Breakthrough cases occur when a fully vaccinated person — someone who has received at least two doses of Pfizer or Moderna, or one dose of Johnson & Johnson — tests positive for Covid-19.
We get lots of reader questions about how to interpret the state’s breakthrough data, and — even though we regularly report on these numbers — why we aren’t including them in our daily Covid-19 updates.
We ran your most frequently asked questions past an expert and reviewed the data ourselves to help explain what breakthrough case data means for Vermont.
What can Vermont data tell us about the vaccine’s effectiveness?
Very little that we don’t know already.
Yes, Vermont publishes data on breakthrough cases, hospitalizations and deaths. But that data is constrained by the state’s small size, older population and earlier-than-average vaccination success — and, as Dr. Tim Lahey explains, “the presumption is that vaccines work the same in Vermont as they work in California.”
Lahey, an infectious disease specialist at the University of Vermont Medical Center, said the stronger evidence comes from vaccine clinical trials and observational studies in recent months, which rigorously follow participants and account for factors such as someone’s age.
One of the largest studies — with 43,000 participants in Los Angeles County during the Delta surge — concluded that unvaccinated participants were 4.9 times more likely to get infected and 29 times more likely to be hospitalized for Covid than their fully vaccinated peers.
Recently released federal data on breakthrough cases shows similar results: As of early September, cases in 14 states across the nation were six times higher in unvaccinated Americans.
As to whether a vaccinated person is less likely to transmit the virus if they’re infected, Lahey said the data was more mixed. Studies have found that vaccinated people who get a breakthrough infection have comparable amounts of the virus in their nose compared to an infected unvaccinated person, but other research has found that vaccinated people are less likely to infect household members in real-world settings.
The bottom line, however, is “if you don’t get infected, you can’t transmit,” Lahey said.
“And so if the risk of infection is several times lower if you’re vaccinated, that means that the risk of transmission is several times lower” in the majority of cases, he said.
How often does Vermont report breakthrough data?
As of Oct. 28, the state regularly releases two reports with information on breakthrough cases: One at the governor’s Tuesday press conferences with some charts on breakthrough data, and another on a twice-monthly basis that includes the raw numbers of vaccinated Vermonters who have gotten infected, gotten sick, or died.
The state does not report this data on a daily basis.
What does Vermont data show?
In general, Lahey agrees with state officials: The data for Vermont shows that Vermonters are less likely to get infected or be hospitalized if they are fully vaccinated.
But there are some caveats.
The most recent data report said that 4,881 vaccinated Vermonters have gotten sick, 134 have been hospitalized and 59 have died. Those numbers may seem high at first glance, but it’s worth noting that the majority of Vermonters, 71% of the population, are fully vaccinated.
When you take the sizes of the vaccinated and unvaccinated populations into account, the chances of an unvaccinated person getting sick are far higher, despite their smaller population.
The gap between unvaccinated and vaccinated Vermonters has changed during the Delta surge. Cases rose from 21 per 100,000 vaccinated Vermonters as of July 31 to 138 now, while unvaccinated cases have gone from 83 to 560 per 100,000 people.
Similarly, hospitalizations for unvaccinated Vermonters have risen from 3 to 24 per 100,000 people during the Delta surge, while breakthrough hospitalizations have risen from 0.47 to 6.
Vermont does not release breakthrough case data by age group, but the recent federal data suggests age plays a role in how likely you are to be hospitalized or die because of Covid, even if you’re fully vaccinated.
Unvaccinated seniors across the nation were the most likely to die of Covid of any group, but vaccinated people in their 80s or older reported a higher rise in deaths than people below 50.
The number of deaths in Vermont is so small that Lahey cautioned against analyzing it, saying it may also be skewed by the state’s population, which is older than the national average.
The rate of deaths between unvaccinated and vaccinated Vermonters has varied widely, from unvaccinated people being seven times more likely to die one week, to it appearing that vaccinated Vermonters were slightly more likely to die in others — but each week has only a handful of deaths to analyze.
Why is the gap narrowing?
That gap in Covid cases between vaccinated and unvaccinated Vermonters remains pretty wide, but the ratio between the two populations has narrowed. Lahey said there could be a few reasons for that shift.
“Protection from infection, and especially hospitalization, remains very strong with vaccines. The glass is way, way more than half full,” Lahey said. “But the glass is not 100% full and it does look like in the summer we saw a subtle drop of those levels of protection” as the more transmissible Delta variant became the dominant strain.
Waning immunity from the vaccine may have played a role as well, he said. For many Americans, the two- or six-month timespan before you need a booster, depending on the vaccine, has already passed.
At the same time, he said, many states reopened their economies and dropped their masking requirements, meaning that vaccinated people were more likely to be exposed to the virus. “The more exposures you get, the more likely you are to get infected, even if you do have vaccine protection,” he said.
“I’m guessing it’s probably all three of those,” he said. “That person in their 70s now is exposed to Delta instead of the original SARS-CoV-2, and their Pfizer vaccine was starting to wear off a little, and their family was a little less likely to wear a mask, and so that just put them at a little bit higher risk.”
How does Vermont compare to the rest of the nation?
As the federal data shows, it appears that Vermont has a higher rate of breakthrough cases than the rest of the nation — its unvaccinated cases are four times higher, versus six times higher.
Lahey said there could be some flaws in the data that make it seem Vermont has a higher breakthrough case rate. The state is fairly small, so the sample size of cases is much smaller as well.
Vermont also has an older population, meaning it may have a higher percentage of breakthrough cases that are more severe and more likely to be detected. Generally, those older Vermonters were also vaccinated earlier than their counterparts in the rest of the country, meaning they are more likely to need a booster, Lahey said.
He said Vermont may in some ways be a victim of success.
“We still did save a bunch of lives” with our high vaccination rate, he said. “But not only, you know, did we get there early — so we’re going to have waning immunity sooner than somebody else — but also we really celebrated it” by dropping all restrictions and most of our social distancing.
Now that we know Delta is causing a surge, he said, “it’s a time to sort of still celebrate, but to make sure that celebrations don’t run us into trouble.”