CHICAGO (NEXSTAR) — A small group of Americans can now get a third COVID vaccine, and appointments are coming online to let them do just that.

Last week, the Food and Drug Administration said people with weakened immune systems can get an extra dose of the Pfizer or Moderna vaccines to better protect them as the delta variant continues to surge.

The Centers for Disease Control and Prevention says that immunocompromised people who are fully vaccinated account for more than 40% of breakthrough cases and could face serious and prolonged illness if they contract the virus.

A third shot is recommended for people who are moderately to severely immunocompromised, including people undergoing cancer treatment, stem cell or organ transplant recipients, people living with HIV or those who are receiving immunosuppressive treatments. A full list of conditions can be found on CDC’s website.

The CDC says about three percent of the U.S. population falls into the moderately or severely immunocompromised category.

NPR reports that patients likely won’t be asked for proof that they are immunocompromised, but people should not assume they are at higher risk if they don’t fall under one of the CDC’s recommended groups. People with questions about their eligibility should consult a doctor.

The CDC recommends patients get the same vaccine given during prior doses.

Meanwhile, county health offices and private medical facilities across the country have been preparing to offer booster vaccines. Major pharmacy chains including Walmart, Walgreens and CVS are now taking appointments, which can be made online.

The director of the National Institutes of Health says the U.S. could decide in the next couple of weeks whether to offer coronavirus booster shots to more Americans this fall.

Dr. Francis Collins told NewsNation Prime that federal health officials are looking at the U.S. numbers regularly but no decision has been made because cases so far still indicate that vaccinated people remain highly protected from COVID-19, including the delta variant.

On Fox News Sunday, he acknowledged there is concern that the effectiveness of the two-dose Pfizer and Moderna vaccines or the one-shot Johnson & Johnson regimen may wane over time. If so, Collins says that may necessitate a booster “maybe beginning first with health care providers, as well as people in nursing homes, and then gradually moving forward” with others, such as the elderly.

Collins says because the delta variant only started hitting hard in July, the “next couple of weeks” of case data will help the U.S. make a decision.

“If it turns out as the data come in, we see we do need to give an additional dose to people in nursing homes, actually, or people who are elderly, we will be absolutely prepared to do that very quickly,” said Dr. Anthony Fauci, who is President Joe Biden’s chief medical adviser.

The Associated Press contributed to this report.