Breakthrough cases

COVID19 vaccines are not perfect; some people have incomplete protection. This means, two weeks after full immunization, you have a small likelihood of COVID19 disease. This phenomenon is called a “breakthrough case”.

And we expected this.

Vaccine efficacy is amazingly high, but not 100%. “Real world” efficacy (against old variants) is 90% for Moderna and Pfizer and 72% for J&J. In other words, if 100 people got COVID19, Moderna could have prevented 90 of them. 10 people would have still gotten some form of the disease, albeit, more mild.

When you have a mass vaccine campaign (848 million doses administered globally; 198 million in the United States), breakthrough cases are going to happen. They just are.

Also, breakthrough cases happen with all other vaccines.

What are the numbers?

Out of 76 million people fully vaccinated, the CDC is investigating 5,800 breakthrough cases (7 out of 100,000 vaccinated). The patterns of breakthrough cases, thus far, are not unexpected:

  • More than 40% have been among people aged 60+

  • 65% have been among women

  • 29% of breakthrough cases have no symptoms

  • 7% of breakthrough cases were hospitalized

  • 74 people have died

A New England Journal of Medicine study found 1% of their vaccinated sample (healthcare workers and the public) had a breakthrough case. However, this is likely an overestimate because the study regularly tested asymptomatic and symptomatic study participants over time.

State specific examples

The CDC isn’t reporting state-by-state numbers, but some states are individually reporting breakthrough cases on a regular basis.

On April 3, Washington State Health Department had fully vaccinated 1.7M people, of which 0.01% (or 217 people) got COVID19. Of these 217 breakthrough cases…

  • The majority experienced mild or no symptoms

  • 26 were hospitalized, in which 5 deaths are currently being investigated (all of which had multiple comorbidities and older than 80)

South Carolina has identified 155 breakthrough cases and Minnesota has reported 89 breakthrough cases (60% had no symptoms), just to name a few others.

What’s fueling the fire?

It looks like variants account for a large majority of breakthrough cases. A recent pre-print study found breakthrough cases in Israel were particularly apparent when individuals were infected with B.1.351 (first discovered in S. Africa) or B.1.1.7 (first discovered in the UK).

This is not surprising given what we know about the variants. B.1.351, P.1 (first detected in Brazil), and B.1.427 (first detected in California) all reduce vaccine efficacy to some degree. The lower the efficacy, the more breakthrough cases.

Another hypothesis circulating is that breakthrough cases may have to do with how much virus someone is exposed to. Scientists are actively investigating.

Should we be worried?

NO. If anything, this is “real world” evidence that the vaccine is working. A breakthrough rate of 0.01% in Washington, for example, is magnitudes better than the rate of COVID19 in Washington without vaccines.

You should go get your vaccine. Is it perfect? No. But it’s sure a lot better than gambling with COVID19 without the vaccine.

As the authors of the Israel study stated: “These results overall suggest that vaccine breakthrough infection is more frequent with variants of concern, yet a combination of mass-vaccination coupled with non-pharmaceutical interventions control and contain their spread.”

Love, YLE