(CNN)As US health officials race to get more Covid-19 shots into arms to control the virus, experts now warn the country will run into another challenge in the next few weeks: vaccine supply will likely outstrip demand.
"While timing may differ by state, we estimate that across the U.S. as a whole we will likely reach a tipping point on vaccine enthusiasm in the next 2 to 4 weeks," the Kaiser Family Foundation said in a new report published Tuesday.
"Once this happens, efforts to encourage vaccination will become much harder, presenting a challenge to reaching the levels of herd immunity that are expected to be needed."
Health officials -- including Dr. Anthony Fauci -- estimate that somewhere between 70% to 85% of the country needs to be immune to the virus -- either through inoculation or previous infection -- to suppress is spread.
So far, roughly 40.1% of the population has gotten at least one Covid-19 vaccine dose, according to data from the Centers for Disease Control and Prevention (CDC). And about 26% of the population is fully vaccinated, that data shows.
A slowing vaccine demand now, experts say, could give dangerous coronavirus variants the opportunity to continue to mutate, spread and set off new surges -- and it could delay the country's return to a semblance of normalcy.
'We have slots going unfilled'
Parts of the US are already seeing fewer people sign up for a shot.
Kristy Fryman, the emergency response coordinator and public information officer for the Mercer County Health District in Ohio, told CNN on Tuesday that vaccine demand in the county is "slowing down."
The county's younger population isn't as eager to get vaccinated, Fryman said, and "have the sense that if they get Covid, it may not be as bad."
Others, she said, are opting to wait "to see how the side effects are."
"We've been going back to the drawing board trying to figure out how to get more people vaccinated but ... we can only do so much," Fryman added.
A little more than 27% of the county's residents have started their Covid-19 vaccinations, according to Ohio's Covid-19 vaccine dashboard.
Earlier in the pandemic, Mercer County was among the hardest hit parts of the state. Now, Fryman said, the county is again reporting a rise in Covid-19 cases.
"It's concerning that we're seeing an increase and that population does not want to get vaccinated," she said.
In Spring Lake, Michigan, emergency room physician Dr. Rob Davidson said Tuesday that local officials there are also growing increasingly concerned over the hesitancy they're seeing.
"We have slots going unfilled, I know in West Michigan and other parts, particularly in rural Michigan," he said.
Experts recommend people continue mask-wearing post-vaccine
For Americans who are fully vaccinated, experts said it's best to keep wearing a mask.
"If you are vaccinated, you are protecting yourself and you probably won't get sick but we don't know how long the virus is going to live in your respiratory system after you catch it," Dr. Jorge Rodriguez, internal medicine specialist and CNN medical analyst, said Tuesday.
"Therefore, you are potentially contagious to others."
As for gatherings, Rodriguez said fully vaccinated Americans should be opting to meet only with others who are also vaccinated.
Experts have highlighted that even as vaccinations climb, it will be important for people to keep following Covid-19 safety measures until the country is able to suppress the spread of the virus.
But as more shots are administered, fewer Americans are practicing public health mitigation measures, according to poll results from Axios-Ipsos published Tuesday. The poll was conducted April 16 to 19 and was made up of a representative sample of more than 1,000 US adults.
About 61% of respondents are social distancing, which is down six percentage points from last month and 13 points from two months ago.
The percentage of people wearing a mask at all times when they leave the house -- 63% -- is the lowest since the summer and down 10 percentage points since two months ago.
And, at a time when Covid-19 cases and hospitalizations are on the rise, the perceived risk of returning to pre-coronavirus life is the lowest it has ever been -- 52%.
Meanwhile, the perceived risks associated with activities like shopping in retail and grocery stores and attending sporting events is also declining.
