care workers, nursing home residents and other older Americans, who were the first to be vaccinated once the shots were authorised last December.
Booster? third shot? What's the difference?
Transplant recipients and other people with weakened immune systems may not have gotten enough protection from vaccines, to begin with. They can now receive a third dose at least 28 days after their second shot as part of their initial series of shots needed for them to be fully vaccinated.
For those with normal immune systems, boosters are given much later after full vaccination – not to establish protection, but to rev it up again.
What questions remain?
Still unknown is whether people should get the same type of shot they got when first vaccinated. And the nation's top health advisers will be looking for evidence about the safety of boosters and how well they protect against infection and severe disease.
Global access to vaccines is also important to stem the pandemic and prevent the emergence of new variants. Booster shots could crimp already tight global vaccine supplies.
What about the unvaccinated?
Dr Melanie Swift, who has been leading the vaccination programme at Mayo Clinic, says getting more shots into people who have not yet been vaccinated at all is "our best tool, not only to prevent hospitalisation and mortality from the delta variant but to stop transmission." Every infection, she says, "gives the virus more chances to mutate into who knows what the next variant could be."
"People who took the vaccine the first time are likely to line up and get their booster. But it is not going to achieve our goals overall if all their unvaccinated neighbours are not vaccinated," Swift says.