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.
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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.
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