The new data from Delaware add to a growing body of early research showing that the shots offer strong protection, even as the highly transmissible delta variant circulates.
Delaware's numbers show not only that vaccinated people make up a small share of cases but also that they have a very low rate of infection. The post-vaccination infection, hospitalization, and case rates are better indicators of vaccine efficacy than the percentage of breakthrough cases, hospitalizations, and deaths out of a state's total, experts say.
In Delaware, people who got the one-shot Johnson & Johnson vaccine have had the highest post-vaccination case rate, the state reported. Of the 43,947 people immunized, 333 had confirmed cases, representing 0.76% of J&J recipients.
Among the more than 274,000 residents who received Pfizer, 1,385 cases were reported, representing 0.51%, according to the data analysis, and 589 cases were reported among the more than 190,000 people who got Moderna, representing 0.31%.
In total, 0.45% of vaccinated Delawareans had tested positive as of Aug. 30, according to state data, or 1 in 222 people. In New Jersey, it was 1 in 286. Data Pennsylvania released Tuesday showed that 1 in 17 unvaccinated people have tested positive in 2021 compared with 1 in 122 vaccinated people.
A handful of other states report highly detailed data on cases among vaccinated people, but Delaware is among the first to link cases to the specific vaccine the person received. State data help fill a nationwide gap, as the CDC tracks only breakthrough cases leading to hospitalization or death.
They are significant numbers to have, said University of Pennsylvania biostatistician Jeffrey D. Morris. Delaware is "one state; it's relatively small," he said, "but to have accurate data for that snapshot will give us a lot."
People who do get infected after being immunized are far less likely to have serious illness, be hospitalized, or die, data have shown.
"What we know for certain is the vaccine is the best tool we have to prevent serious COVID-19 illness and death," Delaware Division of Public Health spokesperson Mary Fenimore said. "The proof of this is that our numbers for hospitalizations and deaths for those who are fully vaccinated is extremely low," with 50 who tested positive having been hospitalized this year and 25 having died.
Breakthrough case data make clear that all vaccines are highly effective, said Jennifer Horney, founding director of the University of Delaware's epidemiology program. "People should feel protected," she added, "but not invincible."
Morris also emphasized the vaccines' efficacy but suggested the United States should consider booster shots for J&J recipients based on Delaware's data and other studies indicating Johnson & Johnson may not perform as highly as the other vaccines.
Pennsylvania, New Jersey, and Philadelphia have not released data linking breakthrough cases to specific vaccines.
Philadelphia announced Wednesday that more than 96% of people hospitalized with the virus in the city this year have been unvaccinated, as have more than 98% of people who died.
On Tuesday, Pennsylvania published its first data on breakthrough cases after previously not tracking them. Pennsylvania's data showed that unvaccinated people were contracting the virus at a rate seven times higher than vaccinated people and dying at a rate eight times higher. Acting Health Secretary Alison Beam said her department planned to link cases to vaccines in the future.
Delaware, a state with just under a million residents, can more easily do so because of its robust contact-tracing program, which reaches most residents who test positive.
The findings, which the state provided to The Inquirer, are in line with other studies on the effectiveness of the vaccines against more transmissible variants.
Pennsylvania's data showed that 97% of the deaths since January were in people who were unvaccinated or partially vaccinated, as were 95% of hospitalizations and 94% of confirmed cases.
The Pennsylvania Department of Health could not immediately respond to questions Wednesday about whether the state would publish more detailed data. Delaware and New Jersey provide weekly reports on breakthrough infections.
If states collected more detailed data, experts said, it would allow scientists to learn more about vaccine efficacy and help inform policy, including booster-shot campaigns.
Breakthrough cases are expected to increase as the number of vaccinated people increases, testing increases due to the delta variant, and offices and schools reopen. It's also possible breakthrough cases are undercounted because vaccinated people are less likely to get tested.
Their existence, however, indicates the vaccines have likely prevented far worse devastation by the delta variant.
"What would the surge have looked like last year if delta had hit us then?" Morris said. "I actually feel pretty confident it would've been almost unimaginable, worse than anything we've seen. The vaccines are taking the edge off what this delta surge would've been."
emccarthy@inquirer.com erinK_mccarthy