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The news is peppered with anecdotal evidence of breakthrough cases. The headlines are terrifying and the personal stories are tragic:

Boston Globe: “79 fully vaccinated Massachusetts residents have died, 303 hospitalized in very rare COVID ‘breakthrough’ cases, officials say”

The Guardian: “COVID outbreak among vaccinated Vegas hospital workers underscores Delta risks”

NBC: “Illinois Coronavirus Updates: Breakthrough COVID Cases Are 2% of State's Deaths This Year”

So, why do breakthrough cases happen?

We shouldn’t think of vaccine protection as binary (yes or no). It’s better to think of protection on a spectrum: On one end the vaccine will protect people in every situation and on the other end it doesn’t protect people at all. And each of us land somewhere in between.

There are many factors that determine where we land:

There’s of course the variants. Depending which variant you come in contact with determines your level of protection. mRNA vaccines’ efficacy is now ~88% against Delta. If 100 people got COVID19, Pfizer/Moderna could have prevented 88 of them. 12 people would have still gotten some form of the disease. On the other hand, if people came in contact with the original strain, 95 (out of a 100) would have been prevented.
Some of us just don’t have immune systems that can build protection. For example, it looks like certain drugs for immunocompromised patients reduce and/or prevent protection. Also, older adults are less protected.
Behaviors, too, can predict your place on this spectrum. If a vaccinated person is exposed to a large enough dose of a virus OR exposed to enough unvaccinated people where transmission is high, the virus can overcome the vaccine and cause infection, even among the sturdiest of immune defenses.
And then there’s just luck. Even if you compare twins, the level of protection can differ. Some people will just create fewer or less strong defense mechanisms (antibodies, T cells, and/or memory B cells). And we don’t really know why.

Nonetheless, should the virus make it through, Dr. Ali Ellebody, an immunologist at Washington University in St. Louis, said it best:

“It becomes a race [against] time. The pathogen rushes to copy itself, and the immune system recruits more defenders. The longer the tussle drags on, the more likely the disease is to manifest.”

It’s important to monitor all breakthrough cases.

If we closely monitor them, we can answer some very critical questions like…

Are breakthrough cases happening at the rate in which we expect?
Who’s most likely to break through?
What is the rate of a mild breakthrough case vs. severe breakthrough case?
Are variants, like Delta, causing more (or more severe) breakthrough cases than other variants?

A study was published describing breakthrough cases between January 1-April 30. During this time among 101 million fully vaccinated Americans, 10,262 breakthrough infections were reported to the CDC. Who were they?

63% female
Median age was 58 years (age range was 40–74 years)
10% were hospitalized
2% died. Median age of patients who died was 82 years (age range was 71–89 years)
Genomic sequence data were available for 5% of breakthrough cases
- 64% were identified as a Variant of Concern (this was before Delta emerged)

Beginning May 1, 2021, though, the CDC transitioned from monitoring all breakthrough infections to investigating only those among patients who are hospitalized or die. The CDC doesn’t have the infrastructure to rigorously investigate all breakthrough cases. They needed to prioritize their operation, so they decided to focus on cases of highest clinical and public health significance.

The CDC continuously publishes the latest count on their website. As of July 6, there were 5,186 severe breakthrough cases. This includes 988 deaths (although it’s important to notice the footnote stating that 255 of these were not directly related to COVID19).

Unfortunately because the CDC does not track asymptomatic or mild breakthrough cases, we have no idea what the true breakthrough rate is in the United States. We just don’t have a clue.

So, we are dependent on other countries that have rigorous surveillance systems in place.

The UK has the best variant surveillance system in the world. The UK alone is responsible for 40% of the world’s COVID19 sequencing. It’s also incredibly fast and the data is made public almost immediately. This helps inform real-time decision-making by public health officials and vaccine sponsors. I continue to be amazed by their efforts.

We can learn a lot from their data- like breakthrough cases due to certain variants. Every week-ish, Public Health England publishes a technical report of the current state of affairs. Here is their latest report from July 9. Within the report, you can find the rate of breakthrough cases per Delta.

From February 1 to June 21, 123,620 Delta cases were sequenced in the UK. Among those, 10,834 cases were among fully vaccinated (i.e. breakthrough cases) and 71,932 cases among the unvaccinated. While this isn’t all of the breakthrough cases, this gives an even closer estimate to the “true” rate of breakthrough cases due to Delta. But even this is among patients who went to the hospital. We still don’t know the asymptomatic and/or mild breakthrough rate.

Bottom Line:

We just don’t know how many breakthrough cases there are in the United States. With Delta or with any other variant. And we may never know. Just know that being vaccinated doesn’t mean you’re immune. It means you have another fantastic layer of protection. Given that we’re now facing exponential spread in the United States, it’s time to add more layers of protection. Like masking up inside.

Love, YLE

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Your Local Epidemiologist

How many breakthrough cases are there?