Breakthrough Infections and Impact of the Delta Variant

With the Delta variant being the most common strain of virus in our community, we’re getting lots of questions about how this has impacted breakthrough infections. Breakthrough infections are defined as infections among people who are fully vaccinated (defined as a positive COVID test 14 or more days after the second dose of the Pfizer or Moderna vaccine, or the single dose of the Janssen vaccine). Let’s take a look at the data:

As a vaccinated person in Dane County, how likely am I to test positive for COVID?

Since the COVID-19 vaccine was introduced in December 2020, the overall case rate (number of new cases for every 100,000 people) declined from January 2021 to June 2021 (Figure 1). While the COVID-19 vaccines were primarily developed to prevent severe illness, including hospitalization and death, there is evidence that vaccinated people are also less likely to become infected with COVID-19, or at least are less likely to have symptomatic infection.

While incredibly effective at preventing severe outcomes, the COVID-19 vaccines are not 100% effective at preventing infection. Each month there has been a small but expected number of breakthrough COVID-19 infections. However, the case rate among people who are fully vaccinated is lower than among people who are not fully vaccinated.

During the first half of 2021 (see blog post), the majority of cases were among people who were not fully vaccinated. In fact, on average only 0.02% of people who were fully vaccinated were diagnosed with COVID-19 between February and June 2021 (there were no infections among fully vaccinated people prior to February 2021). However, the picture changed starting in July, once the Delta variant became the dominant strain of the COVID virus circulating in Dane County (more on Delta, below).

 

Impact of the Delta Variant

On June 14, the CDC classified the Delta variant as a variant of concern. Since approximately July 1, the Delta variant has been the primary circulating strain of virus in Dane County and in Wisconsin. There are a few reasons for why we have experienced higher infection rates in July and August than in previous months among people who are fully vaccinated, as well as among those who are not fully vaccinated.

First, this variant is much more effective at spreading than previous strains of COVID. Second, recent higher rates among people who are fully vaccinated may partially be due to more people getting tested since the arrival of the Delta variant. Third, and this is still under study, may be waning immunity—or immunity that wears off months or years after someone gets vaccinated. The CDC is working to rapidly assess these emerging data to inform recommendations and policies about booster doses.

The risk of infection is still higher among people who are not fully vaccinated. In fact, the COVID-19 vaccines available in the U.S. have still shown to provide strong protection against the Delta variant, particularly in preventing severe disease and death. We have very good vaccine coverage in Dane County (over 70%), including a high percentage of people who have completed the series. The most effective way to protect yourself and our community from being impacted by this variant it to get vaccinated.

As a vaccinated person, how likely am I to be hospitalized or die due to COVID if I do get COVID?

The number of people who are fully vaccinated who have been hospitalized or died due to COVID-19 infection is low (see below for more information about determining hospitalization due to COVID).

From February 2021 through August 17, 2021, there were 1,472 COVID-19 diagnoses among people who were fully vaccinated in Dane County. Of these, 25 people were hospitalized and 3 died; 4 of the hospitalizations and 2 of the deaths occurred before the arrival of the Delta variant. In the same time period, 8,020 unvaccinated people tested positive for COVID, and of those, 174 were hospitalized and 21 died.

While COVID-19 was a contributing factor for their hospitalizations, many of the fully vaccinated individuals who were hospitalized had other comorbidities, or were first hospitalized due to another reason and then COVID-19 contributed to ongoing hospitalization. The median age of fully vaccinated people who were hospitalized was 74 years old, with a range of 26-97 years old.

To put this in perspective, only 25 people out of over 371,000 fully vaccinated Dane County residents were hospitalized due to, or in part due to, COVID-19 infection.

Unvaccinated people are more likely to test positive and much more likely to be hospitalized for COVID. In July, unvaccinated people under the age of 65 were 1.9 times more likely to test positive and 4.6 times more likely to be hospitalized for COVID than vaccinated people under the age of 65 (Figure 3a). Unvaccinated people age 65 and older were 6.1 times more likely to test positive and 9.5 times more likely to be hospitalized for COVID than vaccinated people age 65 and older (Figure 3b).

 

For more information on the rate of breakthrough infections, hospitalizations, and deaths among fully vaccinated people in Wisconsin, please visit the Department of Health Services’ new COVID-19: Illness After Vaccination webpage.

The bottom line: The vast majority of people who are fully vaccinated are not being hospitalized with COVID-19.

In the event of an infection among people who are fully vaccinated, severe outcomes (hospitalization and death) are rare.

“Warning signs” that we’re monitoring

We’re still in the pandemic, of course, and we can’t predict what will happen. We are watching for signs that the situation might be more serious and need more community-wide action.

1. A large increase in hospitalizations. Since we’re a regional hub, our hospitalization rate could go up even if Dane County residents are staying healthy. This is the trend we track on our dashboard—people in Dane County hospitals with COVID, whether or not they are Dane County residents. It’s important that our hospitals have capacity to deal with people who are very sick, whether it’s from COVID-19 or from a heart attack, car crash, or other serious medical incident. We are watching hospitalization trends closely.
2. An increase in children getting very sick. There isn’t strong evidence that the Delta variant is more severe for younger people, but since children under 12 can’t be vaccinated yet, they remain vulnerable to infection, particularly when community levels of COVID are high. This is something we are keeping an eye on at the local and national level.
3. Other variants of concern. It is natural for viruses to mutate, and occasionally these mutations give the virus new characteristics that make it more successful in infecting more people. For example, there’s a small possibility that future variants could fully evade vaccines. This is when we’d need to more heavily rely on our old tools of masks, social distancing, and staying at home while vaccines get updated to match the new variant. We’re working with researchers at the UW to monitor the variants circulating in Dane County, and globally, there hasn’t been any variant identified that can fully evade the existing vaccines.

Data notes: Information about determining hospitalizations and deaths due to COVID-19

The data presented here on hospitalizations is from the Wisconsin Electronic Disease Surveillance System (WEDSS). Information on whether someone was hospitalized can either be entered into WEDSS by the public health investigator after talking to someone with COVID-19 or their representative, or it can be submitted directly by the hospital as part of electronic case reporting. On a routine basis, staff responsible for performing data quality assurance activities review the available information to determine whether the person was hospitalized due to (or in part due to) COVID-19, or whether the person was hospitalized for another reason (e.g., pregnancy) and happened to also test positive for COVID-19. If there is not enough information or the person conducting quality assurance is unsure whether the hospitalization was COVID-19 related the person is counted as hospitalized due to COVID-19. However, if it is clear that the hospitalization was due to another reason, or the person was not admitted overnight or only seen in the emergency room, then the person is not counted as being hospitalized due to COVID-19. If someone cannot be interviewed or the hospital does not submit an electronic case report, then the number of COVID-19 related hospitalizations may be underestimated.

Information about determining whether a death is due to COVID-19 is described on our website, in the section titled “COVID-19 Deaths”.