Booster shots are a key piece of President Joe Biden’s plan to beat the COVID-19 pandemic. Speaking at the White House, Biden said boosters are “likely to provide the highest level of protection yet.”

There has been some confusion about boosters. In August, the administration got ahead of the Food and Drug Administration approval process, setting a booster rollout for Sept. 20. That’s still the goal, but the White House dialed back that plan a bit, saying the administration would be “prepared” by that date, pending a full vetting by regulators.

“The decision of which booster shots to give, when to start them, and who will give them will be left completely to the scientists at the FDA and the Centers for Disease Control,” Biden said Sept. 9.

Until that happens, here’s what we know about the basics of boosters.

Why are boosters needed?

It’s often the case that the antibodies someone gets through vaccination will begin to decline, diminishing their protection against a disease. There’s growing evidence that this is taking place with vaccines against COVID-19. The numbers are preliminary, but an August study in New York found that vaccine effectiveness dropped from 92% in early May to 80% in late July.

Public health leaders voice particular concern about the more infectious delta variant. While all three vaccines approved for use in the U.S. do a good job of keeping people out of the hospital and preventing deaths, the delta variant produces so much virus so quickly, that even vaccinated people can get ill.

The CDC’s website says “with the delta variant, public health experts are starting to see reduced protection against mild and moderate disease.”

A CDC study of nursing home residents reported that among people who got the Moderna or Pfizer vaccines, effectiveness went from 75% before delta to 52% after delta.

Will boosters work?

Again, the numbers are preliminary, but some initial research is promising.

In Israel, which has high rates of vaccination, people over 60 who had been fully vaccinated with Pfizer for at least five months were given a third dose. Researchers saw a 10-fold reduction in the risk of infection and severe disease.

At a September press briefing, leading U.S. epidemiologist Dr. Anthony Fauci said this and other reports out of Israel “support, very strongly, the rationale for such an approach.”

The concept of boosters is well established. They are part of the normal vaccination schedule for diseases such as tetanus and a number of childhood diseases.

“These vaccine series, as we call them, are recommended because you need the extra doses to get longer lasting protective immunity,” Yale infectious-disease specialist Dr. Albert Shaw told a Yale Medicine reporter in August.

Who should get a booster and when?

The CDC says: “The goal is for people to start receiving a COVID-19 booster shot beginning in the fall, with individuals being eligible starting eight months after they received their second dose of an mRNA vaccine (either Pfizer-BioNTech or Moderna).”

That applies to people without any special conditions. The eight-month wait is based on how long the initial protection is expected to be robust. The actual eligibility criteria will be set by the government when, as is expected, a round of booster shots are approved.

But in August, the FDA opened the door sooner for what they call an “additional dose” for people with weaker immune systems.

This is aimed at people receiving active cancer treatment, transplant recipients taking immunosuppressant drugs, and others whose immune systems are specifically at risk.

Technically, the additional dose available for these candidates is not a booster. But given that it is a third shot, it’s easy to see why many people wouldn’t see the distinction. The difference is the timing. Unlike the booster for otherwise healthy people which could have an eight-month wait, the government recommends the additional dose as soon as one month after completing the initial two-shot series.

When boosters open up for the general population, the CDC says the first in line to receive it will be those who were first for the initial doses: healthcare providers, residents of long-term care facilities and other older adults.

Where do boosters stand in the approval process?

Pfizer and Moderna were the first to submit data to regulators, and their vaccines are the furthest along in the vetting process run by the FDA and the CDC’s Advisory Committee on Immunization Practices.

The Pfizer review is on track to meet the Sept. 20 target, with Moderna not far behind, Fauci said in a Sept. 5 interview with CBS News.

“It is conceivable that we will only have one of them out, but the other will likely follow soon thereafter,” Fauci said.

The FDA is also looking at data from Johnson & Johnson, but it is unclear when it will make a decision.

Is a booster the same as the initial vaccination?

For now, yes. In the future, Pfizer and Moderna, with their mRNA approach, could adjust their vaccines fairly quickly to account for new variants or other issues. But for now, any booster will have the same ingredients, dosages and storage requirements as the initial shots.

Will there be enough vaccine boosters for Americans who want them?

Biden said there would be plenty of boosters.

“We’ve bought enough … booster shots, and the distribution system is ready to administer them,” he said Sept. 9.

Given the potential eight-month wait, only those who were fully vaccinated by mid-January would be eligible for boosters by Sept. 20. That’s a small fraction of the 163 million people who have received both shots of Pfizer or Moderna so far.

But there is a debate over whether the U.S. should push boosters at a time when millions of people worldwide haven’t received even a first shot. The World Health Organization has called for a moratorium on boosters until the end of the year.

Is it safe to switch from Pfizer to Moderna, or vice versa, for the booster shot? And what are the Johnson & Johnson options?

The National Institutes of Health has clinical trials underway to answer these questions. One study will give the Moderna booster to participants who received any of the three approved vaccines.

When the FDA cleared the way for immunocompromised people to get an additional dose, it spoke of matching it with the initial vaccine a person received. Any booster guidelines are expected to hold to that.

People who got the Johnson & Johnson shot face a different choice.

Unlike the other two vaccines, Johnson & Johnson is a single-shot regimen, and uses a different approach to building a person’s resistance to the disease. Ongoing studies are looking at the effect of giving a Pfizer or Moderna booster to a Johnson & Johnson recipient. San Francisco General Hospital has begun to give those boosters to people who got the Johnson & Johnson vaccine.

Will boosters, like the COVID-19 vaccines, be free?

Yes, Biden said so.

This article was originally published by PolitiFact, which is part of the Poynter Institute. It is republished here with permission. See the sources for these fact checks here and more of their fact checks here.