CNN  — 

Tuesday brought reopenings in two of the largest states, with New York reaching an important vaccine milestone and joining California in dropping most of its Covid-19 restrictions.

“This is a momentous day, and we deserve it because it has been a long, long road,” New York Gov. Andrew Cuomo said. “We can now return to life as we know it.”

At least 70% of New Yorkers have received at least one dose of a Covid-19 vaccine dose, prompting the lifting of pandemic mitigation measures.

On the other side of the country, California, the first state to shut down last year, ended limits on capacity at most venues and stopped requiring physical distancing and masks for people who have been vaccinated.

“I’m very happy about it. I think as long as we’re safe, it’s great,” San Francisco resident Jim Bloedau told CNN. “Go out and get vaccinated. It’s safe and everybody should take care of themselves and take care of each other.”

The reopenings are occurring in the two states that have seen the most deaths from Covid-19 – California with more than 63,000 and New York with 53,500. Tuesday, the US death toll from the pandemic passed 600,000.

Cuomo noted that at one point, his state had a Covid-19 positivity rate of 48.16% – once the highest positivity rate in the world. The governor said the positivity rate is now 0.40% – the lowest rate in the country, according to Johns Hopkins University.

“We were alone, and it was frightening. It was like living through a science fiction movie … people abandoned New York, but others stayed and others fought,” Cuomo said. “Where are we today? We have the lowest positivity rate in the United States of America. … We went literally from worst to first.”

All state-mandated restrictions were immediately lifted across all commercial and social settings, including the requirements on social gatherings, capacity restrictions, social distancing, cleaning and disinfection protocols, and contact tracing.

Mask requirements will continue in pre-K settings, on public transit and in health care settings, Cuomo said.

“We’re no longer just surviving. We’re not in our homes afraid to go out,” the governor said. “We’re not in our homes disinfecting everything we that we can see. Life is not about survival. Life is about thriving.”

The governor also announced that all of the state’s assets, including the Empire State Building, will be lit in blue and gold Tuesday night. Additionally, firework shows will occur at 9:15 p.m. ET at various sites across the state Tuesday night.

Cuomo warned there are still things to be concerned about as New York recovers.

“We still have to watch these variants of interest. We still have to be careful.”

Variants of concern worry experts

As coronavirus keeps spreading across the globe, it has mutated into more transmissible strains – including the Alpha (B.1.1.7) and Delta (B.1.617.2) variants.

Both strains are now considered “variants of concern” by the US Centers for Disease Control and Prevention, which added the Delta variant now accounts for close to 10% of US cases.

That means there’s “evidence of an increase in transmissibility, more severe disease (e.g., increased hospitalizations or deaths), significant reduction in neutralization by antibodies generated during previous infection or vaccination, reduced effectiveness of treatments or vaccines, or diagnostic detection failures,” the CDC says.

The Alpha variant has become the dominant strain in the United States. But cases of the Delta variant are rising.

Recent studies suggest the Delta variant isn’t just more contagious – it can also cause more severe illness than the Alpha variant.

A study from Scotland published Monday found the Delta variant was associated with about double the risk of hospitalization compared with the Alpha variant.

Getting vaccinated can stave off those variants. But experts say it’s critical to get vaccinated as soon as possible – before the virus mutates into more concerning strains.

It’s ‘crunch time’ to get vaccinated

“I’m extremely worried because the Delta variant is so aggressive in terms of transmission,” said Dr. Peter Hotez, director of the Texas Children’s Hospital Center for Vaccine Development.

“If we remember when we first talked about the UK variant – the B.1.1.7 variant across the British Isles that started out of southeast England – we were horrified about how aggressive it was infecting people. It was a much higher level of transmission,” Hotez said Tuesday.

“This one, the Delta, is even higher than that. And so essentially what’s happening is anyone who’s unvaccinated is at very high risk now of getting this Delta variant.”

Hotez said now it’s “crunch” time for eligible Americans – those ages 12 and older – to get vaccinated.

“This is the time for everyone to get vaccinated, because even if you want to get yourself vaccinated tomorrow or your adolescent child tomorrow, it’s still going to take five to six weeks to get both of those doses of vaccine and then another week after that,” he said.

More contagious variants = more people need to be vaccinated, doctor says

“When a virus is more contagious, you need to have a higher percentage of the population that is protected – immunized – if you’re going to stop the spread,” said Dr. Paul Offit, director of the Vaccine Education Center at the Children’s Hospital in Philadelphia.

“This virus is going to be circulating in the world for a long time.”

And the longer a virus spreads, the more chances it has of spawning variants that are even more contagious, Offit said.

But most Americans aren’t fully vaccinated – leaving them vulnerable to the Alpha and Delta variants.

As of Tuesday, 52.6% of Americans had received at least one dose of vaccine, and 43.9% were fully vaccinated, according to CDC data.

Offit said by the winter, when the virus is likely to surge again, the US will need to get the number of fully vaccinated people up to 80% or higher to protect the population.

The good news: While the spread of more contagious variants may be less inhibited by vaccines, the level of protection still appears to be high, Offit said.

“I think that vaccines will keep you out of the hospital, will keep you out of the ICU, and will keep you from dying,” he said.

Those who’ve had Covid-19 should get vaccinated to be ‘bulletproof,’ researcher says

People who have been infected with Covid-19 appear to maintain their immune response for at least a year, according to a study published in the journal Nature.

Researchers reported that the human immune system’s memory B-cells continue generating protection against Covid-19 for at least a year.

Studies have shown people who recovered from coronavirus infections may be vulnerable to new variants of the virus. But vaccines, especially mRNA vaccines such as those made by Pfizer/BioNTech and Moderna, generate a strong response that protects people against those variants.

An extra boost with a vaccine may expand the protection that recovered patients have, researchers said.

“The data suggest that immunity in convalescent individuals will be very long lasting and that convalescent individuals who receive available mRNA vaccines will produce antibodies and memory B cells that should be protective against circulating SARS-CoV-2 variants,” the researchers wrote.

“The antibodies produced by the memory cells evolved increased breadth and potency,” said molecular immunologist Michel Nussenzweig of The Rockefeller University, who worked on the study.

Nussenzweig said the research should encourage people who previously had Covid-19 to get vaccinated if they haven’t already. “Yes, they should get vaccinated,” he said. “And if they do, they should be bulletproof for SARS-CoV-2.”

CNN’s Arman Azad, Maggie Fox, Naomi Thomas, Sarah Dean, Jen Christensen, Virginia Langmaid and Cheri Mossburg contributed to this report.