The U.S. vaccine rollout is going significantly better than expected, but COVID-19 cases are still going up in various parts of the country as society reopens and unvaccinated citizens mingle more.

“We are seeing so many younger patients getting sicker and infected,” Dr. Calvin Sun, an NYC-based emergency medicine physician, said on Yahoo Finance Live (video above). “A few are getting reinfected. I think it’s 1 or 2%. That’s still too many for me. One in 100 actually can mean families. If you could get reinfected for a young person, you can get reinfected for an older person, a chance I don’t want to take.”

States like Michigan, New York, and New Jersey are experiencing a rise in cases, along with several other states across the country, prompting health officials to call for renewed vigilance.

“We are literally at an inflection point,” Sun said. “I want to be proven wrong.”

'This is really neck and neck right now'

Part of the problem is that many people are either getting impatient waiting for vaccinations or aren’t planning on getting inoculated at all.

This can compromise the U.S.’s chances of reaching herd immunity, which is said to be between 75-80%, according to numerous health experts.

“If everyone delays or something happens with their vaccination program or something happens like the Johnson & Johnson plant where vaccines are no longer in supply, then we’re going to be overrun by these new variants,” Sun said. “There’s a double mutant that just popped up in California that came from India. This is really neck and neck right now. It’s really up to you. We can actually create our destinies at this moment.”

The three vaccines available in the U.S. are from Pfizer (PFE), Moderna (MRNA), and Johnson & Johnson (JNJ). Pfizer’s vaccine is the only one available to those who are ages 16 and up (the others are ages 18 and up).

Younger populations consequently continue to be exposed to unvaccinated individuals, increasing their chances of developing COVID. Furthermore, many states have rolled back their COVID restrictions despite the country being nowhere near herd immunity.

'We’re trying to outrun the mutation of the variants'

Another problem is mutant strains, or variants, of the coronavirus.

Variants out of Britain, Brazil, and South Africa have already caused issues around the world, particularly by making vaccines less effective. Health officials recommend ramping up vaccinations to minimize the number of people who could potentially be carrying a mutated form of the virus.

“If we have these vaccines and then just like in a movie, the enemy develops a new defense system to mutate for it, that’s how evolution workers,” Sun said. “We’re trying to outrun the mutation of the variants with these vaccines.”

COVID strains are able to mutate the longer they’re floating around, creating a race for herd immunity.

“I don’t want to have that false sense of security when more variants pop up and it could be that one where we let down our guard and it overruns us,” Sun said. “But we can prevent it if enough people get vaccinated. No matter how virulent this new mutant is, if there aren’t enough people to infect because we’re already protected... the new mutant can’t spread as well.”

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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