Why are the currently approved COVID-19 vaccines administered as two separate doses?


SciLine tracks common science questions that reporters have about the coronavirus pandemic – and reaches out to our network of scientific experts for quotable comments in response. Reporters can use the comments below in news stories, with attribution to the scientist who made them.

This article was published on
January 7, 2021

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What our experts say

Context and background


Media briefing

Media Release

Expert Comments: 

Dave O’Connor, PhD

The idea behind two separate doses is that the first dose ‘primes’ the immune response and the second dose acts as an amplifier, making the immune response against the virus stronger. Such a strategy is why vaccine manufacturers often design studies to have multiple doses. This doesn’t mean that one dose can’t be effective, but it is impossible to test every combination of doses, timings, routes, and vaccine compositions at once. The Moderna and Pfizer vaccine Phase 3 trials were designed to use each company’s ‘best guess’ strategy first. Fortunately, both of these were extraordinarily successful.

Deborah Fuller, PhD

The first dose primes the immune system and generates memory cells, thus getting the body ready to respond quickly to an infection. Antibody and memory cell responses are induced after the first dose, but the level is usually lower and could wane quickly below thresholds needed for protection. The second dose, also called a booster, expands the number of immune cells that were primed by the first dose, resulting in a stronger antibody response that is generally more durable.  Administering two doses of COVID-19 vaccines increases the likelihood of achieving maximum prevention of disease and increases the percentage of people vaccinated that develop protective levels of immunity, leading to more durable immunity.


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