Top US infectious disease expert Dr. Anthony Fauci has renewed his pandemic warnings and cautioned vaccinated Americans about an even worse variant that could be coming after the COVID-19 Delta variant that is taking hold across the United States.

Fauci said those who remain unvaccinated are responsible for the coronavirus mutating, which has led to Delta. The variant has caused sharp rise in the COVID cases and could lead to another surge in the fall.

There�s a tenet that everybody knows in virology: a virus will not mutate unless you allow it to replicate, Fauci said, adding, Fortunately for us, the vaccines do quite well against Delta, particularly in protecting you from severe disease.

But if you give the virus the chance to continue to change, you�re leading to a vulnerability that we might get a worse variant, and then that will impact not only the unvaccinated, that will impact the vaccinated because that variant could evade the protection of the vaccine.

The administration of President Joe Biden is scrambling to promote vaccines amid the rise in the coronavirus cases in recent weeks.

Some cities, such as Los Angeles and Las Vegas, have reimposed mask mandates in response to the new cases. New York City also became the first this month to announce proof of vaccines will be required at certain venues, gyms, and restaurants.

Fauci also said he hoped the Food and Drug Administration (FDA) would grant full approval of the coronavirus vaccines - now under emergency use authorizations - within the next month. Once that happens, Fauci said he thinks businesses, colleges and universities, and other local entities would feel empowerment to enact more of their own mandates.

The time has come, the White House�s chief coronavirus medical adviser said. We�ve got to go the extra step to get people vaccinated. You want to persuade them, that�s good ... but for those who do not want, I believe mandates at the local level need to be done.

The coronavirus cases in the United States rose 40 percent in the past week, while hospitalizations rose 36 percent in the same period. The virus cases among children are also on the rise as the school year set to begin across the country.

Randi Weingarten, president of the American Federation of Teachers, said she supported a vaccine mandate for teachers, in part to help protect children who could not yet be inoculated. She also noted that immunizations - and immunization requirements - have been in schools since the 19th century.

The US Centers for Disease Control and Prevention has changed its guidance to recommend all children wear a mask when they return to school. However, some Republican-led states have prohibited cities and local entities, including schools, from mandating masks.

Fauci�s warning came as a top American public health official said the United States is failing, as the country was recording its highest daily number of COVID-19 cases in six months.

Francis Collins, director of the National Institutes of Health, said, In that regard, yes, we are failing. We should not really have ever got to the place we are.

The US is now averaging more than 100,000 new COVID-19 infections a day, as the Delta variant has spread throughout the country. In late June, the US was averaging about 11,000 cases a day. Total daily new cases have now surged to 118,000, their highest since February.

Millions of Americans, especially in conservative areas, are skeptical of vaccines despite scientific data.