Millions of Americans are getting the green light for a limited supply of the third dose of a COVID-19 vaccine Friday morning.

The announcement Thursday night by the Food and Drug Administration applies to several million Americans who are especially vulnerable because of organ transplants, certain cancers, or other disorders. Several other countries, including France and Israel, have similar recommendations.

The announcement comes as the extra-contagious delta version of the coronavirus surges through much of the country, pushing new cases, hospitalizations and deaths to heights not seen since last winter.

U.S. regulators say transplant recipients and others with severely weakened immune systems can get an extra dose of the Pfizer or Moderna COVID-19 vaccines to better protect them.

There’s little data on how well a third dose works, and if it causes any safety problems such as an increased risk of organ rejection.

Other small studies have similarly found that some transplant recipients respond to a third dose while others still lack enough protection.

Mount Sinai South Nassau's Vaxmobile is the region's first COVID-19 vaccination mobile unit. They will be making stops next weeks at specified locations in Nassau County. 

Walk-ups are permitted while supplies last, but making an appointment ahead of time is encouraged. 

COVID RESOURCES: COVID-19 vaccine resources and case data