The U.S. surpassed 400,000 coronavirus deaths Tuesday afternoon, and to honor that bleak milestone President-elect Joe Biden held a memorial service Tuesday and unveiled a touching light display in honor of those victims Tuesday evening.

After 5 p.m. Tuesday, 400 lights surrounding the Lincoln Memorial’s reflecting pool were lit to honor the 400,000 people in the U.S. who have died from COVID-19. On Tuesday, Johns Hopkins University released the latest data that showed the U.S. has recorded 401,288 deaths since the pandemic began, far outpacing the rest of the world. Brazil is second in coronavirus deaths. India is second in overall cases to the 24.2 million U.S. cases.

The president-elect was joined by his wife, Jill Biden, Vice President-elect Kamala Harris and her husband Doug Emhoff.

“Tonight, we grieve and begin healing together,” Harris said. “Though we may be physically separated, we the American people are united in spirit.”

Gospel singer Yolanda Adams belted a soothing version of Leonard Cohen’s “Hallelujah” during the memorial. Biden also offered comments as several others attended the memorial that captured a beautiful image of lights on the Lincoln Memorial pool.

“To heal, we must remember. It’s hard sometimes to remember, but that is how we heal. It is important to do that as a nation,” Biden said during the ceremony. “Between sundown and dusk, let us shine the lights and the darkness the sacred pool of reflection to remember all we lost.”

Globally, more than 96 million coronavirus cases have been reported, and last week, the worldwide death toll surpassed 2 million.

The U.S. recorded 1.5 million new COVID-19 cases in the last seven days, according to the most recent data from Johns Hopkins, an 11% drop from the previous week. Cases declined in 35 states week-over-week, and 18 states saw a drop in the number of deaths, CNN reported.

But with a longer-range view, last week’s apparent improvement falls much closer to average. During the last month, the number of new COVID-19 cases recorded each day has ranged from nearly 101,000 to more than 302,000; over the past seven days, new cases averaged about 218,000 daily.

Last week, Biden unveiled a massive economic stimulus plan, a plan the Democratic president-elect is one “we need to move fast on” to provide another round of coronavirus relief to the nation.

The president-elect said his proposal, dubbed the “American Rescue Plan,” would impact millions, add jobs and reduce the instances of coronavirus.

Biden, who will be inaugurated Wednesday, plans to ask Congress for $1.9 trillion to assist with vaccination and COVID-19 reduction efforts as well as economically assist individuals and small businesses. His plan requires $415 billion, specifically for vaccination, testing, tracing and sick leave.

He also would like to give Americans direct aid, via $1,400 stimulus checks and additional $400 payments to the unemployed, adding up to about $1 trillion. An additional $440 billion will assist small businesses, local government and transit.

Biden takes office as the battle against the virus that causes COVID-19 has taken a new turn: Mutations are rapidly popping up, and the longer it takes to vaccinate people, the more likely it is that a variant that can elude current tests, treatments and vaccines could emerge.

The coronavirus is becoming more genetically diverse, and health officials say the high rate of new cases is the main reason. Each new infection gives the virus a chance to mutate as it makes copies of itself, threatening to undo the progress made so far to control the pandemic.

On Friday, the World Health Organization urged more effort to detect new variants. The U.S. Centers for Disease Control and Prevention said a new version first identified in the United Kingdom may become dominant in the U.S. by March. Although it doesn’t cause more severe illness, it will lead to more hospitalizations and deaths just because it spreads much more easily, said the CDC, warning of “a new phase of exponential growth.”

“We’re taking it really very seriously,” Dr. Anthony Fauci, the U.S. government’s top infectious disease expert, said Sunday on NBC’s “Meet the Press.”

“We need to do everything we can now ... to get transmission as low as we possibly can,” said Harvard University’s Dr. Michael Mina. “The best way to prevent mutant strains from emerging is to slow transmission.”

So far, vaccines seem to remain effective, but there are signs that some of the new mutations may undermine tests for the virus and reduce the effectiveness of antibody drugs as treatments.

“We’re in a race against time” because the virus “may stumble upon a mutation” that makes it more dangerous, said Dr. Pardis Sabeti, an evolutionary biologist at the Broad Institute of MIT and Harvard.

Younger people may be less willing to wear masks, shun crowds and take other steps to avoid infection because the current strain doesn’t seem to make them very sick, but “in one mutational change, it might,” she warned. Sabeti documented a change in the Ebola virus during the 2014 outbreak that made it much worse.

In March, just a few months after the coronavirus was discovered in China, a mutation called D614G emerged that made it more likely to spread. It soon became the dominant version in the world.

Now, after months of relative calm, “we’ve started to see some striking evolution” of the virus, biologist Trevor Bedford of the Fred Hutchinson Cancer Research Center in Seattle wrote on Twitter last week. “The fact that we’ve observed three variants of concern emerge since September suggests that there are likely more to come.”

One was first identified in the United Kingdom and quickly became dominant in parts of England. It has now been reported in at least 30 countries, including the United States.

Soon afterward, South Africa and Brazil reported new variants, and the main mutation in the version identified in Britain turned up on a different version “that’s been circulating in Ohio ... at least as far back as September,” said Dr. Dan Jones, a molecular pathologist at Ohio State University who announced that finding last week.

“The important finding here is that this is unlikely to be travel-related” and instead may reflect the virus acquiring similar mutations independently as more infections occur, Jones said.

That also suggests that travel restrictions might be ineffective, Mina said. Because the United States has so many cases, “we can breed our own variants that are just as bad or worse” as those in other countries, he said.

Some lab tests suggest the variants identified in South Africa and Brazil may be less susceptible to antibody drugs or convalescent plasma, antibody-rich blood from COVID-19 survivors — both of which help people fight off the virus.

Government scientists are “actively looking” into that possibility, Dr. Janet Woodcock of the U.S. Food and Drug Administration told reporters Thursday. The government is encouraging development of multi-antibody treatments rather than single-antibody drugs to have more ways to target the virus in case one proves ineffective, she said.

Current vaccines induce broad enough immune responses that they should remain effective, many scientists say. Enough genetic change eventually may require tweaking the vaccine formula, but “it’s probably going to be on the order of years if we use the vaccine well rather than months,” Dr. Andrew Pavia of the University of Utah said Thursday on a webcast hosted by the Infectious Diseases Society of America.

Health officials also worry that if the virus changes enough, people might get COVID-19 a second time. Reinfection currently is rare, but Brazil already confirmed a case in someone with a new variant who had been sickened with a previous version several months earlier.

“We’re seeing a lot of variants, viral diversity, because there’s a lot of virus out there,” and reducing new infections is the best way to curb it, said Dr. Adam Lauring, an infectious diseases expert at the University of Michigan in Ann Arbor.

Loyce Pace, who heads the nonprofit Global Health Council and is a member of Biden’s COVID-19 advisory board, said the same precautions scientists have been advising all along “still work and they still matter.”

“We still want people to be masking up,” she said Thursday on a webcast hosted by the Johns Hopkins Bloomberg School of Public Health.

“We still need people to limit congregating with people outside their household. We still need people to be washing their hands and really being vigilant about those public health practices, especially as these variants emerge.”

The Associated Press contributed to this report.