WASHINGTON — The United States reached another sobering milestone in the COVID-19 pandemic, with 600,000 lives lost to the disease as of Tuesday, according to Johns Hopkins University data. While daily deaths have decreased in recent months, hundreds of Americans are still dying every day from the disease that"s largely preventable by vaccines. Over the past week, the average number of COVID-19 deaths each day was 343, according to data Monday from Johns Hopkins. 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. Vaccines have been shown to be effective against these variants, but experts warn it is critical to get vaccinated as soon as possible. "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. "We are going to need to have a highly vaccinated population for years, if not longer. This virus is going to be circulating in the world for a long time." The longer a virus spreads, the more chances it has of spawning variants that are even more contagious, Offit said. But most Americans are not fully vaccinated — leaving them vulnerable to the highly contagious Alpha variant and the troubling Delta variants, which researchers say is associated with increased hospitalization rates. As of Monday, 52.5 percent of Americans had received at least one dose of vaccine, and 43.7 percent were fully vaccinated, according to data from the US Centers for Disease Control and Prevention. Offit said by the winter when the virus is likely to surge again, the US will need to get the number of fully vaccinated up to 80 percent 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. 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." California was the first state to issue a stay-at-home order during the pandemic. Now, 15 months later, it is fully reopening. Starting on Tuesday, capacity limits and social distancing requirements for all businesses will be lifted. Large events, such as concerts, conventions and sports will still have some restrictions — including vaccine verification requirements for those attending indoor events with 5,000 people or more and recommendations for outdoor events with more than 10,000 attendees. About 72 percent of Californians are at least partially vaccinated, and about 47 percent of the state"s residents are fully vaccinated, Gov. Gavin Newsom said on Monday. There will be no so-called vaccine passport, but Newsom plans to announce an electronic version of state"s vaccine cards later this week. California has invested $116.5 million in incentives for vaccinations, including gift cards and cash prizes. On Tuesday, state officials will hold a drawing where 10 vaccinated Californians will each win a $1.5 million prize. Fully vaccinated people can go without a mask in most situations, but those who are unvaccinated will still be required to wear masks in indoor public places. And masks will be mandated for everyone in certain places, including on public transportation, inside hospitals and inside jails. Businesses can require masks at their discretion, and Cal/OSHA is set to adopt new rules for face coverings in the workplace. But because that isn"t expected until the end of June, Newsom indicated he will sign an executive order later this week "to clear up any ambiguity." — CNN