WASHINGTON, Feb. 22 (Xinhua) -- The COVID-19 vaccine rollout across the United States has contributed significantly to the sharp drop in key COVID-19 indicators, a leading expert told Xinhua in an interview on Monday.

"The vaccine rollout is the major reason for the decreasing of new cases, deaths and hospitalizations," said Zhang Zuofeng, professor of epidemiology and associate dean for research with the School of Public Health at the University of California, Los Angeles.

The effect began to show 28 days after the start of vaccine rollout in the country on Dec. 14 last year, and was quite obvious after nine weeks, Zhang noted.

"The average daily COVID-19 incidence on Dec. 14 was about 652 infections per 1 million people, and it dropped to about 257 infections per 1 million people on Feb. 15, a 60 percent decrease in the ninth week after the start of vaccine rollout in the country," Zhang told Xinhua.

On a global scale, the number of new COVID-19 cases per day dropped by about 51.3 percent nine weeks after the initial vaccine rollout, Zhang said.

COVID-19 cases have been falling sharply for five weeks, hospitalizations for four, and deaths for two, according to The COVID Tracking Project.

The U.S. death toll from the coronavirus is approaching 500,000 on Monday, a milestone that underscores the grave threat the virus still poses nationwide even as more Americans get vaccinated.

Over 64.1 million vaccine doses have been administered across the United States as of Monday, data of the U.S. Centers for Disease Control and Prevention (CDC) show.

Zhang predicted that based on the current vaccination speed, about 75 percent of the U.S. population will be vaccinated by July or August.

Last week, the vaccine rollout in the country had been hindered by power outages amid the winter storms in much of the country, with nearly all states experiencing shipment delays.

Officials pledged to work "double time" to make up for vaccine delays.

U.S. President Joe Biden's national vaccination campaign aims to administer 100 million doses of two-stage coronavirus vaccines in his first 100 days in office.

Though key COVID-19 indicators in the United States continue to fall, experts warn the public not to let down guard as the country sees an uptick in coronavirus variants infections.

"The vaccines could provide some protection against the variants," Zhang noted. "If we can complete mass vaccination to reach general immunity before the variants take hold, the situation could get in control."

"Otherwise, if the vaccine rollout lags behind the spread of variants, and the variants evolve to a pandemic, it will bring huge disaster," he noted.