Cullen Veasley, 17, is receiving his second dose of the Coronavirus Disease (COVID-19) vaccine at a pop-up mobile vaccination clinic owned by the Detroit Health Department with the Detroit Public Schools Community District at Renaissance High School in Detroit, Michigan, will be held 02/26/2021.

Emily Elconin | Reuters

Seventy percent of U.S. adults received at least one shot of a Covid vaccine on Monday, about a month behind President Joe Biden’s July 4th goal, according to the CDC.

The 70% target set by Biden in May is viewed by federal health officials as a critical step in achieving what is known as herd immunity – when enough people in a particular community have antibodies to a particular disease.

While the milestone is a significant achievement for the nation, it should be viewed as a lower bound rather than an upper bound, especially as the highly contagious Delta variant spreads, health experts say.

“We have to have at least 80% of the population vaccinated to really have any form of herd immunity,” said Dr. Paul Offit, a voting member of the Food and Drug Administration’s Advisory Committee on Vaccines and Related Biological Products, in a recent interview. “It’s a pretty contagious virus.”

Dr. Natasha Bhuyan, a GP at One Medical in Phoenix, said that while the nationwide 70% mark was remarkable, local communities with lower vaccination rates were still worrying.

“Even if America hits 70% or 75%, if we continue to have zip codes and neighborhoods at 40 or 50%, there is still a risk of breakouts and hot spots,” she said. “Even if we hit the 70% milestone, we can still celebrate, but we should celebrate it with caution.”

The Centers for Disease Control and Prevention’s updated data comes almost a week after the agency flipped its previous course and advised fully vaccinated Americans living in areas with high rates of Covid infection to return to wearing face masks indoors. According to a CNBC analysis, the guidelines cover about two-thirds of the US population.

While the Delta variant hits unvaccinated people the hardest, some vaccinated people could carry higher amounts of the virus than previously thought and pass it on to others, said CDC director Dr. Rochelle Walensky last week. She added that the variant “behaves uniquely differently from previous virus strains”.

U.S. health officials claim the Covid vaccines manufactured by Pfizer-BioNTech, Moderna, and Johnson & Johnson offer high levels of protection against the variant, particularly against serious illness and death. Still, the pace of vaccination in the US has slowed in recent months.

According to the CDC, the nation reports an average of about 660,000 vaccinations per day on Sunday, far from the peak of more than 3 million daily vaccinations in mid-April, but 14% more than a week earlier.

The number of first vaccine doses has risen faster than the overall rate in the past few days, which means new people are getting their first vaccinations. According to the CDC, an average of about 432,000 first doses were given daily for the past seven days, 24% more than a week earlier. The states with the lowest vaccination rates and the worst outbreaks have the largest increases in first doses, a CNBC analysis found.

Vaccination rates vary widely across the country. According to CDC data, 20 states and the District of Columbia beat the 70 percent adult mark in one shot, with Vermont, Hawaii, Massachusetts, and Connecticut each surpassing the 80 percent mark.

Other states are lagging behind, and 12 have less than 60% of adults with at least one shot. Mississippi has the lowest rate with 50% of adults, followed by Wyoming with 52.2% and Louisiana with 53.6%.

To increase the number of vaccinations given, some state and local officials have either offered vaccination incentives or enforced mandates.

Biden last week called on state and local officials to offer residents $ 100 in cash as an incentive for a vaccination, his government’s latest attempt to vaccinate more Americans.

Biden also urged school districts across the country to host pop-up vaccination clinics in the coming weeks and directed the federal pharmacy program partners to work with schools. He also announced that a Covid reimbursement program that pays back small and medium-sized businesses that have offered paid vacations to their employees to vaccinate will be expanded to include family members and children of employees.

– CNBC’s Kevin Breuninger contributed to this report.