Dr. Soumya Swaminathan, chief scientist at the World Health Organization, urged people to keep practicing public health measures like mask wearing and social distancing while COVID-19 vaccine production scales up enough to reach billions of people worldwide.

"So, we have to be here a little bit patient, the vaccines are going to come, they're going to go to all countries but meanwhile we mustn't forget there are measures that work," Swaminathan said.

People around the world will need to keep practicing fundamental public health measures at least "for the rest of this year at least," according to Swaminathan.

"Even as vaccines start protecting the most vulnerable, we're not going to achieve any levels of population immunity, or herd immunity in 2021," she added.

-ABC News' Christine Theodorou contributed to this report.

President-elect Joe Biden got his second dose of the two-dose Pfizer/BioNTech vaccine on Monday in Newark, Delaware.

After two doses, the Pfizer vaccine is 95% effective.

U.S. Rep. Bonnie Watson Coleman, D-N.J. announced Monday that she has received a positive result for COVID-19 from a rapid antigen test and is awaiting results from a polymerase chain reaction (PCR) test.

Coleman said she was forced to shelter in place with unmasked colleagues last week when a mob of President Donald Trump's supporters stormed the U.S. Capitol building in Washington, D.C.

"Following the events of Wednesday, including sheltering with several colleagues who refused to wear masks, I decided to take a Covid test," Coleman, 75, wrote on Twitter. "I have tested positive."

The congresswoman, who previously received one dose of the Pfizer/BioNTech vaccine, said she is experiencing mild, cold-like symptoms and is isolating at home.

-ABC News' Mariam Khan contributed to this report.

Japan has identified a new variant of COVID-19, Dr. Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said during a Monday news conference.

Japanese health officials alerted the WHO of the new variant, which is different than COVID-19 variants first identified in the U.K. and in South Africa, over the weekend. High transmission of the virus in much of the world is giving it more chances to mutate and evolve into new variants.

"Most notably, transmissibility of some variants of the virus appears to be increasing," Tedros said. "This can drive a surge of cases and hospitalizations, which is highly problematic for health workers and hospitals already close to breaking point," he added.

"At present, the variants do not seem to show increased severity of disease," Tedros said.

-ABC News' Christine Theodorou contributed to this report.

Mississippi has allotted its entire supply of COVID-19 vaccines and doesn't expect more doses until mid-February, the state health department announced Wednesday.

"Neither the county health department drive-through sites, nor the [University of Mississippi Medical Center] vaccine scheduling website was designed to accommodate the monumental surge we are currently experiencing," the Mississippi State Department of Health said in a statement. "At this time, we have no additional vaccine, and every appointment is tied to an actual vaccination."

New appointments are expected to resume after an anticipated vaccine resupply in mid-February, the department said.

On Tuesday, the state opened vaccine eligibility to people ages 65 and older and those with certain chronic health conditions, which led to an influx of calls to the state's COVID-19 hotline, officials said. Previously, vaccinations were prioritized for health care workers, long-term care residents and those over the age of 75.

"We decided to open vaccines to many more people. We knew it would cause a rush, but believe it is more fair than having government arbitrarily limit access," Gov. Tate Reeves said on Twitter Wednesday evening.

As of Tuesday, the state had administered 62,744 vaccinations over the last four weeks, Reeves said during a press briefing.

ABC News' Will Gretsky contributed to this report.