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Illustration: Annelise Capossela/Axios

There's no evidence that anyone who's been vaccinated against the coronavirus needs a booster yet. Deciding who needs one — and when — could be complicated.

Driving the news: With vaccination rates plateauing at the same time the more transmissible Delta variant of the virus spread across the U.S., it has raised new fears among Americans, including many vaccinated individuals who worry about how long they'll remain protected against COVID.

Big picture: We're still waiting on a lot of data to come in. But emerging evidence shows booster shots will strengthen the immune responses of the vaccinated.

  • But it's far from clear whether it will be necessary for anyone to receive a third mRNA shot.
  • There are also new questions about whether people who received Johnson & Johnson's one-dose vaccine, which has a lower efficacy rate than the Pfizer and Moderna shots, may need a booster.

State of play: Boosters would be given in two main scenarios: A new variant arises that is vaccine-resistant, or data eventually shows that vaccinated populations' level of protection wanes over time.

  • But protection against the virus doesn't just disappear all at once. And it's possible that the mRNA vaccines could be 70% effective against a new variant, for example, which is still relatively good but less than their current efficacy of more than 90%.
  • Less protection could mean that more people could get sick, but with only mild infections that infrequently lead to hospitalization or death.

Giving Americans a third shot isn't cost-free, financially or politically.

  • “If you’re going to give a third dose, that is a massive logistic exercise — administratively, logistically, and in terms of the cost. So it’s not a trivial decision," said Cornell virologist John Moore.
  • Plus, a large part of the world is still waiting for their first round of shots. Booster shots aren't even America's most pressing problem: Getting more people vaccinated with the initial round of shots is.
  • Yes, but: If there's evidence that virus immunity is decreasing, that could create plenty of domestic political pressure for another round of shots.

What we're watching: Some populations are more likely to need a booster than others.

The bottom line: Vaccine protection and disease severity exist on a spectrum, and there's no data-based answer as to when a booster would be necessary.

Go deeper

19 hours ago - Health

Pfizer CEO: Company will submit data for children's vaccine to FDA in "days"

Pfizer CEO Albert Bourla. Photo: John Thys/POOL/AFP via Getty Images

Pfizer plans to submit data about its COVID-19 vaccine for children ages 5 to 11 to the Food and Drug Administration "pretty soon," CEO Albert Bourla told ABC's "This Week" on Sunday.

Why it matters: The start of the school year saw a rise in COVID-19 infections among kids, and heightened the focus on when the vaccine will be available for children.

Sep 26, 2021 - Health

New York prepares for staff shortages from health vaccine mandate

New York Gov. Kathy Hochul during a news conference Tuesday in New York City. Photo: Mark Kauzlarich/Bloomberg via Getty Images

Gov. Kathy Hochul (D) announced Saturday she would declare a state of emergency if there were health worker shortages due to New York's upcoming COVID-19 vaccine mandate.

Why it matters: Hochul moved to reassure concerns of staffing shortages in the health care sector in a statement that also outlined plans to call in medically trained National Guard members, workers from outside New York and retirees if necessary when the mandate takes effect Monday.

Sep 25, 2021 - Health

Federal judge upholds Cincinnati health care system's COVID vaccine mandate

Photo: Jason Armond/Los Angeles Times via Getty Images

A federal judge ruled Friday that a health care provider serving the Northern Kentucky and Greater Cincinnati region can issue mandates requiring its more than 10,000 employees to get vaccinated or risk termination.

Why it matters: It's the latest ruling to uphold U.S. private employers' right to issue vaccine mandates, and comes after President Biden signed an executive order requiring vaccinations or once-a-week testing for companies with more than 100 employees.