New York City Mayor Bill de Blasio unveiled a two-phased vaccination plan for the city during a news conference Wednesday.

In Phase 1, when the vaccine is in limited supply, de Blasio said health care personnel, front-line and essential workers, and vulnerable groups will be prioritized.

Phase 2 comprises of the general public, the mayor said.

"The vaccine will be a crucial part of our rebirth and open the economic bridge to our recovery," de Blasio said.

He added that he does not have an exact date of when the phases would happen, and it remains contingent on an approved vaccine, but promised “it will be sooner rather than later.”

New York City's rollout plan is in line with proposed federal prioritization plans, as the push for first dibs on a vaccine is already underway.

Boston will suspend all in-person learning for its public school system, effective this Thursday, Superintendent Brenda Cassellius announced on Wednesday in a letter to employees obtained by ABC News' local affiliate WCVB.

The move came after local health officials said that the citywide seven-day COVID-19 positivity rate had increased to 5.7% -- a jump from 4.5% last week, and 4.1% two weeks before.

“We remain committed to providing in-person learning opportunities to our students as soon as it is safe to do so, and will continue to prioritize out students with the highest needs for in-person learning,” Cassellius wrote.

Boston will resume in-person learning for high-need students in public schools once the seven-day positivity rate is below 5% for two consecutive weeks. A phased-in approach will also begin for the districts youngest students when the positivity rate is below 4% for two consecutive weeks.

The seven-day average of new cases in Massachusetts has been on the rise for nearly seven weeks, increasing by 86% just in the last month.

ABC News' Arielle Mitropoulos contributed to this report.

Russia registered a record 317 deaths from COVID-19 in the last 24 hours, according to the country's coronavirus response headquarters.

The previous record of 286 fatalities from COVID-19 in a 24-hour reporting period was set just last week.

An additional 15,700 new cases of COVID-19 were also confirmed nationwide in the past 24 hours, down from a peak of 16,319 the previous day.

Nearly 28% of the new cases -- 4,389 -- and almost 20% of the deaths -- 63 -- were reported in the capital, Moscow.

The cumulative totals now stand at 1,447,335 cases and 24,952 fatalities, according to Russia's coronavirus response headquarters.

Although Russia has been breaking its own records for daily case counts and deaths almost every day since Oct. 9, authorities there are resisting shutting down businesses again. Few measures have been imposed in Moscow, the epicenter of the country's COVID-19 outbreak and recent surge.

The Eastern European country of 145 million people has the fourth-highest tally of COVID-19 cases in the world, behind only the United States, India and Brazil, according to a real-time count kept by Johns Hopkins University.

ABC News' Alina Lobzina contributed to this report.

U.S. Surgeon General Jerome Adams said Wednesday that a "herd immunity" approach to combating COVID-19 could "lead to many complications/deaths."

Adams posted the comment on his official Twitter account, along with a link to a recent article from The Journal of the American Medical Association entitled "What is Herd Immunity?"

"The summary: Large numbers of people would need to be infected to achieve herd immunity without a vaccine; this could overwhelm health care systems and lead to many complications/deaths," Adams tweeted. "So far, there is no example of a large-scale successful intentional infection-based herd immunity strategy."

Instead, Adams urged people to "wear masks," "wash hands" and "watch distances."

The surgeon general's comments come after the White House embraced a controversial declaration by a group of scientists calling for an approach that relies on "herd immunity."

The so-called Great Barrington Declaration, which claims on its website to have been signed by more than 9,000 medical and public health scientists around the globe, opposes lockdowns and argues that authorities should allow the novel coronavirus to spread among young, healthy individuals while protecting the elderly and the vulnerable.

Dr. Anthony Fauci, the nation’s top expert on infectious diseases, has called the concept "ridiculous" and "total nonsense."

A new study out Wednesday from the CDC found that multiple brief exposures within 6 feet of someone infected can heighten the risk of COVID-19 transmission.

Previously, the CDC had advised that a good rule of thumb is that you can contract the virus if you spend at least 15 minutes within 6 feet of a person with COVID-19.

In the new study, however, researchers found that a corrections officer working in a Vermont prison appears to have contracted the virus during "multiple brief encounters" with six incarcerated people who had COVID-19 but didn't know it yet. The six people were still awaiting the results of their tests.

The encounters were very brief (one minute or less), but the corrections officer overall had 22 encounters adding up to about 17 nonconsecutive minutes of possible exposure.

This scenario shows that in addition to 15 consecutive minutes within 6 feet, it might also be possible to contract the virus if you have multiple, shorter exposures that collectively add up to more than 15 minutes.

ABC News’ Sony Salzman contributed to this report.