How common are breakthrough infections in Virginia? It depends how you crunch the numbers.

‘A lot of the data is really messy right now’

By: - August 24, 2021 12:02 am


For the last month, the rise of the highly infectious Delta variant has prompted growing anxiety over so-called “breakthrough” infections — commonly understood as cases of COVID-19 among individuals who are fully vaccinated.

In Virginia, though, it’s not easy to understand how often those cases are occurring. According to the state’s recently updated COVID-19 dashboard, 0.2 percent of more than 4.7 million fully vaccinated Virginians are known to have tested positive for COVID-19. Meanwhile, some localities are reporting different — and much larger — figures. 

In late July, for example, the Prince William Health District announced that 25 to 30 percent of the area’s recently reported infections were breakthrough cases. Just a few days later, the Rappahannock Area Health District reported that 17 percent of its recent cases were among fully vaccinated residents. Those local numbers seem to back up data from other states, where breakthroughs have accounted for anywhere from 18 to 28 percent of identified cases in recent weeks, according to The New York Times.  

So why do statewide numbers look so much lower? It mostly comes down to the way the data is being reported.

In both Prince William and Rappahannock, local health officials were reporting breakthrough cases as a percentage of all total positive COVID-19 infections throughout much of July. National data, too, has often presented breakthrough cases among fully vaccinated individuals as a percentage of total infections in recent weeks — when Delta was quickly becoming dominant across the United States.

At the state level, though, Virginia has done things differently. For much of July and August, the state Department of Health reported breakthrough cases as the percentage of all COVID-19 infections since January. That roughly seven-month time frame includes a long period before Delta accounted for the vast majority of new coronavirus cases and when many residents were unvaccinated.  

On Monday, the state updated its dashboard to “more accurately represent the impact of vaccinations on infection rates in the commonwealth,” according to a release from VDH. But the new data still doesn’t provide breakthrough cases as a percentage of total infections. Instead, the department compares the weekly rate of new infections among fully vaccinated, partially vaccinated and unvaccinated Virginians. 

The dashboard does include Virginia’s total number of breakthrough cases from Jan. 17 to Aug. 14 — 10,712 as of Monday, according to VDH — and a week-by-week breakdown. But without including the overall number of new cases every week, it’s impossible to calculate the percentage of breakthrough infections in the same way that’s been done by some localities and other states.

Why is VDH doing things this way?

There’s concern among health experts that the growing focus on breakthrough cases will minimize the success of vaccines. Part of that is linked to the lack of specificity in much of the available reporting. The term “infection” does nothing to indicate the severity of the case — whether it was totally asymptomatic or whether it was severe enough to require medical attention, said Dr. John Swartzberg, a clinical professor emeritus of infectious diseases and vaccinology at the University of California, Berkeley School of Public Health.

“It’s a nuanced concept,” he said. “The lay public uses the word ‘infection’ to mean you’re sick. But infection just means you have the virus that’s replicating in you. It doesn’t say anything about whether you have symptoms or not.”

In Prince William, for example, epidemiologist Sean Morris emphasized that the vast majority of breakthrough cases were mild or asymptomatic. And given the wide availability of at-home testing options, it’s likely there’s some degree of undercounting when it comes to those cases. That’s especially true for immunized people who contract the virus but show no signs of illness.

A woman is tested for COVID-19 in a parking lot in Richmond where the health department set up a temporary walk-up testing center. (Ned Oliver/Virginia Mercury)

“In the pre-vaccine era, we know around 40 percent of people who get infected don’t get symptoms,” Swartzberg said. “But we have no idea what percentage of people fully vaccinated are getting asymptomatic infections because we’re not — in any kind of robust, prospective way — surveying that population.”

So even with a rise in breakthrough cases, the vast majority of vaccinated people aren’t coming down with severe illness. In many cases, they’re not even developing the kind of moderate symptoms that made the virus so concerning earlier in the pandemic. There’s also a fear of conflating the percentage of breakthrough cases with the effectiveness of vaccines, according to Dr. Lilian Peake, Virginia’s state epidemiologist. While the former can help gauge how frequently fully vaccinated infections are occurring, it doesn’t give a good sense of how well vaccines are working on a much wider level. 

