What data would be necessary to determine whether delaying or even eliminating a second dose to give more people first doses would be a net benefit?


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

I believe that we could gather much of this data from real-world follow-up of existing programs. People will skip their second doses; some vaccine distributors won’t have the necessary number of doses at the specified interval. Hopefully state and federal governments are going to evaluate how many people receive the second dose at the specified interval and how many people skip their second dose entirely and make this information available publicly. It would also be a good idea to test a subset of individuals at the time they receive their first and second vaccines. Within the same community, a lower rate of SARS-CoV-2 in people getting their second vaccine relative to those who are receiving their first vaccine would suggest that the initial vaccine dose is protecting from infection.

Deborah Fuller, PhD

Not all vaccines require two doses to achieve sufficient levels of immunity to provide protection. While a booster dose maximizes the response, in some cases a single shot may be able to induce sufficient immunity to provide protection. The data supporting this ‘dose sparing’ approach for the currently approved COVID-19 vaccines is evidence from phase 3 clinical trials showing there was a significant level of protective efficacy achieved within two weeks after only the first dose. However, since the second dose was administered within 3-4 weeks after the first dose, it’s not known how durable the immunity after a single dose will be and whether it will last long enough to provide persistent protection during the delay. Another concern is protection in the elderly. Due to aging of the immune system, the elderly will generally respond less well to vaccination. Even if a single dose provides sufficient immunity in younger people, the elderly may still require both doses.

For most vaccines, spacing between doses can be stretched out without compromising the potency of the booster dose. In some cases, allowing a longer resting period of even up to 6 months between doses can generate stronger immunity after the second dose than a shorter, several week waiting period. The spacing of 3-4 weeks between doses is the minimum spacing needed to allow the first dose to effectively prime the immune system and was likely implemented with a view toward inducing maximum immunity in the shortest possible time.


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