The former director of the U.S. Centers for Disease Control and Prevention said that although he expects a COVID-19 vaccine to be approved soon, he also anticipates "some bumps in the road."

"When you vaccine millions of people, some people get really sick after the vaccination and you don't know whether that was the vaccine or that was just coincidence. So that has to be studied carefully or you're going to get all sorts of wild rumors flying around," Dr. Tom Frieden, who is now the president and CEO of Resolve to Save Lives, told ABC News chief anchor George Stephanopoulos in an interview Tuesday on "Good Morning America."

"There may be production problems, there may be rumors, there may be people who don't want to take it even if you do have the vaccine," he added. "So this is probably, George, the single most complicated vaccination program in American history."

Although a vaccine may be right around the corner, Frieden said people must remain vigilant this winter by wearing masks, washing hands, practicing social distancing and avoiding indoor gatherings.

"We have to double down on safety protocols or we're going to see the worst season we've yet had for COVID," he warned. "We can all do more."

The European Medicines Agency (EMA) said Tuesday that it could decide by Dec. 29 whether to recommend granting a conditional marketing authorization for a COVID-19 vaccine developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech.

Meanwhile, the European drug regulator said its assessment of another COVID-19 vaccine developed by American biotechnology company Moderna, which has also applied for a conditional marketing authorization, could be completed by Jan. 12.

In the European Union, conditional marketing authorizations allow for the approval of medicines or vaccines "that fulfill an unmet medical need on the basis of less complete data than normally required," according to the EMA. However, the data must show that the benefits outweigh any risks, and companies must provide further data from ongoing or new studies once a conditional marketing authorization has been granted.

The EMA said its reviews of both vaccine candidates "will proceed under an accelerated timeline" and that decisions could be issued "within weeks, depending on whether the data submitted are sufficiently robust and complete to show the quality, safety and effectiveness of the vaccine."

"Such a short timeframe is only possible because EMA has already reviewed some data on the vaccine during a rolling review," the agency said in a statement Tuesday. "These timelines are based on the type of data assessed so far in the context of the rolling review and may be subject to change as evaluation proceeds."

If the EMA decides to recommend granting conditional approval, the agency said the European Commission will then fast-track its decision-making process with a view to permitting a conditional marketing authorization that's valid in all EU and European Economic Area member states "within days."

U.S. pharmaceutical giant Pfizer and its German partner BioNTech said they have submitted an application for conditional approval of their COVID-19 vaccine with the European Medicines Agency.

The submission, which occurred Monday, completes the rolling review process that the two companies initiated with the regulator on Oct. 6.

"Today’s announcement marks another key milestone in our efforts to fulfill our promise to do everything we can to address this dire crisis given the critical public health need," Pfizer chairman and CEO Albert Bourla said in a statement Tuesday. "We have known since the beginning of this journey that patients are waiting, and we stand ready to ship COVID-19 vaccine doses as soon as potential authorizations will allow us."

If the vaccine candidate, called BNT162b2, is approved, the companies said it could potentially be available for use in Europe before the end of the year.

"As a company located in the heart of Europe, today’s milestone is important to us as we continue to seek to enable a worldwide supply upon potential approval of BNT162b2," BioNTech CEO and co-founder Ugur Sahin said in a statement Tuesday. "We will continue to work with regulatory agencies around the world to enable the rapid distribution, should the vaccine receive the approval, contributing to the joint efforts to let the world heal and regain its normal pace of life."

There were 157,901 new cases of COVID-19 confirmed in the United States on Monday, according to a real-time count kept by Johns Hopkins University.

It's the 28th straight day that the country has reported over 100,000 newly diagnosed infections. Monday's count is down from a peak of 205,557 new cases last Friday.

An additional 1,172 fatalities from COVID-19 were also registered nationwide on Monday, less than the all-time high of 2,609 new deaths on April 15.

COVID-19 data may be skewed in the coming days and weeks due to possible lags in reporting over Thanksgiving followed by a potentially very large backlog from the holiday.

A total of 13,545,017 people in the United States have been diagnosed with COVID-19 since the pandemic began, and at least 268,087 of them have died, according to Johns Hopkins. The cases include people from all 50 U.S. states, Washington, D.C. and other U.S. territories as well as repatriated citizens.

Much of the country was under lockdown by the end of March as the first wave of pandemic hit. By May 20, all U.S. states had begun lifting stay-at-home orders and other restrictions put in place to curb the spread of the novel coronavirus. The day-to-day increase in the country's cases then hovered around 20,000 for a couple of weeks before shooting back up over the summer.

The numbers lingered around 40,000 to 50,000 from mid-August through early October before surging again to record levels, crossing 100,000 for the first time on Nov. 4 and reaching 200,000 for the first time on Nov. 27.