The continued spread of the Delta variant of COVID-19 has put vaccination rates in the U.S. under the microscope.

Currently, 58.4% of those in the U.S. over the age of 18 are fully vaccinated, and 67.2% have received at least one dose of the vaccine. However, this varies significantly by state and, in many areas with low vaccination rates, more cases are emerging.

According to data from Johns Hopkins, states with lower vaccination rates are seeing an average of 6 cases per 100,000 every day, while states with higher rates are reporting an average of just 2.2 cases per 100,000.

It’s why public health experts are calling on Americans — both vaccinated and unvaccinated — to continue wearing masks in crowded or indoor settings until things settle down.

“Err on the side of caution,” Dr. Anthony Santella, professor of health administration and policy at the University of New Haven, told Yahoo Finance Live on Wednesday.

“That’s really my bottom line here. We’ve worn masks for so long. To wear them indoors or in crowded places just for the next few months isn’t a bad idea," he added.

'You never really know who's vaccinated'

Santella isn’t the only one encouraging others to be cautious. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID) and a member of the White House coronavirus task force, revealed that despite being fully vaccinated, he is still wearing masks in areas with low vaccination rates.

Santella is taking the same approach, and stated that he carries his mask everywhere with him.

“I use my best judgment now,” he said. “I have the luxury of being a public health scientist and I study infectious and communicable diseases, so I know a little bit more than the average person. But, I judge the situation I’m in or the surrounding area I'm in.”

Though cases are down in states with high vaccination rates, that’s not the situation with states on the opposite end of the spectrum.

For example, states like Missouri, Arkansas, Nevada, and Utah are seeing some of the biggest increases in the number of cases — yet their full vaccination rates are all below 50%, and their economies are fully open. The blurred lines make things even more difficult in terms of public safety.

“You never really know who’s vaccinated just because there are these local jurisdiction and state guidelines that say ‘if you’re unvaccinated you can do this,’ ‘if you’re vaccinated you can do this,’” Santella said. “You can’t make assumptions about people just based on if you see them wearing a mask or not.”

In the meantime, Santella and others in the public health space are urging those who are unvaccinated to reconsider opting out of getting the shot.

“While we’re in a good place in terms of the United States’ pandemic response, we can’t let down our guard,” Santella added.

'Not there yet'

COVID-19 variants like the Delta strain and the new Lambda version are calling into question the efficacy of the coronavirus vaccines, though some studies have indicated the vaccines are mostly effective against the Delta variant.

“The Delta variant is an important part of our conversation,” Santella said. “It’s very transmissible. It’s causing severe illness and hospitalizations, particularly amongst those who are unvaccinated, but this shouldn’t surprise us too much because this is what viruses do — they evolve. They mutate as they replicate and spread through the community.”

In other words, the more time that people are unvaccinated, the more opportunities the virus gets to mutate and create another variant.

Until there is widespread vaccination on a global level, Santella advised against traveling to other countries, or even parts of the U.S. with low vaccination rates, in order to curtail community spread.

“While travel is safe, you have to decide what’s best for you,” he added. “I know everyone really is fatigued when it comes to all things COVID, but we’re so close to being in a place where we can resume our normal daily activities as if they were pre-pandemic. But we’re just not there yet.”

Adriana Belmonte is a reporter and editor covering politics and health care policy for Yahoo Finance. You can follow her on Twitter @adrianambells and reach her at adriana@yahoofinance.com.

READ MORE:

• Doctor: US faces COVID-19 'pandemic within a pandemic’ as Delta variant rises

• Delta variant will 'spread like wildfire' in communities with low vaccination rates, doctor says

• Coronavirus vaccine: 'We really need to step up our game,' biocontainment expert says

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