For the first time in over a year, the US records a daily average of fewer than 20,000 new Covid-19 cases By Christina Maxouris, Holly Yan and Madeline Holcombe, CNN

(CNN)The US just recorded a seven-day average of fewer than 20,000 new daily Covid-19 cases for the first time since March 2020. 

The daily average of new cases dropped to about 17,248 as of Monday, according to data from Johns Hopkins University. However, that number might be lower than reality, as some cases from the weekend and the Memorial Day holiday might not have been reported yet. 

Still, it's a stunning milestone that comes after more than a year of loss and suffering across the country and the world. And it's one worth pausing for, to acknowledge both that devastation but also the progress the US has made. 

In March of last year, Covid-19 infection and hospitalization numbers started climbing rapidly -- and deaths followed. At least 80% of the country's population was under stay-at-home orders. 

That was the first of several crushing surges. More than 33 million Americans have been infected with coronavirus, according to Johns Hopkins University, and more than 594,000 have died -- both numbers likely undercounts of the pandemic's true toll. 

But now, the US is heading in the right direction, thanks to a powerful ally in the battle against the pandemic: Covid-19 vaccines. 

"Cases, hospitalizations and deaths are all declining because of the millions of people who have stepped forward and done their part to protect their health and the health of their communities to move us out of this pandemic," US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said in a recent White House briefing. 

Tuesday, New York reported its lowest positivity rate since the pandemic's start, Mayor Bill de Blasio said, attributing the milestone to vaccinations and the "willpower of New Yorkers.” 

"This is ... another clear piece of evidence that New York City is coming back strong," the mayor said. "Let's drive Covid out of New York City once and for all." 

More than 50% of the US population has received at least one Covid-19 vaccine dose, CDC data shows, and more than 40% of the country is fully vaccinated. 

Governors nationwide have eased Covid-19 restrictions, and nearly every state that had a mask mandate has now lifted it. But the pandemic certainly isn't over 

The US can push its Covid-19 numbers lower and help prevent Covid-19 outbreaks if more Americans are inoculated. 

But vaccination rates across the country are uneven. Some communities lag far behind others, in some cases because of ongoing hesitancy, while in others because of challenges with access. And vaccines are available only to those 12 and older. 

"We all have more work to do," White House Covid-19 Response Team senior adviser Dr. Marcella Nunez-Smith said recently. "We have to continue to ensure everyone who is a 'yes' does not face barriers to vaccination.” 

Moderna seeks full FDA approval for its emergency authorized vaccine 

While new Covid-19 cases keep dropping as more Americans get vaccinated, Moderna said Tuesday it's seeking full approval for its vaccine from the US Food and Drug Administration. 

Since December, Moderna's two-shot vaccine has been distributed under an FDA emergency use authorization for Americans 18 and older. 

On April 13, the company announced its vaccine maintained over 90% efficacy six months out -- the length of follow-up time needed to apply for FDA approval. 

Moderna is the second company to seek such approval in the US. On May 7, Pfizer announced it was starting its own application for people 16 and older, following an April 1 announcement that its clinical trials showed over 91% efficacy after six months. 

Experts say they expect vaccine protection will last much longer than six months, to be confirmed as more data comes in. 

Moderna said it will keep submitting trial data "on a rolling basis over the coming weeks with a request for a Priority Review." A priority review asks the FDA to take action within six months, compared to the 10 months designated under standard review. 

Both Pfizer and Moderna are also studying their vaccines in children as young as 6 months. Last month, the FDA granted Pfizer's vaccine an emergency use authorization for children 12 to 15. 

Full FDA approval could motivate some vaccine-hesitant Americans to roll up their sleeves, according to research released Friday by the Kaiser Family Foundation. 

But in practical terms for the public, there's not a big difference between emergency use authorization and full FDA approval, said Dr. Paul Offit, a member of the FDA's Vaccines and Related Biological Products Advisory Committee. 

"Frankly, the only real difference was in length of follow-up" for efficacy, Offit said. 

Both the Moderna and Pfizer vaccines have shown to be extremely safe in both clinical trials and in the real world, he said. Throughout the history of vaccines, he said, any serious side effects have happened within two months after inoculation. 

"The effectiveness and efficacy data in the Phase 3 trials and now in the real world ... is excellent," Offit said. 

The first big holiday with millions fully vaccinated 

Meanwhile, for the first time in more than a year, millions of vaccinated Americans safely enjoyed close holiday gatherings without masks on Memorial Day. 

In California, "it feels very, very close to normal," Santa Monica resident Bob Alfera said. "And it's nice to see people really all in a good mood." 

But the majority of Americans still aren't fully vaccinated -- threatening the possibility of yet another post-holiday Covid-19 spike. 

Vaccinated Americans are celebrating the 1st big holiday safely without masks. But for some, returning to normal is not so easy 

Last year's Memorial Day parties included throngs of unvaccinated revelers crammed around each other. The following month, at least 16 states paused or rolled back their reopening plans to combat surges. 

Health experts hope vaccinations will blunt a post-holiday spike this year. But vaccines only work if people take them. 

"It's great news that people can see their friends, they feel comfortable to travel because they're vaccinated," former Harvard Medical School professor William Haseltine said. 

"The bad news is if you are not vaccinated, you are still at risk, and your risk is about as high as it was before." 

Unvaccinated people are still at risk as the US celebrates Memorial Day weekend maskless 

Vacationers also enjoyed Miami Beach, Florida, over the weekend, but the mayor said he worried "too many people are coming" to the scenic city. 

"The virus is still here," Mayor Dan Gelber said. "The volume of people that have been coming here is very unprecedented." 

US sees lowest child case numbers in months 

The United States also reported the lowest number of new weekly Covid-19 cases among children since early October, with roughly 34,500 new child cases reported last week, the American Academy of Pediatrics said in a Tuesday report. 

As of May 27, nearly 4 million children had tested positive for the virus since the pandemic's start. 

Children made up between 6% and 19.6% of those who were tested for Covid-19, according to the states that reported numbers, and between 5.2%-34.6% of children tested were positive for the virus, depending on the state. 

"At this time, it still appears that severe illness due to COVID-19 is rare among children," the report said. "However, there is an urgent need to collect more data on longer-term impacts of the pandemic on children, including ways the virus may harm the long-term physical health of infected children, as well as its emotional and mental health effects." 

CNN's Michael Nedelman, Jen Christensen, Laura Ly, Rebekah Riess, Naomi Thomas, Sahar Akbarzai, Pete Muntean and Greg Wallace contributed to this report.