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As the U.S. vaccine rollout gains momentum, six states are still lagging behind the rest. Alabama, Arizona, Arkansas, Georgia, Mississippi and South Carolina have administered fewer than 2,000 shots per 100,000 residents, according to the latest data from the CDC. The Trump administration announced Tuesday it would shift distribution protocol to allocate more doses for states administering more shots to avoid spoiling the vaccine. Across the country, more than 9.3 million vaccine doses have been administered, out of a total 27.6 million doses distributed.

Here are some of the biggest developments Wednesday:

• Brazilian trial finds China's Sinovac vaccine to be 50.4% effective, lower than previously reported
• What we know about the Covid risks for children and schools
• Ohio researchers identified two new Covid strains that likely originated in the U.S.

The U.S. is recording at least 247,600 new Covid-19 cases and at least 3,340 virus-related deaths each day, based on a seven-day average calculated by CNBC using Johns Hopkins University data.

The following data was compiled by Johns Hopkins University:

• Global cases: More than 91.88 million 
• Global deaths: At least 1.96 million
• U.S. cases: More than 22.90 million
• U.S. deaths: At least 382,120

J&J’s one-shot vaccine safe, generates promising immune response in early trial

Johnson & Johnson's one-dose coronavirus vaccine is safe and appears to generate a promising immune response in both young and elderly volunteers, according to early-to-mid stage trial data published in the New England Journal of Medicine.

Most of the volunteers produced detectable neutralizing antibodies, which researchers believe play an important role in defending cells against the virus, after 28 days, according to the trial data. By day 57, all volunteers had detectable antibodies, regardless of vaccine dose or age group, and remained stable for at least 71 days in the 18-to-55 age group.

The most common side effects were fever, fatigue, headache, muscle aches and pain at the injection site, according to the trial data.

If J&J's vaccine is authorized by the Food and Drug Administration, it would be the third approved for use in the U.S. behind the Pfizer-BioNTech vaccine and Moderna's. Pfizer's vaccine was authorized on Dec. 11, and Moderna's was authorized a week later on Dec. 18.

–Berkeley Lovelace Jr.

New York has turned the Javits Convention Center in Manhattan into a mass Covid-19 vaccination site, a key step toward speeding up the rollout of the life-saving doses.

The center was commissioned as an emergency hospital in the early days of the pandemic.

"It's been a challenging year for all of us – especially the events industry. But we are honored to play a role in vaccinating New Yorkers & defeating this invisible enemy," representatives for the center said in a tweet.

—Sara Salinas

California will expand vaccine eligibility to everyone 65 and over

California Gov. Gavin Newsom said the state will expand its vaccine eligibility to include everyone over the age of 65. The shift follows a guideline change from the CDC and similar moves by New York and New Jersey.

California has administered nearly 900,000 of the 3.4 million doses it has received to date, representing roughly 2,200 administered shots per 100,000 residents, according to the CDC.

Parts of the state have seen devastating surges in Covid-19 cases in recent weeks, overwhelming health networks and leading to scores of virus deaths.

—Sara Salinas

U.S. has now administered 10 million Covid vaccine doses

The United States has administered more than 10 million doses of Covid-19 vaccines, though there still remains a significant gap between the number of vaccines given and the 29.4 million doses that have now been delivered, according to updated data from the Centers for Disease Control and Prevention.

The new milestone comes a day after the federal government announced new efforts to pick up the pace of vaccinations. The U.S. will no longer keep a reserve of second doses for the two-dose vaccines and advised states to expand vaccine eligibility to everyone age 65 and older and to those with underlying health conditions.

—Noah Higgins-Dunn

U.S. reports record daily Covid deaths as experts warn of worsening outbreak

The United States reported a record 4,327 coronavirus fatalities on Tuesday, the second time in just a week the nation's daily death toll has surpassed 4,000, according to data compiled by Johns Hopkins University.

Medical experts say the U.S. is now in its post-holiday surge, and the situation will like worsen before it improves.

"We're in a very difficult situation, and it's getting worse," White House health advisor Dr. Anthony Fauci said Tuesday during the Schmidt Futures' Forum on Preparedness.

"I hope that as we get towards the end of January that we'll see a peaking and a turning around, particularly if people hang in there and don't get discouraged by Covid-19 fatigue and let down on their public health measures," Fauci said.

—Noah Higgins-Dunn

Marco Rubio sends letter to Biden calling for $2,000 Covid relief checks

GOP Sen. Marco Rubio of Florida wants President-elect Joe Biden to push for $2,000 stimulus checks on his first day in office.

"It would send a powerful message to the American people if, on the first day of your presidency, you called on the House and Senate to send you legislation to increase the direct economic impact payments to Americans struggling due to the pandemic from $600 to $2,000," Rubio wrote in a letter to Biden dated Tuesday.

Biden, who supports $2,000 direct payments to combat the Covid-19 economic downturn, is expected to lay out his economic relief agenda on Thursday. He will be inaugurated on Jan. 20.

The Democratic former vice president, whose party will soon control both houses of Congress, has said his plan will call for trillions of dollars in spending. Rubio urged Biden not to let the stimulus checks "get caught up in the normal political games by adding a wish list of far-left or other unrelated priorities to this legislation."

—Tucker Higgins

Nebraska Gov. Pete Ricketts on declining hospitalizaitons

Nebraska Gov. Pete Ricketts joins CNBC's "Squawk on the Street" to discuss the state's response to Covid-19 and the hospitalization numbers in the state.

Ohio researchers identified two new Covid strains that likely originated in the U.S.

Ohio State University researchers said they've discovered two new Covid-19 variants that likely originated in the U.S. — one of which became the dominant strain in Columbus, Ohio, in late December and January, reports CNBC's Will Feuer.

