The US Food and Drug Administration on Thursday authorized third doses of both the Pfizer/BioNTech or Moderna coronavirus vaccines for certain immunocompromised people: solid organ transplant recipients or “those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise.”

The US Centers for Disease Control and Prevention has signed off on the authorization and now recommends an additional dose of vaccine for these certain people.

But these doses will be considered part of the initial vaccination series for people who have not mounted an adequate immune response. A booster dose is something different. It helps refresh a waning immune response, or helps the body fight an evolving pathogen.

And many Americans who aren’t now eligible for an additional shot are wondering when it will be their turn.

Who is eligible for an additional shot now?

The CDC said immunocompromised people are more likely to get breakthrough infections than people with healthy immune systems, and the recommendation on Friday allows certain immunocompromised people to get a third dose of Covid-19 vaccine.

“This EUA is intended to be for people with moderate to severe immunosuppression and not persons with chronic conditions for which there might be mild associated immunosuppression,” the CDC’s Dr. Amanda Cohn told the meeting of the Advisory Committee on Immunization Practices.

The third doses were recommended for immunocompromised people as young as 12, who are included in Pfizer’s EUA. People as young as 18 can get a third dose of Moderna’s vaccine. The CDC urges people to get the same vaccine they got originally.

Booster shot brings reassurance to some immunocompromised patients

People with compromised immunity are one stop closer to getting a booster dose of the Pfizer or Moderna coronavirus vaccine and it’s bringing reassurance to some patients.

There’s not enough data to discuss the possibility of an extra dose of Johnson & Johnson’s one-shot Covid-19 vaccine, the FDA said.

The CDC said patients and doctors should decide who needs an extra dose and what the timing of that dose should be. No prescription or doctor’s note will be needed — people must attest to their need for the third dose — and third doses are already available at major pharmacies and doctors’ offices.

What about boosters for everyone?

Covid-19 vaccine boosters are not currently needed for the general US population, but the administration is checking the data daily and will be ready if that changes, US Surgeon General Dr. Vivek Murthy said Thursday.

“We’re trying to understand if there’s a decrease in protection that’s manifesting as a significant increase in breakthrough infections, particularly breakthrough hospitalizations and deaths,” Murthy told CNN’s Erin Burnett.

Murthy said the administration is looking at data from pharmaceutical companies, private health care systems and other countries, including the UK, Israel and Canada.

“We look at that data very regularly, closely. When we see the threshold met, that’s when we’ll recommend boosters for additional people,” Murthy said. “That’s an if question. It’s a when question, and the data is going to drive our decision on that.”

However, the Biden administration is expected to lay out a Covid-19 vaccine booster strategy for all vaccinated Americans in September — with possible details on a phased rollout, starting with the most vulnerable.

National Institutes of Health Director Dr. Francis Collins acknowledged on Fox News Sunday that boosters may be needed “maybe beginning first with health care providers, as well as people in nursing homes and then gradually moving forward.”

The US can look to Covid-19 vaccination in Israel for an idea of what may need to be considered a month or so in the future, Andy Slavitt, former White House Covid-19 adviser, said Thursday. Israel has authorized Covid-19 vaccine booster doses for adults older than 50, becoming one of the first countries in the world to make such a move.

“Israel began their vaccination process a little bit ahead of us, so I think in many respects, we’ve been looking to them — and to a certain extent, the UK — for what we can expect, as well as the people who participated in the original clinical trials of the vaccines in 2020,” Slavitt told CNN’s Anderson Cooper on Thursday. “Those three things together, generally speaking, give us a sense of what to predict for the future.”

Some researchers and health officials suspect antibodies against coronavirus produced through the vaccines may diminish over time — possibly after a year or more — and may not protect as well against coronavirus variants that could emerge.

In July, Israel’s health ministry said in a statement that it had seen efficacy of Pfizer’s vaccine drop from more than 90% to about 64% as the B.1.617.2 or Delta variant spread.

That might mean a vaccinated person would need a booster dose of vaccine to stay protected against the original coronavirus strain and newly emerging variants — including the Delta variant — in a way similar to how a tetanus booster is recommended every 10 years or different flu vaccines are recommended each year.

In the case of Covid-19 vaccines, it remains unknown for how long immune protection lasts, but vaccine developers and health officials know it may not be forever — and that emerging variants could evade immunity.

“There is a little nuance with Covid-19 vaccines,” Dr. William Moss, professor and executive director of the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health, told CNN in May.

While typical booster doses use the same vaccine formula someone previously received to remind the immune system about immunity to a pathogen, Moss said, any future boosters for the Covid-19 shot could use altered vaccine formulas.

For instance, the companies Pfizer and BioNTech said in a statement that while a third dose of their coronavirus vaccine “has the potential to preserve the highest levels of protective efficacy against all currently tested variants including Delta, the companies are remaining vigilant and are developing an updated version of the Pfizer-BioNTech COVID-19 vaccine that targets the full spike protein of the Delta variant.”

What happens if you skip the booster?

Whether booster coronavirus vaccine doses are modified or not, missing a booster dose — if one is recommended in the future — could leave someone less protected against Covid-19.

“A person who skipped a booster is placing themselves at higher risk of getting infected, and getting disease from the SARS-Coronavirus-2, but I would also expect that they’re going to have some partial immunity and so they may be protected against more severe disease,” Moss said in May. SARS-CoV-2 is the virus that causes Covid-19.

“They’re just at higher risk of infection and disease than someone who got the booster, but they have more immunity than someone who was never vaccinated.”

Scientists are also currently investigating whether it makes a difference if someone gets the same type of vaccine as a booster as the original dose administered.