The United States is dealing with a massive surge in COVID-19 cases due to the fast-spreading and highly contagious Delta variant, but one expert fears that the worst may be still yet to come.Dr. Anthony Fauci spoke out this week, warning that there could be even deadlier strains of coronavirus coming in the future. He and other experts have offered sometimes dire warnings about the dangers of low vaccination rates, which could give the virus the ability to mutate into more powerful or contagious forms.Fauci Issues WarningFauci warned this week that the low vaccination rates in the United States and across the globe are giving the virus more opportunities to mutate and become more deadly."[Q]uite frankly, we’re very lucky that the vaccines that we have now do very well against the variants — particularly against severe illness," Fauci said, via Yahoo News.“If another one comes along that has an equally high capability of transmitting but also is much more severe, then we could really be in trouble,” he added.New Strain EmergesEven as the Delta variant is spreading rapidly across the globe, public health experts warn that there could already be something worse on the way. As Yahoo News noted, officials in South Korea announced that they had detected new cases of what is known as the Delta Plus variant. This new strain has the potential to be even more contagious than the Delta variant, though more study is needed. It was not yet clear how effective the current vaccines could be at preventing infections with the new strain.States Struggle With Vaccination RatesWith COVID-19 cases going up across the United States and the more contagious Delta variant becoming the dominant strain, some states are ramping up vaccination efforts and taking steps to prevent further spread.Some of these states and cities have issued new rules and restrictions on those who have not gotten vaccinated. That included new mask mandates in many areas, the prospect that students will return in the fall wearing masks, and even some more drastic measures in the nation's largest city.New RestrictionsAs Fox 5 New York noted, New York City just mandated that people must be vaccinated in order to enter many public spaces, including restaurants. The city's mayor issued a warning to those not getting the vaccine that they could soon be missing out."This is a miraculous place, literally full of wonders," Mayor Bill de Blasio said. "If you’re vaccinated, all that’s gonna open up to you. If you have the key, you can open the door. If you’re unvaccinated, unfortunately, you will not be able to participate in many things."