Coronavirus infections are rising dramatically all over the U.S. as the highly contagious Delta variant spreads.

The big picture: Some “breakthrough” infections are happening to vaccinated people, but this rising tide of cases and hospitalizations is mainly a threat to those who aren’t vaccinated. And in some parts of the country, most people aren’t vaccinated — so the virus can still do serious damage.

Where it stands: Nationwide, the average number of new cases per day was up 55% over the past week.

• New cases increased in 46 states, and many of those increases are substantial.
• Florida is now averaging just under 6,500 new cases per day — by far the most of any state, and a 91% jump from the week before.
• New cases more than doubled over the past week in Mississippi — from about 320 per day to about 660 per day. The state has one of the lowest vaccination rates in the country; just 34% of its residents are fully vaccinated.

Earlier this summer, the U.S. seemed to have COVID-19 on the ropes. But now the Delta variant is sweeping through the country.

“Breakthrough infections” — people who have contracted the virus even after being vaccinated — are getting a lot of attention as cases mount. But it’s clear that those infections are not the primary driver of this new surge in cases, and that vaccinated people are much, much safer than unvaccinated people.

• 97% of people hospitalized for COVID-19 infections are unvaccinated, the CDC said last week, and federal officials have previously said that about 99% of people who die from the virus weren’t vaccinated.

By the numbers: More than 160 million Americans are fully vaccinated.

• Of those 160 million people, just 3,733 have subsequently been hospitalized for a severe COVID-19 infection, according to the CDC’s most recent update, and 791 have died from the virus.
• Clinical trials showed both the Pfizer and Moderna vaccine to be 94-95% effective at preventing serious illness and death. There will be people in the other 5-6%. That does not mean the vaccines don’t work; those cases are noteworthy precisely because they are rare.
• Real-world evidence consistently shows that the vaccines continue to offer strong protection against the Delta variant.

More evidence of the vaccines’ effectiveness came just this week, in a study — which has not yet been peer-reviewed — of health care workers in India.

• In this study of roughly 28,000 vaccinated health care workers, just 5% developed symptomatic infections after being vaccinated. Only 83 people had to be admitted to a hospital, and none died.

The bottom line: The vaccines are the most effective weapon against this pandemic, but they only work if we use them.