(CNN)Covid-19 infections may be on the decline, but the urgency to continue vaccinating the US population is far from over, one expert said.
"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," said Dr. Paul Offit, a member of the US Food and Drug Administration's Vaccine Advisory Committee.
As the virus spread across the globe, it mutated into more transmissible strains -- including the Alpha and Delta variants. Though vaccines are shown to be effective against these variants, experts warn it is crucial to get ahead of them with widespread immunization.
If the virus continues to spread, it has an increased chance of spawning variants that are even more contagious, Offit said.
"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," he added.
As of Monday, 52.5% of Americans had received at least one dose of vaccine, but only 43.7% 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% or higher to protect the population.
The good news, Offit said, is that although the spread of more contagious variants may be less inhibited by vaccines, the level of protection still appears to be high.
"I think that vaccines will keep you out of the hospital, will keep you out of the ICU, and will keep you from dying," Offit said.
Immune system protection may extend a year after infection, study finds
Fortunately, 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 that people who recovered from coronavirus infections may be vulnerable to new variants of the virus, but also that vaccines, especially mRNA vaccines like 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," molecular immunologist Michel Nussenzweig of The Rockefeller University, who worked on the study, told CNN.
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."
The first state to shut down fully reopens
California was the first state to issue a stay-at-home order over coronavirus concerns. And now, 15 months later, it is fully reopening.
As of Tuesday, capacity limitations and social distancing requirements for all businesses will be lifted. Though large events, like 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.
California has administered nearly 40 million vaccines, and about 72% of the state's population is at least partially vaccinated, Gov. Gavin Newsom said Monday. Approximately 47% of the state's residents are fully vaccinated.
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 a whopping $116.5 million in incentives, offering gift cards and cash prizes. Tuesday, state officials will hold a drawing where 10 people who have been vaccinated will each win a $1.5 million grand prize.
Unvaccinated people will still be required to wear face coverings in indoor public settings, but those who are inoculated can go without a mask in most situations.
Masks will still be mandated in certain places, including on public transportation and indoors in hospitals and 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's Holly Yan, Virginia Langmaid and Cheri Mossburg contributed to this report.