Two of the nation’s top health agencies authorized and expanded COVID-19 vaccine eligibility to allow older Americans to receive a second booster shot in a flurry of federal action.

The FDA approved a second dose of either the Pfizer-BioNTech or Moderna shot for people 50 and older at least four months after their first dose on Tuesday.

The authorization was signed off on by the Centers for Disease Control and Prevention several hours later.

The additional booster is especially important for people 65 and older, as well as those 50 and older who have underlying medical conditions that increase their risk of severe disease from COVID-19, according to the agency’s director, Dr. Rochelle Walensky.  “At this time, they are the most likely to benefit from an additional booster dose,” she said in a statement.

Anyone 50 and older who has received a primary COVID-19 vaccine series and a booster dose from any approved or authorized provider is eligible for a second booster of Pfizer-BioNTech or Moderna at least four months after the first.

Despite the CDC’s expansion of booster eligibility, the agency stresses the importance of the second shot for those 65 and older, as well as those 50 and older with underlying medical conditions.

Anyone who is immunocompromised and over the age of 12 is eligible for a second dose of the Pfizer-BioNTech vaccine, and immunocompromised adults can now get the Moderna vaccine as well.

The initial vaccination course for immunocompromised people is three doses. They’ve been able to get a first booster, or fourth dose, since last fall and are now eligible for a fifth, according to CDC spokeswoman Kristen Nordlund.

According to the CDC, studies have shown that people with certain medical conditions are more likely to develop severe COVID-19 disease.

Obesity, diabetes, chronic lung, kidney, or liver disease, heart disease, smoking, pregnancy, and a variety of mental health conditions are among these medical conditions.

The CDC reviewed each medical condition to ensure it met the criteria for inclusion on the list, but recommends that people discuss their risk with their health care provider because some medical conditions may not be on the list. According to Nordlund, people who received initial doses of the Johnson & Johnson vaccine can get a second booster dose. The J&J vaccine was originally intended to be a one-shot shot, but studies have shown that two doses are required to provide the same level of protection as the other two approved vaccines.

Since last fall, J&J recipients have been eligible for a second shot, and the CDC recommends that they get an mRNA vaccine for their second dose. They can get a third shot from either Pfizer-BioNTech or Moderna, according to Nordlund.

The nation’s top two pharmacies have announced that second booster doses will be given to eligible recipients this week. A CVS spokesperson said that eligible patients will be able to schedule the additional shot “shortly” online or through the pharmacy app, but did not say when they will be able to do so.

Walk-in appointments are available at Walgreens pharmacy from Wednesday to Friday, “as store capacity allows,” according to a spokesperson. Starting Friday, Walgreens will allow customers to schedule appointments online or via the mobile app.

There is no evidence that a fourth dose will provide significant benefit to healthy people, nor does it specify how long the fourth dose should last.

However, some evidence suggests that a second booster shot may be beneficial in combating fading immunity after the third shot. The FDA cited a study in which 154 Israeli health-care workers were given a fourth shot with the Pfizer-BioNTech vaccine and 120 were given a second boost with Moderna.

Two weeks later, both groups had at least a tenfold increase in COVID-19-fighting antibodies, whereas antibody levels in those who didn’t receive an additional booster continued to fall. In neither group, any new safety concerns were raised.

Other studies, some of which have not been peer-reviewed or published, show that vaccine effectiveness declines three to six months after the first booster, and that a second booster can improve protection against severe disease.