Dr. Anthony Fauci has a sobering prediction: "Things are going to get worse."

The White House chief medical advisor made those remarks amid rising Covid cases nationwide, due largely to the virus's newly dominant and more transmissible delta variant, during an interview with ABC's "This Week" on Sunday. Though it's tough to imagine a situation more dire than the country's current surge, "we're looking to some pain and suffering in the future because we're seeing the cases go up," Fauci said.

Delta has run rampant through the U.S. in recent weeks, surpassing last summer's peak of new daily cases and hitting the country's relatively large population of unvaccinated people — 50% as of Thursday afternoon — particularly hard. Experts suggest the U.S. needs a 90% vaccination rate to reach herd immunity, given delta's keen ability to spread.

If that doesn't happen, Fauci told McClatchy D.C. on Wednesday, the virus will keep spreading through the fall and winter — giving it "ample chance" to develop another, worse variant.

So, just how bad is "worse"? Here's what could transpire in the coming months, and what can be done to stop it:

New (and worse) variants could mean booster shots

So long as a virus can spread, it can mutate and create more dangerous variants. And while the Covid vaccines in use appear to work well against current variants, "there could be a variant that's lingering out there that can push aside delta," Fauci told McClatchy.

If a more vaccine-resistant variant emerges, people could need booster shots. Countries like Israel, Germany and France have already started administering third doses of mRNA vaccines as boosters — though the World Health Organization said Wednesday that it's too soon to move forward with boosters until vaccine inequalities are addressed around the world.

"We're talking about boosters in countries that have access to vaccine," Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told CNBC Make It. "Whereas we still have people all over the world who don't have any access, at this point, to even the first dose."

Most of the virus's current ability to spread across the U.S. — which has plentiful vaccine supply — is due to America's large population of unvaccinated people. About 30% of the adult population in the U.S. has not received at least one dose, and roughly 33% of eligible children ages 12 -17 have yet to receive a shot.

New data from the CDC has also raised concerns about breakthrough cases, where vaccinated people can occasionally transmit the delta variant to other people. The CDC only tracks breakthrough cases that lead to severe hospitalizations and death, but most breakthrough cases tend to be mild or asymptomatic — leading some experts to say the agency could be missing crucial real-time data on their prevalence and ability to foster new variants.

"The worry has to be that something new is going to evolve, call it epsilon or some other variant, and we need to be monitoring very carefully for that," Dr. Ezekiel J. Emanuel, vice provost for global initiatives and co-director of the Health Transformation Institute at the University of Pennsylvania, said in a briefing with the Infectious Diseases Society of America on Tuesday.

"Unfortunately, if you're missing breakthrough infections, you may be missing some evolution here, that would be very important for us to follow," Emanuel said.

Cases could climb to 100,000 or 200,000 a day

A key indicator that Covid will continue to worsen, according to Fauci: The nation's seven-day average for new daily cases is currently rising.

"Remember, just a couple of months ago, we were having about 10,000 cases a day," Fauci told McClatchy. "I think you're likely going to wind up somewhere between 100,000 and 200,000 cases." The average is currently higher than the peak last summer, before vaccines were authorized and in use. The seven-day rolling average of daily cases was 84,389 on Aug. 2, according to the CDC's data. Last year, the CDC reported about 68,700 new cases per day.

There's evidence that the surge is motivating people to get vaccinated. Louisiana, which has the highest rate of Covid cases per capita, saw the number of people getting vaccinated quadruple in recent weeks.

But even with this vaccination bump, people won't be considered "fully vaccinated" for a while. The CDC defines "fully vaccinated" as two weeks after the second dose of a two-dose regimen like Moderna or Pfizer, or two weeks after getting the Johnson & Johnson single-shot vaccine. "Even if we vaccinated everyone today, we're not going to see an effect until the middle to end of September," Fauci told McClatchy.

Social distancing and restrictions could come back

Last year's full lockdowns are unlikely to return because the country's vaccine supply is strong, and experts know more now about how Covid spreads, Fauci told ABC. But as long as vaccines remain unavailable to everyone — like school-aged children, for example — nonpharmaceutical prevention measures like masking and social distancing may come back.

In late July, the CDC walked back its mask guidance for fully vaccinated people, recommending that everyone wear masks in indoor public settings in counties where there is "substantial or high transmission," as determined by the agency's data tracker. Masks are also a good idea in areas with high vaccination rates, Althoff said, to help prevent breakthrough cases and dampen further transmission.

"Masking protects yourself and others, and with a variant that is really transmissible, masking indoors is really important right now," she said.

Restrictions on large indoor and outdoor gatherings could also return — because while canceled parties and six-feet markers in stores are certainly inconvenient, they could help prevent more restrictive measures.

"Nobody wants to go back to what we had before with the lockdowns," Althoff said.

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