As the number of coronavirus infections among unvaccinated Americans continues to rise, Anthony S. Fauci, the nation’s top infectious-disease expert, said on Sunday that the country is in a “unnecessary predicament” at this point in the pandemic.

The highly virulent delta variant, which continues to spread rapidly across the country, is to blame for the latest surge in infections. Although the Centers for Disease Control and Prevention’s official advice has not changed, Fauci hinted that a return to indoor mask mandates for vaccinated people and booster shots may be required to stop the virus’s spread. Fauci echoed CDC Director Rochelle Walensky, who stated that the majority of infections have occurred among the country’s unvaccinated population.

“It is truly a pandemic among the unvaccinated, so this is primarily an issue among the unvaccinated, which is why we’re out there practically pleading with the unvaccinated to go out and get vaccinated,” said Fauci, who described the disease as still “in retreat” among vaccinated people.

More than 163 million people have been fully vaccinated, accounting for less than half of the eligible population. Vaccination rates have varied greatly from state to state, with 30 states vaccinating less than half of their eligible populations.

As government officials continue to review data on vaccine efficacy, Fauci discussed the possibility of booster shots, saying that a recommendation to do so for certain vulnerable populations, such as organ transplant and cancer patients, is “likely.” The CDC is investigating options for distributing additional doses to these populations, amid growing concerns about the virus’s waning immunity.

With the resurgence of infections, many local governments have begun to reintroduce more stringent coronavirus restrictions. Mask mandates have been implemented in both St. Louis and Los Angeles County, requiring everyone, including those who have been vaccinated, to wear face coverings indoors and on public transportation. Many health officials have also stated that mask mandates may be necessary to combat what has been dubbed a “pandemic of the unvaccinated.” Fauci acknowledged that federal officials are “actively considering” issuing updated guidance.

“When you look at what’s going on locally, in the trenches, in places like Los Angeles County, local officials have the ability and discretion, and the CDC agrees with that ability and discretion,” Fauci said. According to new projections from the Covid-19 Scenario Modeling Hub, infections will continue to rise throughout the summer and fall. According to the model’s most likely scenario, the most recent wave of infections would peak in mid-October, with approximately 60,000 new cases and 850 deaths per day.

“If you look at the modeling that has been done over the last 18 months historically, it has been pretty accurate for the most part,” Fauci said.

Later, President Biden’s chief medical adviser added, “We have the tools to blunt that and make that model wrong, but if we don’t vaccinate people, the model will predict that we’re going to be in trouble as we continue to get more and more cases.”

With vaccine hesitancy and outright resistance persisting in many communities, Fauci emphasized the importance of other officials continuing to promote vaccinations.

He also praised Republican officials such as Arkansas Gov. Asa Hutchinson, Florida Gov. Ron DeSantis, and Rep. Steve Scalise (La.) for recent remarks encouraging people to get vaccinated, which come after months of opposition from many in the GOP. Arkansas, Florida and Louisiana have some of the lowest vaccination rates in the country.

“We’ve got to get more people who relate well to the individuals who aren’t getting vaccinated to get out there and encourage them to get vaccinated,” Fauci said.