On July 7, 2021

What we can and can’t say about Vermont’s vaccine breakthrough cases

By Erin Petenko/VTDigger

As vaccinations rise throughout the state — hitting 81.9% of eligible Vermonters on June 29 — readers have written to VTDigger to ask: Just how many vaccinated Vermonters have ended up with Covid-19 anyway?

The Vermont Dept. of Health has the answer: 239 Vermonters since Jan. 1. But the small sample of data, plus other caveats, make it difficult to answer other questions about vaccinated Vermonters’ Covid-19 risk.

The federal Centers for Disease Control and Prevention is “taking a really hard look at vaccine breakthrough, hospitalization and death,” said Dr. Lynn Blevins, surveillance epidemiologist for the state health department. “They have much better numbers than Vermont does — we just easily aren’t able to do this analysis and get any kind of conclusive answer with our small numbers.”

Breakthrough cases are Covid-19 cases that occur in fully vaccinated Vermonters, defined as someone two weeks past their last dose of the vaccine.

About 1% of cases since Jan. 1 have been among fully vaccinated Vermonters, health department data shows. Blevins said it’s fair to say that’s “pretty small.”

But it gets tricky to calculate the rate of Covid-19 cases among fully vaccinated Vermonters, she said, and how that compares to unvaccinated Vermonters. For one thing, Covid-19 cases have fluctuated wildly since Jan. 1, rising to a new peak in April and then reaching new lows in June.

At the same time, the percentage of vaccinated Vermonters has changed drastically, Blevins said.

“There’s no calendar date where you can say that the whole population was unvaccinated and then the whole population was vaccinated. We have this gray area where people are partially vaccinated and becoming fully vaccinated,” she said. “It becomes a much more complicated analysis to do.”

It’s also difficult to use Vermont-specific data to figure out the risk of hospitalization and death among fully vaccinated Vermonters. The health department reports that 11 people with breakthrough cases have been hospitalized, or about 5%.

It would appear at first glance that breakthrough cases have a greater hospitalization rate than other Covid-19 cases, 3% of whom have been hospitalized. But again, Blevins said the sample size here is too small to draw clear conclusions about risk.

“That’s a much more unstable estimate than our overall hospitalization rate because that’s got thousands of cases,” she said.

It’s also possible that the hospitalization rate among breakthrough cases appears higher because a higher percentage of those cases are symptomatic, Blevins said. Vaccinated Vermonters may be less likely to get tested if they’re asymptomatic because of health guidelines and surveillance testing requirements.

While there are no clear takeaways from Vermont’s breakthrough data, the state has established science on the effectiveness of the Pfizer, Moderna and Johnson & Johnson vaccines.

“This is why we give vaccine efficacy studies, and these are large studies that give us that number,” Blevins said.

Clinical studies early in the vaccine approval process — conducted in more controlled environments — showed each vaccine is effective at preventing severe illness. Real-world follow-up studies have confirmed that effectiveness.

For example, the CDC found that real-world use of the Pfizer and Moderna vaccine between January and March was 94% effective.

“These are really good vaccines,” Blevins. “They’re highly effective. We do expect to see some vaccine breakthroughs because no vaccine is perfect.”

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