Doses vs. Regimens: Tracking vaccinations by first and second dose, versus regimen initiated and completed

There are a lot of places to find vaccination data: health departments, media and even your Facebook feed. It can be confusing, especially when not everyone is using the same metrics or the same language to describe the progress towards full community vaccination. Let’s look at an example of that today.

Some data sources post vaccine data for the number of “first doses” and “second doses” given, while some show data for “regimen initiated” and “regimen completed.” What is the difference?

This difference in language is primarily driven by the introduction of the single-dose Johnson & Johnson vaccine to the market.

“First dose” and “second dose” came into use because the first two vaccines approved in the U.S. (Pfizer and Moderna) both require two doses to provide full protection from COVID-19. In that context, “first doses administered” indicates the number of people who have gotten their first dose of a two-dose series of shots. “Second doses administered” indicates the number of people who have received their second dose of a two-dose series.

Tracking first and second doses was a way to identify what proportion of the population was partially or fully vaccinated. The language tracks doses, but the intent is to track level of immunity and completion of vaccination.

Once the Johnson & Johnson vaccine was available in the region, tracking first and second doses became less useful. This is because a person who receives Johnson & Johnson will not need a second dose to be fully vaccinated.  To reflect this, many state and local health departments changed how they tabulate and describe their data to directly measure the number of people who have initiated or completed their vaccinations. 

People receiving Pfizer or Moderna are considered “initiated” after their first dose and added to “completed” after their second dose. People receiving Johnson & Johnson are included in counts for both “initiated” and “completed” at the time of their first dose.

Not all data sources have updated their dashboards or data to reflect the new best practice of tracking vaccine initiations and completions rather than doses. This may change as single-dose vaccination becomes more widely available.

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