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

(Source: WMC)
Published: Aug. 24, 2021 at 11:28 AM EDT
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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.

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