The U.S. mutations — like a new strain first detected in the U.K. — appear to make Covid more contagious but don't seem likely to diminish the effectiveness of the vaccine, researchers said.

One of the new strains, found in just one patient in Ohio, contains a mutation identical to the now-dominant variant in the U.K. However, the "Columbus strain," which the researchers said has become dominant in the city, includes "three other gene mutations not previously seen together in SARS-CoV2."

The researchers have not yet published their full findings but said a non-peer-reviewed study is forthcoming.

—Melodie Warner 

What we know about the Covid risks for children and schools

A growing body of evidence appears to indicate that children attending school do play an important role in community transmission of Covid-19. It comes at a time when the science around the link between kids and the spread of the coronavirus is still evolving.

The polarizing issue of whether schools should stay open amid the pandemic is far from clear cut. The World Health Organization has recommended that policymakers take a risk-based approach to maximize the benefit for the population.

"Studies from interventions across hundreds of countries across the globe have consistently shown that school closures are associated with a reduction in R (or reproduction rate), and openings with an increase," Dr. Deepti Gurdasani, a clinical epidemiologist at Queen Mary University of London, told CNBC via email.

"Acknowledging the impact of schools on community transmission is crucial to ensuring we minimise risk of transmission within schools, as well as from schools into the community."

—Sam Meredith

Dollar General offers extra pay to encourage Covid vaccines

Dollar General will offer four hours of pay to workers who get a Covid-19 vaccine, The Wall Street Journal reports.

The discount retailer, which has about 157,000 employees, will also provide paid time off to anyone who experiences adverse reactions to the shots. The vaccine-incentive program won't have a material impact on the company's finances, a spokesman said to The Journal.

Dollar General was deemed an essential retailer and saw higher sales as shoppers restocked their pantries more frequently during the pandemic. The company also launched a new store concept and accelerated plans to open more locations.

—Melodie Warner 

U.S. stocks open flat amid political uncertainty

U.S. stocks opened flat as the market digests the latest inflation data, political uncertainty and a still-raging pandemic, reports CNBC's Fred Imbert and Pippa Stevens.

The Dow Jones Industrial Average dipped 31 points, or 0.1%. The S&P 500 hovered just above the flatline and the Nasdaq Composite climbed 0.2%.

—Melodie Warner 

NJ governor says state will soon expand vaccine eligibility to everyone over 65

New Jersey Gov. Phil Murphy told CNBC Wednesday the state will soon expand vaccine eligibility to everyone over 65, in line with new CDC guidelines to speed up the country's vaccine rollout.

New Jersey will make that change within the next couple of days, Murphy said. Neighboring New York state made a similar announcement Tuesday.

"We were actually getting there on our own," Murphy said on "Squawk Box." "We had hospitals saying, 'Listen, we want to open up to beyond just our health care workers.' We were already working toward that objective, and the CDC's blessing, the incoming Biden administration support for this — all of those converge. And we're going to take that step shortly."

—Sara Salinas

The latest on U.S. hospitalization numbers

U.S. hospitalizations are showing the earliest signs of a plateau, hovering around 130,000 patients for a few days after a months-long dramatic rise. Health systems continue to be overwhelmed by that volume, while also trying to balance a vaccine rollout.

—Sara Salinas

Target will shut stores Thanksgiving Day 2021, after closing them during pandemic

The pandemic is already shaking up next holiday season. Target said Wednesday that its stores will be closed on Thanksgiving Day of 2021 after it opted to keep them closed during the health crisis.

Other retailers, including Walmart, Best Buy and Kohl's, also kept their stores closed this past Thanksgiving — but have not yet announced plans for the 2021 holiday. They also tried to thin crowds over the peak shopping season by kicking off sales as early as October and moving more deals online.

The new cadence of the holidays didn't hurt Target's sales. Comparable sales grew by 17.2% in November and December as digital sales more than doubled. Family sleepwear sets, gingerbread house kits and Christmas tree ornaments were among the top sellers as people spent a cozy holiday at home during the pandemic.

—Melissa Repko

Scammers claim to sell Covid-19 vaccines on the dark web in return for bitcoin

Cybersecurity firm Check Point has uncovered a number of people on the so-called dark web claiming to sell Covid-19 vaccines.

The vendors were asking for payments in bitcoin but did not deliver the goods. Listings for vaccines had price tags as high as $1,000 worth of bitcoins.

A number of listings also appeared to contradict official medical guidance on doses. 

Check Point said the number of coronavirus vaccine ads on the dark web has surged since the last time they checked in December. The spike could be the result of growing excitement and lengthening waits around vaccines.

"We believe this is because of a spike in demand from individuals who don't wish to wait weeks or months to receive their vaccination from their countries' governments," the company said in a blog post.

—Arjun Kharpal

Brazilian trial finds China's Sinovac vaccine to be 50.4% effective

Brazil's Butantan Institute said the coronavirus vaccine developed by China's Sinovac Biotech was just 50% effective, according to multiple media reports.

That efficacy rate is lower than what the institute had earlier announced.

Butantan said last week the CoronaVac vaccine was 78% effective among volunteers with "mild" to "severe" infections, but on Tuesday said the overall efficacy rate fell to 50.4% after including "very mild" cases that did not require medical assistance, The Wall Street Journal reported.

Still, the trial results met the minimum 50% efficacy rate stipulated by Brazilian health regulator Anvisa for Covid vaccines. The regulator will meet Sunday to decide on whether to approve CoronaVac for emergency use, Reuters reports.

—Yen Nee Lee

Read CNBC's previous live coverage here:

Covid updates: Operation Warp Speed chief resigns; U.S. lowers age for vaccine eligibility