Although reports of breakthrough COVID-19 cases occurring among fully vaccinated Americans are garnering much attention, as the country experiences a viral resurgence, new data illustrates just how rare these breakthrough infections are likely to be, and further shows that the vast majority of those becoming severely ill are the unvaccinated.
With more than 156 million Americans fully vaccinated, nationwide, approximately 153,000 symptomatic breakthrough cases are estimated to have occurred as of last week, representing approximately 0.098% of those fully vaccinated, according to an unpublished internal Centers for Disease Control and Prevention document obtained by ABC News. These estimates reflect only the adult population and do not include asymptomatic breakthrough infections.
Substantial vaccination coverage amid increasing COVID-19 case rates are driving an increase in "expected" symptomatic breakthrough infections in recent weeks, the CDC wrote in the document.
Experts stress that no vaccine can provide 100% protection, but they are still very effective at preventing severe illness and death.
“The risk to fully vaccinated people is dramatically less than that to unvaccinated individuals. The occurrence of breakthrough cases is expected and, at this point, is not at a level that should raise any concerns about the performance of the currently available vaccines,” Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Pennsylvania State University, told ABC News.
“Some vaccinated folks may still get infected, some may still transmit. And the more vaccinated people there are, the more breakthrough cases we’ll see,” he added.
Coronavirus cases are now at their highest point since early May, according to CDC data, with the U.S. average nearly quadrupling since June to 47,000 new cases a day, largely driven by the highly infectious delta variant, which now accounts for more than 83% of new cases nationwide.
Virus-related hospitalizations have also increased, with more than 27,000 patients hospitalized around the country, though that number is still significantly lower than in January, when over 125,000 patients were receiving care at one time.
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.
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.
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."