Pharmaceutical company Pfizer Inc. says its latest data shows its experimental COVID vaccine prevented mild to serious forms of the disease and its treatment is now 95% effective in older patients and has no side effects.

Meanwhile, the U.S. death toll from COVID-19 surpassed 250,000 lives lost as of late Wednesday. COVID-19 has claimed at least 250,016 lives in the U.S. It has recorded about 11.5 million infections since the pandemic emerged, according to a Reuters tally of public health care data. The U.S. is the world's worst affected country.

Pfizer will submit its potential coronavirus vaccine for U.S. regulatory approval "within days" after the first set of results from the advanced-phase tests - raising optimism that a safe and effective treatment might soon be available.

The treatment's efficacy - up from 90% announced last week - was found to be consistent in all age brackets: a fundamental concern for a virus that heavily affects older people and ethnicities, according to health experts.

Nearly 79,000 COVID-19 patients were reported in U.S. hospitals as of Wednesday, the highest number yet for a single day and up from about 75,000 Tuesday, the Reuters' count showed.

Health experts say greater social mixing and indoor gatherings during the holiday season, combined with colder weather, might accelerate the increase which is threatening to overwhelm already strained health care systems.

Pfizer said no significant side effects were seen among the 41,135 adults given two doses, Pfizer - which developed the treatment with BioNTech - said in a joint statement. The most common reactions were that 3.7% of the subjects experienced fatigue and 2% got a headache.

According to Pfizer chief executive Albert Bourla, the results were a critical step in this "historic eight-month journey to bring forward a vaccine capable of helping end this devastating pandemic," The New York Times quoted him as saying.

The final phase clinical tests are being conducted at 150 locations in the U.S., Turkey, Brazil, Germany, South Africa and Argentina. It will continue for another two years to determine how long immunity lasts and if the vaccine has any long term safety issues.