As the comeback COVID-19 continues to ravage hospitals across the United States, health authorities Friday warned against the coronavirus outbreak morphing into a battle against vaccination as they pleaded to the public to roll up their sleeves and get their jabs.

"There is a clear message that is coming through: this is becoming a pandemic of the unvaccinated," Centers for Disease Control and Prevention (CDC) director Rochelle Walensky told reporters.

The agency reported more than 33,000 new cases Thursday, bringing the seven-day average up to 26,306, a 70% rise on the week before. The seven-day average of hospital admissions is about 2,790 per day, an increase of 36%. And after weeks of declines, the seven-day average of deaths was 211, an increase of 26%.

The spikes are focused in communities with low vaccination rates, and "unvaccinated Americans account for virtually all recent COVID-19 hospitalizations and deaths" said Jeff Zients, White House COVID-19 response coordinator.

The new wave is driven by the delta variant, which now accounts for more than 80% of new cases, according to the covSpectrum tracker.

A recent study in the journal Virological shows delta grows more rapidly inside the body compared to past strains, and people who are infected shed much more of it in the air, greatly increasing the likelihood it will be transmitted.

Vaccines, including those made by Pfizer-BioNTech, Moderna and Johnson & Johnson, remain highly effective against the variant, but the U.S.' immunization campaign has drastically slowed in recent weeks.

President Joe Biden set July 4 as a target for 70% of adults to have received one or more doses, but as of July 15, the figure was still only 67.9%. At the present rate, the goal would not be achieved until the end of the month.

Parts of the country that voted for former Republican President Donald Trump in the 2020 election have significantly lower vaccination rates than those which voted for Democrat Biden, and are now at the center of the surges. Hot spots include Missouri, Arkansas and Louisiana.

But health officials are hopeful that, since 80% of the most vulnerable age group of over-65s are fully vaccinated, the rise in hospitalizations and deaths would not be as dramatic as the spike in cases. This would follow the pattern seen in Israel and Britain, highly vaccinated countries struck by delta waves.

An expert panel convened by the CDC will next week be examining whether immune-compromised people, whose bodies mounted a subpar response to COVID-19 vaccines, may require a third dose, said Walensky.