Effectiveness, on the other hand, is calculated through carefully designed studies that compare the risk of vaccinated people contracting COVID-19 with the risk of unvaccinated people contracting disease— and how well vaccines reduce that risk in the vaccinated group.

“It’s looking at a whole population, not just cases,” Peake said. And while those types of studies are difficult to conduct at the state level, Virginia’s reporting highlights the much greater risk of disease among the unvaccinated. 

During the week of Aug. 14, for example, unvaccinated Virginians developed new infections at a rate more than eight times higher than the vaccinated. The rate of unvaccinated hospitalizations was nearly 14 times higher. And in supplementary data, state epidemiologists calculated the state’s “attack rates” — the percentage of breakthrough infections compared with natural infections among unvaccinated Virginians. Even with growing numbers of breakthroughs, it’s clear infections among the unvaccinated are driving the state’s latest surge in cases.

“And for Delta, it’s more transmissible, so it’s spreading more quickly because more people can get infected from a single case,” Peake said. “Because it’s spreading so much more quickly, you need to have more people who are vaccinated to really slow it down.”

Individual risk

While it can be helpful to underscore differing rates of infection among vaccinated and unvaccinated people, there’s been criticism of the relative lack of data when it comes to breakthrough cases. The CDC stopped tracking all but the most severe on May 1, and many states don’t produce any of their own reporting. Virginia’s data is actually considered more rigorous than most, given that it’s updated regularly and includes numbers on infections in addition to hospitalizations and deaths.

As a result, though, some public health experts say we’re minimizing the true frequency of breakthrough cases. And while vaccines are still very effective at reducing severe illness, including hospitalization and death, according to both national and international studies, there’s growing evidence that they’re less effective at reducing Delta infections. It also appears that individual protection begins to wane over time — a prime reason why booster shots are now recommended by federal and state officials. 

A basket of syringes containing the Moderna COVID-19 vaccine ready to be used at Richmond Raceway in Richmond, Va., February 2, 2021. (Parker Michels-Boyce/ For the Virginia Mercury)

“I think part of the problem is that a lot of the data is really messy right now,” Swartzberg said. In Virginia, for example, VDH staff had to hand-match positive test results with vaccination status to determine the number of breakthrough infections. The department has improved its data processing, but VDH couldn’t immediately confirm whether it’s analyzing every newly identified case in the state. 

That messiness, on both the state and national level, can make it harder to calculate individual risk. Much is still unknown about the Delta variant and breakthrough infections, including how often fully vaccinated people develop asymptomatic cases and how easily they transmit the virus in those instances. Virginia is still in the process of boosting its genetic sequencing to determine how extensively Delta is spreading through the state. 

It’s also still unclear how many breakthrough cases, in addition to hospitalizations and deaths, occur among Virginians with underlying health conditions. Peake said it’s an analysis that VDH is hoping to get underway, but right now, those unanswered questions make it all but impossible to determine the level of individual risk for vaccinated individuals. Currently, it’s not even clear how much Delta is to blame for breakthrough cases — though it’s certainly more transmissible than earlier iterations of the virus.

“There are three explanations and one is Delta,” Swartzberg said. “But another is that people’s behaviors changed and they started doing riskier things. And the third possibility is waning immunity. My guess is that it’s a combination of these three things, but I think we’ll have a much better idea once we have some distance from this.”

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Kate Masters
Kate Masters

Kate grew up in Northern Virginia before moving to the Midwest, earning her degree in journalism from the University of Missouri. She spent a year covering gun violence and public health for The Trace in Boston before joining The Frederick News-Post in Frederick County, Md. Before joining the Mercury in 2020, she covered state and county politics for the Bethesda Beat in Montgomery County, Md. She was named Virginia's outstanding young journalist for 2021 by the Virginia Press Association.