BALTIMORE – Imagine a race where not only is your opponent finding ways to run faster, but hurdles pop up unpredictably and the finish line keeps moving.

This is where COVID-19 vaccination efforts are, health experts say, seeking to quickly immunize more people against as a coronavirus that has mutated into faster-spreading variants.

Despite the challenges that poses, many believe there is reason for hope that at some indefinable point, we will reach herd immunity – when so much of the population is immune that the virus’ ability to spread drops dramatically.

“I see a lot of optimism for where we are,” said Gypsyamber D’Souza, an epidemiology professor at the Johns Hopkins Bloomberg School of Public Health. “That doesn’t mean there aren’t barriers – there are multiple barriers,” she said. “It just makes our race a little harder.”

The stepped-up pace of vaccinations, in which about 3 million doses are administered every day in the U.S., has many envisioning a return to some semblance of normalcy after more than a year shadowed by the pandemic and the restrictions imposed to curtail it.

About 20% of the country is fully vaccinated, and while there is some disagreement on the threshold for herd immunity, experts often place it at 70% of the population having some kind of immunity. That would include both those who have been vaccinated and those with antibodies from previous infections – groups with some overlap because doctors recommend even those who have had the disease get vaccinated.

As the focus remains on vaccinations, a recent rise in COVID-19 infections and hospitalizations in several states has raised fears of a fourth surge in cases.

Concern is centered on the so-called British variant of the coronavirus, known as B.1.1.7, which is now responsible for most of the new infections in the U.S. The variant is considerably more contagious and fatal than the original virus – although current vaccines are effective against it and overall COVID deaths remain on a downward trend.

“It’s another reason to have more people vaccinated,” said Dr. James Campbell, a pediatric infectious disease specialist at the University of Maryland School of Medicine.

The more people who are immune to the virus, the less chance it has to continue mutating into ever more dangerous variants, he said.

Timetables vary for when we might achieve herd immunity, and depend on some largely unknowable factors: How quickly can vaccines get distributed? How many of those currently resisting them might change their minds? When will the FDA approve vaccines for use on children? Will more jurisdictions lift pandemic restrictions, and how closely will people adhere to existing ones and help prevent transmission?

“We’re seeing a lot of states removing mask mandates or opening up businesses too soon,” said Dr. Wilbur H. Chen, an adult infectious disease physician at the University of Maryland Medical School. “Or people not adhering to mask mandates.

“I think people are tired of this so there might not be the compliance we need,” said Chen, who advises Republican Gov. Larry Hogan and serves on the national committee that makes recommendations to the CDC on how to use and distribute vaccines.

Still, Chen said he is optimistic that rising vaccine rates will prevent a repeat of this past winter, when COVID cases and deaths spiked.