The fast-spreading Delta variant of the coronavirus which emerged in Rhode Island just a few weeks ago is raising alarms in other parts of the country with lower vaccination rates.

Nationally, the Delta variant makes up about 83% of new coronavirus cases, up from about 50% the week of July 3, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said Tuesday. The dramatic increase in the variant has been blamed for a surge in new COVID-19 cases and hospitalizations in places such as New York City. 

“The best way to prevent the spread of COVID-19 variants is to prevent the spread of disease, and vaccination is the most powerful tool we have,” Walensky said during a U.S. Senate hearing Tuesday.

In Rhode Island, health officials say they are monitoring the data locally and nationally “very closely” while continuing to follow the federal CDC guidance.

Gov. Daniel J. McKee said Tuesday that he is not expecting any major problems with the Delta variant in Rhode Island that would cause the state to have to “move backward” in restricting businesses or gatherings. McKee said Rhode Island is in good shape with vaccinations, citing data showing that about 80% of adults have had at least one shot. 

Addressing those concerns in an interview Thursday with NPR's All Things Considered, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said recently on NPR’s All Things Considered studies continue to show that vaccines are not only effective against the virus, they're also highly effective at preventing serious disease or hospitalization.

But a single-dose of the two-dose Pfizer-BioNTech vaccine “barely” offers any protection against the Delta variant, according to a peer-reviewed report published in the journal Nature and reported by The Washington Post.

Almost 61% of all Rhode Islanders are now fully vaccinated, including 71% of all adults (age 18 and older). But vaccination rates vary widely in different parts of the state. 

Fewer than half of all Woonsocket residents have received at least one-dose of the vaccine, tied with Tiverton for the lowest vaccination rate in the state, according to state health department data. And unlike the more affluent, seaside community, the City of Woonsocket is more densely populated and many residents live in multi-generational households where the risk of the virus spreading is greater.

The state’s vaccination drive peaked in early April at nearly 9,200 doses per week, and declined steadily through mid-June, when it dipped below 1,000 doses, according to an analysis of state health data by The Public’s Radio. As of mid-July, the seven-day average has fallen below 700 doses.

The Pfizer vaccine is about 80% effective in preventing infection from the Delta variant, Fauci said at a recent briefing. A U.K. study found it was 96% effective in preventing hospitalizations due to the virus.

The Rhode Island Department of Health has so far identified 14 cases of the highly contagious Delta variant of the coronavirus which emerged in the state just a few weeks ago.  But that may be an undercount, state health officials said, since it takes a few weeks to perform  genetic sequencing on specimens from positive tests. 

State health officials also have tracked 1,046 “breakthrough” COVID-19 cases among residents fully vaccinated against the virus, representing less than two-tenths of 1% of the more than 626,000 people in the state immunized. That’s “testament to the tremendous effectiveness of the vaccine,” Joseph Wendelken, a health department spokesman, said in an email. 

In New York City, state health officials have reported during the first six months of this year about 1.1% of cases of the virus in people who had been fully vaccinated, according to the New York Times.

  • Ian Donnis and the Associated Press contributed to this report. 

Correction: A earlier version of this story incorrectly reported the source of the undercount.

Health reporter Lynn Arditi can be reached at Follow her on Twitter @LynnArditi