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According to the White House COVID-19 Task Force, severe breakthrough infections remain rare, and nearly all of these hospitalized patients -- 97% -- are unvaccinated.

Earlier this week, the popular summer destination of Provincetown, Massachusetts, was thrust into the spotlight after at least 551 COVID-19 infections were confirmed, many of them breakthroughs, following a busy July Fourth weekend.

Confirmed cases among Massachusetts residents, stemming from the Provincetown cluster, have been found to be predominantly symptomatic, with 69% of affected individuals reported to be fully vaccinated, according to local officials. Apart from three hospitalizations -- two in state and one out of state -- symptoms from cases associated with this cluster are known to be mild and without complication, Alex Morse, the town manager for Provincetown, said.

\"The transmissibility of the delta variant raises the likelihood of sporadic 'super spreader' events among vaccinated people, especially when indoors and in close proximity without masks. These events raise the risk to those unvaccinated while the vast majority of the breakthrough cases will be mild or asymptomatic,\" Brownstein added.

Statewide in Massachusetts, state health officials report there have been at least 5,166 breakthrough infections as of July 17. More than 4,800 of these infections resulted in no hospitalization or death. A total of 80 of these breakthrough cases resulted in death, representing 0.0015% of individuals fully vaccinated -- and 272 cases resulted in hospitalization, representing 0.006% of those fully vaccinated.

The hospitalizations and deaths that do occur among fully vaccinated individuals tend to occur among people who are older or those with serious underlying medical conditions for whom the vaccines may have reduced efficacy, experts said.

\"PHOTO:
Paul Hennessy/SOPA Images via ZUMA Wire via Newscom
PHOTO: A nurse prepares to administer a dose of COVID-19 vaccine to a patient at a mobile vaccination event at the downtown Orlando campus of the University of Central Florida and Valencia College in Orlando, Fla., July 24, 2021.
>

Ankoor Shah, principal senior deputy director at the Washington D.C. Department of Health, said during a Thursday press conference that the district had 200 fully vaccinated breakthrough cases of COVID-19, out of a total record nearly 376,000 fully vaccinated people, representing “only point .05 percent, which just strengthens our confidence on how great these vaccines are.”

And in New Jersey, the total number of breakthrough cases, so far, is 5,678 out of a total of 4.8 million people vaccinated by July 12, according to state data. Forty-nine fully vaccinated individuals have died as a result of COVID-19.

\"It is important to point out that 49 deaths due to COVID-19 among 4.8 million fully vaccinated state residents is slightly greater than one in 100,000 fully vaccinated individuals. That means vaccines are about 99.999% effective in preventing deaths due to COVID-19,\" Dr. Ed Lifshitz of the New Jersey Department of Health said in a statement to ABC News.

Additionally, 27 of these individuals had pre-existing conditions, Lifshitz said, and many had more than one condition.

Of concern to some experts is the decline in daily COVID-19 tests, which makes it more difficult to track the spread of the virus. The nation is now recording just under 600,000 COVID-19 tests a day, which has ticked up slightly in recent weeks but is still much lower than at the country's peak in January, when U.S. was recording over 2 million tests a day. In addition, the CDC has, since May, ceased reporting asymptomatic or mild breakthrough cases.

According to Brownstein, the combination of the overall testing decline, the mild nondescript nature of breakthrough infections, and the general perception that vaccines are protective, means that any count of breakthrough infections is likely an underestimate.

Hence, he said, “given the efficacy of the vaccines, we recognize that even more cases will be asymptomatic, so these data only show part of the story. While asymptomatic cases are not of clinical relevance, they do help understand important patterns of transmission in the community.”

Experts concur that even with lower case levels than this past winter, the pandemic is not yet over, and it is critical to track the disease in order to attempt to slow its spread.

In a recent editorial in the Journal of the American Medical Association, experts urged the CDC to “re-energize” its testing services in light of the highly transmissible delta variant, “because without vigorous testing, the nation cannot be sure whether declining cases are a function of decreased numbers of infections or reduced numbers of tests.”

“As long as the virus is circulating, with or without causing illness, it can change and mutate, including into new strains that may be even harder to control,” Samuel V. Scarpino, managing director of pathogen surveillance at the Rockefeller Foundation, told ABC News.

“To get ahead of the pandemic we need to track the virus more closely and collect high-quality information on how and where COVID-19 is changing,\" he said. \"This high-quality, detailed information is crucial for COVID-19 and future pandemics.\"

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