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Should someone still get the first dose of a COVID-19 vaccine, without assurance that a second dose will be available?

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For COVID-19 vaccines that are designed to have two doses, it is important to get both doses to maximize protection. In late 2020, the U.S. Food and Drug Administration (FDA) authorized emergency use of COVID-19 vaccines from Moderna and Pfizer / BioNTech, both of which are designed to be implemented in two-doses.  Studies have shown each of these vaccine candidates to be relatively safe and ~95% effective at preventing symptomatic COVID-19 disease in adults after both doses. There are many reasons why a second dose may become delayed or unavailable due to issues like limited vaccine supply and other logistical challenges. As a result, there are proposals to ration vaccine doses or to initially give a single dose to as many people as possible. The reaction of scientists to these proposals is mixed, because of limited data on the impacts of changing recommended vaccine dosing. According to data provided by Moderna, one exploratory analysis of participants who received just one dose of its vaccine suggested that the efficacy in protecting against symptomatic COVID-19 could be around 73%, in the short-term. Efficacy in protecting against symptomatic COVID-19 after the first dose of the Pfizer vaccine was about 52.4%, with most of the cases happening in the days immediately following the first dose. From day 10 after the first dose until the second dose, the efficacy in protecting against symptomatic COVID-19 was around 89%.  It is important to note that the second dose was given on day 21 in these Pfizer / BioNTech trials, so there is limited data on how well the first dose would protect someone after day 21.   On January 4, 2021, the FDA issued a statement about following the authorized vaccine dosing schedules, saying: “We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19. These are all reasonable questions to consider and evaluate in clinical trials. However, at this time, suggesting changes to the FDA-authorized dosing or schedules of these vaccines is premature and not rooted solidly in the available evidence.” More research is being done to help answer the question about how beneficial it is to change a recommended vaccine dosing schedule in order to stretch limited supplies to as many people as possible. 

This article was published on
January 12, 2021

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For COVID-19 vaccines that are designed to have two doses, it is important to get both doses to maximize protection. In late 2020, the U.S. Food and Drug Administration (FDA) authorized emergency use of COVID-19 vaccines from Moderna and Pfizer / BioNTech, both of which are designed to be implemented in two-doses. 

Studies have shown each of these vaccine candidates to be relatively safe and ~95% effective at preventing symptomatic COVID-19 disease in adults after both doses.

There are many reasons why a second dose may become delayed or unavailable due to issues like limited vaccine supply and other logistical challenges. As a result, there are proposals to ration vaccine doses or to initially give a single dose to as many people as possible. The reaction of scientists to these proposals is mixed, because of limited data on the impacts of changing recommended vaccine dosing.

According to data provided by Moderna, one exploratory analysis of participants who received just one dose of its vaccine suggested that the efficacy in protecting against symptomatic COVID-19 could be around 73%, in the short-term.

Efficacy in protecting against symptomatic COVID-19 after the first dose of the Pfizer vaccine was about 52.4%, with most of the cases happening in the days immediately following the first dose. From day 10 after the first dose until the second dose, the efficacy in protecting against symptomatic COVID-19 was around 89%. 

It is important to note that the second dose was given on day 21 in these Pfizer / BioNTech trials, so there is limited data on how well the first dose would protect someone after day 21.  

On January 4, 2021, the FDA issued a statement about following the authorized vaccine dosing schedules, saying: “We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19. These are all reasonable questions to consider and evaluate in clinical trials. However, at this time, suggesting changes to the FDA-authorized dosing or schedules of these vaccines is premature and not rooted solidly in the available evidence.”

More research is being done to help answer the question about how beneficial it is to change a recommended vaccine dosing schedule in order to stretch limited supplies to as many people as possible. 

Context and background

Some healthcare organizations are starting to offer registration or appointments for patients to receive the first dose of a COVID-19 vaccine, without any assurances about when a second dose may become available. This can be due to limited supplies of the vaccines, supply chain challenges with refrigeration requirements during storage and transport, and uncertainties that remain about vaccine distribution and allocation for the near future. 

People who are initially offered only a single dose out of the two doses may have legitimate questions about whether they should accept. While the data is limited, initial studies suggest that a single dose can potentially offer some protection in the short-term, but until more is known, the U.S. FDA recommends for both doses to be given rather than deviate from what was rigorously tested in clinical trials. Unfortunately, real situations around the world may make it difficult to avoid delays in providing second doses at this time. Some experts are suggesting that some protection through a single dose initially is better than no protection while waiting for the ability to give both doses on schedule, but there is not yet consensus because more research needs to be done on this topic.

The current constraints require a complex balancing act, considering the need to vaccinate people based on what is recommended by rigorous data as well as the need to reduce vaccine hesitancy. Anyone who has been offered a vaccine dose but has questions about taking it is encouraged to consult with their healthcare provider.

Resources

  1. FDA Statement on Following the Authorized Dosing Schedules for COVID-19 Vaccines (U.S. FDA)
  2. How can countries stretch COVID vaccine supplies? Scientists are divided over dosing strategies (Nature)
  3. COVID-19 vaccination: What’s the evidence for extending the dosing interval? How effective is just one dose? (BMJ)
  4. Safety and efficacy of the BNT162b2 mRNA COVID-19 vaccine (NEJM)
  5. COVID-19 Greenbook Chapter 14a (U.K. Government)
  6. Safety and efficacy of the BNT162b2 mRNA covid-19 vaccine (NEJM)
  7. COVID-19 Greenbook chapter 14a (U.K. Government)

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