LOS ANGELES - Millions of unused COVID-19 doses in the United States are set to go wasted as the country's vaccination rates continued to decline amid an uptick of new infections.

Since the emergency use authorization of the first COVID-19 vaccines in the country last winter, federal data show that US states received a staggering 720 million COVID-19 doses, and more than 570 million of those shots have been administered, according to a report of ABC News.

However, state-provided data found that millions of those shots have not ended up in arms, largely due to a significant decline in the number of individuals willing to get vaccinated.

Many vaccine doses now left unused in refrigerators or discarded in trash cans across the country.

Millions of COVID-19 vaccine doses have either gone to waste, remain unused, or will expire in the coming weeks and months, according to the ABC News report, citing officials from health departments in states and state-provided data.

"It is a tremendous loss of opportunity for these vaccines to not make it into the shoulders of those who need them," said C. Buddy Creech, director of the Vanderbilt Vaccine Research Program and professor at the Department of Pediatrics at Vanderbilt University.

"Not only is it a financial loss for the purchaser of vaccines - the US government - but also a significant health loss for those who are not yet protected from COVID and its complications," he said.

About 66.1 percent of the US population is fully vaccinated against COVID-19 as of Monday, according to the latest data of the US Centers for Disease Control and Prevention (CDC).

COVID-19 boosters have been authorized in the United States for adults since November. However, CDC data show less than 50 percent of eligible Americans have received their first booster.

Vaccination rates continued to drop in the country amid a declining demand for vaccines and hesitancy, leading to the waste of unused millions of doses currently in state stockpiles and at risk of expiring.

As of April 20, the 7-day average number of administered vaccine doses per day was 470,903, a 13.2 percent decrease from the previous week, according to the latest CDC data.

The country is witnessing an increase of new COVID-19 infections recently again, with an average of about 44,000 new cases and 310 new deaths each day, CDC data show.

"The pool of people that are unvaccinated is likely to remain unconvinced of the importance of vaccine-induced protection," said John Brownstein, an epidemiologist at Boston Children's Hospital and an ABC News contributor, adding that convincing those still hesitant to get the shot will be a difficult feat.

Some politicians and public health experts have raised concerns over the ethical implications of so many shots going to waste.

"The wastage of millions of doses is a stark reminder of the privilege we have had in accessing vaccines while the majority of the world had to wait months. Extraordinary resources and financial investment will ultimately go to waste," Brownstein said.