The US Food and Drug Administration authorized an additional Covid-19 vaccine dose for certain immunocompromised people on Thursday.

The FDA amended the emergency use authorization for the Pfizer/BioNTech and Moderna Covid-19 vaccines to allow for an additional dose for certain people with compromised immune systems. That group includes "specifically, solid organ transplant recipients or those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise," the agency wrote in a statement Thursd

The country has entered yet another wave of the COVID-19 pandemic, and the FDA is especially cognizant that immunocompromised people are particularly at risk for severe disease," Acting FDA Commissioner Dr. Janet Woodcock said in a statement. "After a thorough review of the available data, the FDA determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna Vaccines."

Currently three coronavirus vaccines are authorized for emergency use in the United States -- the two-dose Pfizer/BioNTech vaccine for people 12 and older and the two-dose Moderna vaccine and single-dose Johnson & Johnson vaccine for people 18 and older.

All three are used under emergency use authorization by the FDA, 

The CDC's Advisory Committee on Immunization Practices is scheduled to meet on Friday to discuss booster doses of Covid-19 vaccines and additional doses for some immunocompromised people, according to a meeting agenda posted online. The committee is also scheduled to vote on Friday on whether to recommend additional doses of the vaccine for immunocompromised people.

A recent study by Johns Hopkins researchers found that vaccinated immunocompromised people are 485 times more likely to end up in the hospital or die from Covid-19 compared to the general population that is vaccinated.

Based on an estimate by the CDC, about 9 million Americans are immunocompromised, either because of diseases they have or medications they take.

"Emerging data show that certain people who are immune compromised, such as people who have had an organ transplant and some cancer patients, may not have had an adequate immune response to just two doses of the Covid vaccine," CDC Director Dr. Rochelle Walensky said during a White House Covid-19 Response Team briefing on Thursday. "To be clear, this is a very small population. We estimate it to be less than 3% of adults."

It has been known for months that Covid-19 vaccines might not work well for this group. The hope was that vaccination rates overall would be so high so that the "herd" would protect them. But it didn't work out that way, because about a third of eligible people in the US have not received even one dose of a Covid-19 vaccine.

The FDA also recommends immunocompromised individuals maintain other precautions, including physical distancing and masking.

"In addition, close contacts of immunocompromised persons should get vaccinated, as appropriate for their health status, to provide increased protection to their loved ones," according to the FDA's statement on Thursday.

Additionally, FDA recommends immunocompromised people who contract or are exposed to Covid-19, check with their doctor about monoclonal antibody treatments.