Dr. Rochelle Walensky, the new director of the U.S. Centers for Disease Control and Prevention, called for optimism and adherence to science in her first message to the American people on Wednesday.

"Better, healthier days lie ahead. But to get there, COVID-19 testing, surveillance, and vaccination must accelerate rapidly," Walensky said in a statement. "We must also confront the longstanding public health challenges of social and racial injustice and inequity that have demanded action for far too long."

Dr. Anne Schuchat, the CDC's principal deputy director, will be conducting a comprehensive review of the agency's existing COVID-19 guidance and will update those recommendations based on the best available scientific evidence, so that Americans can make informed decisions about their lives, Walensky added.

ABC News' Anne Flaherty contributed to this report.

The U.S. has experienced "a second week of encouraging data trends and projections" which "lends greater confidence that the country may, as a whole, be rounding a corner," according to researchers with PolicyLab at the Children's Hospital of Philadelphia.

In its weekly report based on an analysis of COVID-19 data, PolicyLab said much of the country appears "to be in a transition period during a winter surge."

Current positivity rates are down nationally and "projections from Southern California to Arizona suggest while overall case incidence may continue over the next four weeks, these areas might expect declines in overall transmission rates in many counties," the report stated.

Researchers noted caution "not to overstate ... optimism," citing evidence for possible increased transmission in winter vacation destinations such as Vail, Colorado, as well as Park City, Utah, and Sun Valley, Idaho.

Despite some positive outlook, the report said incidence in coastal areas of the Southeast may continue to worsen into February as people attempt to escape the winter weather.

Variants of the coronavirus could also be a factor in the "potential to increase case incidence again," the report stated.

ABC News' Brian Hartman contributed to this report

Wednesday marks one year since the Centers for Disease Control and Prevention confirmed the first case of COVID-19 in the United States.

It wouldn't be until several months later that scientists identified the virus that caused COVID-19 in blood samples from people in various U.S. states as early as December 2019.

Since the first confirmed case 365 days ago, more than 24.2 million people in the U.S. have tested positive for COVID-19, which means that approximately one in every 13 Americans have contracted the disease, according to data collected by Johns Hopkins University.

At least 402,400 lives in the U.S. have been lost to COVID-19, representing approximately 19.5% of the worldwide death toll from the disease. That means one in every 823 Americans have now died from COVID-19.

New York remains the worst-hit U.S. state in terms of COVID-19 deaths -- with more than 37,000 confirmed fatalities -- followed by Texas, California and Florida.

Since the start of the pandemic, nearly 763,000 people in the U.S. have been hospitalized with COVID-19. Just under 124,000 people nationwide are currently hospitalized with the disease. In the last two weeks, that number has declined by 5.6%, according to data compiled by The COVID Tracking Project, a volunteer-run effort to track the U.S. outbreak.

January has already proven to be one of the worst months on record for the U.S. outbreak. In the first 19 days of 2021, the country has confirmed more than 4.15 million cases and over 55,000 deaths from the disease.

Although the numbers are currently impacted by the holiday weekend, the U.S. continues to see a drop in new infections, now averaging approximately 197,000 newly confirmed cases per day, according to The COVID Tracking Project.

ABC News' Brian Hartman and Arielle Mitropoulos contributed to this report.

U.S. Surgeon General Jerome Adams resigned from his post Wednesday at the request of President-elect Joe Biden.

"I've been asked by the Biden team to step down as Surgeon General," Adams wrote on his official Twitter account.

In a lengthy statement that was posted on Facebook, Adams reflected on his role in the COVID-19 response.

"In the face of a once in a century pandemic, I sought to communicate the rapidly evolving science on this deadly adversary, and arm people with the knowledge and tools they needed to stay safe," he said. "I wasn’t always right -- because no one was, and this virus continues to humble all of us -- but I was always sincere in my efforts to speak to every day Americans, and address the terrible health inequities this virus exposed."

Biden has nominated former Surgeon General Vivek Murthy back to the position and as a senior adviser in the COVID-19 response. Murthy's nomination will need to be confirmed by the Senate.

Mexican President Andrés Manuel López Obrador announced on Twitter Sunday evening that he contracted the coronavirus.

Obrador said his symptoms are mild and he is undergoing medical treatment.

"As always, I'm optimistic. We'll all get by," the 67-year-old president tweeted.

Obrador had repeatedly refused to wear a mask and resisted calls for mask mandates and other restrictions despite the growing number of cases in the country.

-ABC News' Anne Laurent and Kirit Radia