The continued spread of the Delta variant of COVID-19 could pose a serious risk to the U.S., particularly the unvaccinated population, experts say.

Currently, the variant accounts for 1 in 4 cases in the U.S., according to estimates from the Centers for Disease Control and Prevention (CDC). But that number could keep rising if vaccination rates don’t start picking up quickly enough. Among those ages 18 and up in the U.S., 57.4% are fully vaccinated, while 66.5% has received at least one shot, according to the latest CDC data. However, there are major disparities across different states.

“We have the short term — the Delta outbreaks, the Delta surges — which are already being seen in places like Missouri, where you have the rise of cases, rising ICU bedding problems, rising hospital admissions,” Dr. Anand Swaminathan, a New Jersey-based emergency medicine physician, told Yahoo Finance Live last week. “And then, there is the delayed consequence. As the Delta variant spreads, it can create other variants.”

CNN reported last week that Missouri has the highest number of cases — 29% — traced to the Delta variant, a mutant strain that originated in India and has since made its way through other countries. Among those ages 18 and up in the state, 56% have received at least one dose of the vaccine, while 49% are fully vaccinated, according to a New York Times analysis of data updated on Thursday. Missouri’s 7-day moving average of new cases is 898, a number that has been slowly increasing over the past month.

“You have some counties in Missouri that have as low as 20% with at least one shot in the arm,” Swaminathan said. “That is a huge problem because Delta or any other variant is going to spread like wildfire in those communities. So we’re really seeing the stark contrast. We’re seeing a divide between those areas that are more vaccinated and are getting even closer to herd immunity and those states and counties that are really lagging behind.”

Other states like Wyoming, Arkansas, and Nevada are also seeing an uptick in the number of COVID cases, according to the Mayo Clinic. Concurrently, less than 50% of the population in each of those three states has received at least one dose of a coronavirus vaccine.

“You contrast that to somewhere like New York where about 70% of people have at least one shot and we’re seeing a 40% decline in cases,” Swaminathan said. “It’s a huge, stark difference if people didn’t believe that vaccines were really important. This is a microcosm of exactly that.”

According to The New York Times' vaccine tracker, 60% of New York's population has had at least one dose of the vaccine, and cases had declined 25% in the 14-day period prior to Thursday.

“What this tells us is that this variant is going to continue to mutate, which means that unchecked spread in the U.S. is going to lead to other problems,” he added. “There’s the immediate threat of Delta itself. And then there’s the down-the-line threat of it mutating into something that may possibly be able to evade our vaccines.”

'We knew this was going to happen'

COVID hotspots usually have low vaccination rates, but reopening measures may also play a role.

In many of these states, governors lifted public health restrictions despite low vaccination rates. Every state with vaccination rates below 50% have already fully reopened with no mask mandates for indoors or outdoors, despite some states waiting until they hit a higher threshold of vaccinated residents.

“We knew this was going to happen where you have low vaccination rates and you forgo public health measures,” Swaminathan said. “You’re going to see surges within the state itself. We’re going to see these microsurges because different counties have different levels.”

There are a variety of reasons why many Americans are choosing to forgo vaccines, including fears over the vaccine’s safety or efficacy, deciding that it violates their personal liberties, unfounded conspiracy theories, or other concerns.

Data from the Kaiser Family Foundation indicates that 6% of those unvaccinated would only opt to get the shot if it became mandatory, while 14% will "definitely not" get vaccinated.

“We need to do better communication, letting them know that these vaccines are safe and they are the best way to protect themselves and their community from COVID,” Swaminathan added.

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 [email protected].

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