Despite ample vaccine supplies, outbreaks among unvaccinated populations in the US and the appearance of the Delta variant promote infection in fully vaccinated individuals. These breakthrough infections must be carefully tracked.

As the percentage of Americans considered fully vaccinated reached 58 percent, and Covid-19-related deaths surpassed three quarters of a million, concerns over breakthrough infections continue to grow.  Breakthrough infections tend to be mild, and the risk of secondary transmission from these infections, while real, especially in high-risk settings, is low compared to those who have not been fully vaccinated. What do breakthrough cases mean in the context of ending this pandemic? The answer is neither simple nor short.

How are states reporting breakthrough infection data?

Between March 2020 and March 2021, The COVID Tracking Project collected data on Covid-19 tests, cases, hospitalizations, and deaths from all 50 U.S. states, the District of Columbia, and five U.S. territories. These metrics were inconsistently defined and reported by the 56 jurisdictions. The same seems to be true of the tracking of metrics associated with breakthrough cases, resulting in a similar patchwork of data and creating an incomplete picture of the true incidence of Covid-19 infections in fully vaccinated individuals in the U.S.

Breakthrough Infections Are Rare, But Important to Understand

Understanding breakthrough infections starts with knowing the “incidence” of breakthrough infections. Technically, incidence is the rate, or frequency of a disease in a select period of time.  In order to properly calculate incidence, widespread testing–both diagnostic and viral genome sequencing–is critical, as well as the ability to link a positive test to the vaccination status of the individual. Right now, there isn’t a clear or standardized approach to linking these important pieces of information on state or national dashboards. These data would also help illuminate the real-world data of vaccine effectiveness and detect emerging viral variant trends.

The occurrence of breakthrough infections is tied to the dynamics and variability in vaccine uptake.

On a county-by-county level the pattern of vaccination in the U.S. is far from uniform. While 9% of counties have rates below 30% of their total population fully vaccinated, only 3% have rates above 70% of their total population fully vaccinated, and vast disparities still remain along economic, racial, and geographic lines.

Read the full article about data for tracking breakthrough infections by Dr. Jacqueline Houtman,  Rebecca Glassman, Dr. Lindsey Shultz,  Jessica Malaty Rivera, Emily Bass,  Dr. Rick Bright at The Rockefeller Foundation.