A U.S. study of a potential COVID-19 vaccine is being expanded to include older adults, the age group which is said to be most at risk from the new virus.

The shot, which is being made by the National Institutes of Health and Moderna Inc., is now being tested in healthy young and middle-aged adults in the Seattle and Atlanta areas.

Moderna announced on Thursday that the study is expanding to include older adults. They are being divided into two age groups – 51 to 70 and those over 70.

NIH said it is seeking 60 older adults, in order to have 105 participants in the initial phase.

Additionally, Moderna also announced that it has received funding from the U.S. government to expedite development of the shot code-named mRNA-1273.

Earlier this week, NIH infectious disease chief Dr. Anthony Fauci said the safety study showed “no red flags,” and that he hoped the next testing phase could begin around June.

The NIH’s shot is one of three leading candidates in the international search for a COVID-19 vaccine.

A possible vaccine made by CanSino Biologics is now in the second phase of testing in China. Meanwhile, another potential option, made by Inovio Pharmaceuticals, began its first U.S. study last week and received funding to begin similar test vaccinations in South Korea.