While testifying at a Senate hearing Wednesday, Robert Redfield stood by his timeline on when most Americans would be vaccinated. He said the expectation is that millions of doses will be ready by April and that it could take until summer 2021 to get the vaccine to most Americans.

“I think that’s going to take us to April, May, June, possibly July to get the entire American public completely vaccinated," said Redfield, director of the CDC.

When Dr. Anthony Fauci testified he said that about 50 million doses of all the viable vaccine candidates will be available in November, and more in December, and that those doses will be prioritized to health care providers and vulnerable populations.

ABC News' Stephanie Ebbs contributed to this report.

At a Senate hearing on COVID-19, Dr. Anthony Fauci on Wednesday addressed some lesser-publicized side effects seen among some COVID-19 "long-haulers": heart inflammation and cognitive abnormalities.

"A disturbing number of individuals" who have recovered from COVID-19 and "apparently are asymptomatic," "when they have sensitive imaging technologies such as magnetic resonance imaging or MRI," they're found to "have inflammation of the heart," Fauci said.

Fauci also said a symptom among COVID-19 "long-haulers" is "cognitive abnormalities," like the inability to concentrate.

"These are the kinds of things that tell us we must be humbled that we do not completely understand the nature of this illness,” stressed Fauci, director of the National Institute of Allergy and Infectious Diseases.

Robert Redfield, director of the Centers for Disease Control and Prevention (CDC), also testified Wednesday, noting that young people -- 18 to 25 year olds -- are making up 26% of new infections.

ABC News' Anne Flaherty contributed to this report.

As COVID-19 cases rise, the Navajo Nation is re-issuing a strict stay-at-home order and a 57-hour weekend lockdown. The increase in cases is in the Sage Memorial Hospital service area in Arizona and in satellite chapters in the Eastern Navajo Agency.

Many of these new cases are a result of family gatherings and people traveling to areas outside of the Navajo Nation and returning with the virus, Navajo Nation President Jonathan Nez said in a statement.

"These new cluster cases are very troubling because we do not yet know the extent to which these individuals came into contact with people in the general public,” Nez said Tuesday.

The lockdown begins at 8 p.m. on Sept. 25 and ends at 5 a.m. on Sept. 28. There is also a daily curfew from 9 p.m. to 5 a.m. on weekdays.

The Navajo Nation previously implemented public health emergency orders that restricted in-person gatherings and traveling off the Nation.

“We have told our people repeatedly that there remains substantial risk if you choose to travel off the Nation and hold family gatherings. Cities and towns near the Navajo Nation continue to see large increases in daily COVID-19 cases. It only takes a few positive cases to lead to another surge and we all know that our health care system cannot handle another large surge," Nez said.

As of Sept. 22, there have been more than 10,000 COVID-19 cases in the Navajo Nation, with at least 548 deaths.

In the city that was hit hardest by the coronavirus pandemic, cases of COVID-19 are on the rise in six New York City neighborhoods, according to local health officials.

Due to an uptick in the Borough Park, Kew Gardens, Midwood, Edgemere-Far Rockaway, Flatbush, Bensonhurst and Williamsburg communities, the city announced a targeted response to slow the spread in these areas as part of its "Get Test Tuesday" initiative.

The Midwood, Borough Park and Bensonhurst neighborhoods saw a 4.71% increase in COVID-19 cases from the week ending Aug. 1 to the week ending Sept. 19. In that same period, Far Rockaway saw a 3.69% increase, Kew Gardens a 2.24% increase and Williamsburg a 2% increase.

The increases in these areas make up 20% of all cases citywide.

"At this point in time, these increases could potentially evolve into more widespread community transmission and spread to other neighborhoods unless action is taken," the city said in a statement.

To help combat the rising cases in these areas, Dave Chokshi, commissioner of health of the City of New York, said the city would increase its presence, communication and add testing in these neighborhoods.

"... we'll launch robocalls and WhatsApp messages, communications with houses of worship, core-four palm cards to businesses, distribution of masks, gloves, and hand sanitizer. We'll place additional ads in community newspapers and we'll offer new point-of-care testing resources in these six neighborhoods at both Urgent Care and community provider offices," Chokshi said at a press conference Tuesday. "We aim to be seen and heard, so sound trucks will broadcast core four messages in these neighborhoods as well."

Since the beginning of the pandemic, New York City has had more than 23,000 coronavirus-related deaths and 236,000 cases.

Mayor Bill de Blasio said with these upticks, combined with cooler weather, it's important for New Yorkers to continue to avoid large indoor gatherings.

"There are clear rules from the state on this," de Blasio said at a press conference Tuesday. "Large gatherings are still a problem both legally and in terms of the health problem they create. So, we need people to avoid that."

Dr. Blythe Adamson, a former member of the White House coronavirus task force, told "GMA3" on Wednesday, "One of things that we just learned recently -- and published in a new study today in Clinical and Infectious Diseases -- is that being indoors, you're 20 times more likely to catch COVID from an infectious person than if you were around them outdoors."

Adamson is now adviser to Testing for America, a nonprofit established to help solve the testing crisis.

Adamson stressed that the U.S. must "be flexible and adapt, so as we learn more about the science, we're willing to change our public policies so that they match the best science."

"There's a lot of work for us to continue to do over the next couple of months. As we move forward into flu season, it's even more important that we're able to distinguish between a viral infection that's from influenza or coronavirus," she said.