WASHINGTON (Reuters) — The U.S. government and two of the nation’s largest pharmacy chains kick off a nationwide campaign to vaccinate nursing home residents against COVID-19 on Monday, a week after the first vaccines authorized in the country began being administered to health care workers.

The program is the latest effort to control a pandemic that has killed more than 300,000 people in the country and is straining the capacity of health care systems in some states.

The United States has two authorized vaccines against the virus, one developed by Pfizer Inc and German partner BioNTech SE, which was cleared for use on Dec. 11, and one by Moderna Inc that was approved on Friday.

Some 2.9 million shots of the Pfizer vaccine were distributed last week and mostly given to health care workers. The first Moderna injections are expected to begin being administered on Monday. About 7.9 million doses of the two shots are being distributed nationwide this week.

The U.S. government is urging states to prioritize those at greatest risk from the virus because vaccine supplies remain limited. A top U.S. official said on Saturday that the government may not meet its original goal of inoculating 20 million Americans by the end of the year, with some of those planned doses dragging into January.

Many states have put health care workers and nursing home residents first in line for vaccinations, in keeping with recommendations from the U.S. Centers for Disease Control and Prevention.

Older people in nursing homes have accounted for a disproportionate number of deaths in the pandemic.

Walgreens Boots Alliance Inc and CVS Health Corp are leading a program, in partnership with the federal government, that aims to vaccinate some 7 million people in more than 70,000 long term care facilities.

The companies had already began dosing some residents with Pfizer’s vaccine last week but their full rollout begins Monday.

Walgreens said it will provide vaccinations this week in about 800 facilities in 12 states. CVS did not specify how many facilities they would visit, but said they also plan to begin administering doses in a dozen states.

Some states have opted to wait for access to the Moderna shot, which is easier to transport and store, before vaccinating nursing home residents, the companies said.

Moderna’s shots are also seen as easier to distribute to hard-to-reach rural areas. Pfizer’s shot requires special handling and must be kept at minus 70 degrees Celsius. Both have been shown to be nearly 95% effective in preventing COVID-19.

Walgreens plans to administer the vaccine to about 3 million residents and staff at 35,000 long term care facilities over the coming weeks as more shots become available. CVS eventually plans to vaccinate around 4 million residents and staff at over 40,000 long term care facilities in the same timeframe.

On Sunday, outside advisors to the CDC recommended that 30 million frontline essential workers should be the next group to receive priority for vaccination, including first responders, teachers, public transit staff and grocery store workers.