Understanding Breakthrough Infections

 Breakthrough infections—when someone tests positive after they’ve been fully vaccinated for COVID-19—have been mentioned more and more in the news lately, especially in relation to the Delta variant. How concerned should we be and what do we know about breakthrough infections in Dane County?

Definition and background

A breakthrough infection is defined by CDC as a positive COVID-19 test 14 or more days after completing all recommended doses of an FDA-approved COVID-19 vaccine.

It’s important to remember that vaccine breakthrough infections are expected: no vaccine is 100% effective in preventing infection in vaccinated people. There is evidence that vaccination makes illness less severe for those who are vaccinated but still get sick. The COVID vaccines authorized for use in the US offer protection against all variants of concern currently circulating—including the Delta variant. However, we will see some vaccine breakthrough cases among both people infected with variants of concern and people infected with other COVID strains.

CDC has studied breakthrough infections, and they haven’t found unexpected patterns among people who experience a breakthrough infection. The CDC has stopped tracking breakthrough infections, but states like Washington continue to monitor. Washington has found that around 0.05% of vaccinated people have subsequently tested positive for COVID after full vaccination—that means 99.95% of vaccinated people have not gone on to test positive!

Dane County data

Let’s walk through Dane County data.

• Looking at 2021, we see the total confirmed cases were highest in January and lowest in June.
• The number of breakthroughs among confirmed cases column tells us that we see the number of breakthrough infections per month ranging from 0 in January, when only a few thousand people had been vaccinated, to 87 in May, when over 300,000 people had been vaccinated.
• While cases have been decreasing, the number of people fully vaccinated has been increasing.
• The number fully vaccinated each month column tells us that April was our biggest vaccination month—106,047 people were fully vaccinated in April! The cumulative fully vaccinated column keeps track of the total number of people fully vaccinated by adding up each month’s total.
• The last column, percent breakthroughs among cumulative vaccinated is important—it answers the big question of “How common are breakthrough infections?” Answer: uncommon. Overall, we find that 0.07% of fully vaccinated people have tested positive for COVID. This means that 99.93% of people vaccinated in Dane County have not gotten COVID after being vaccinated.

Seeing the big picture

Sometimes when we see alarming headlines like “Vaccinated people are getting COVID!” it makes us question the 99% effectiveness of vaccines. When this happens, we need to pause and consider the bigger picture. It’s also important to understand a concept called “base rate bias”. In simple terms, it’s the idea that the more vaccinated a population, the more we’ll hear of the vaccinated getting infected. You can take a deeper dive into base rate bias in this newsletter.

Do we see any evidence of immunity decreasing over time?

What about people who have been vaccinated for a long time—are they more likely to experience breakthrough infections? We took a look and did not see evidence of this! If you were vaccinated back in January, you’re no more likely to experience a breakthrough infection than if you were vaccinated in June.

Total adds up to 258 because this analysis was done at a later date than the first table.

We see most breakthroughs identified within 60 days after being fully vaccinated. We have seen breakthroughs that happen right after the two-week post-vaccine mark (these are possibly people who were infected with COVID before getting vaccinated and didn’t know it), all the way to 4-5 months after getting fully vaccinated.

Do things change with the Delta variant?

Since we analyzed these data in June, the Delta variant has increased in prevalence in both Dane County and Wisconsin. The Delta variant is a mutation of COVID that is more contagious and spreads more easily than the original COVID virus. Still, the COVID-19 vaccines are effective in protecting fully vaccinated people from catching and spreading the virus. The Delta variant may reduce efficacy somewhat—researchers are still studying this—but in all cases, the vaccines are highly effective in protecting fully vaccinated people from hospitalization and death due to COVID. We will continue to monitor these data as the variant landscape changes. Stay tuned for an upcoming blog on the Delta variant!

The bottom line: It’s rare for people to test positive for COVID after getting vaccinated.

It does happen because no vaccine is 100% effective; in Dane County we’ve had seven people later test positive for COVID out of every 10,000 people vaccinated. However, even if you test positive after vaccination, your outcome will be far better than without. Fewer than 10 breakthrough infections out of 350,162 vaccinated people have resulted in hospitalization in Dane County. Vaccines remain our best defense against COVID—variants included.