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Comparing COVID-19 Booster Vaccinations (COV-BOOST)  - Young Adults Fractional Dosing Sub-Study

What is the purpose of this research trial?

There are now a number of vaccines that have been approved in the UK to prevent COVID-19 and other vaccines that are still in UK clinical trials, which may be approved later in the year. Millions of people have now received their first 2 vaccinations, which we call a “prime-boost” course. There were 3 vaccines used by the NHS to deliver an initial prime-boost during the first part of the NHS immunisation campaign: ChAdOx1-nCov19 (commonly known as the “Oxford vaccine” or ‘AstraZeneca Vaccine’), BNT162b2 (commonly known as the “Pfizer vaccine”) and mRNA-1273 (commonly known as the “Moderna vaccine”). The NHS has now recommended all adults receive a 3rd dose booster vaccine against COVID-19 at least 3 months after they received their second dose.

Young adults have a stronger immune response to vaccines than older adults, and results from the first stage of COV-BOOST and other COVID-19 vaccine studies have suggested that lower doses of BNT162b2 (Pfizer) or mRNA-1273 (Moderna) may give as good an immune response in young adults as higher doses. They may also be associated with fewer side effects or lower rates of already rare adverse events. Giving lower doses may also allow existing stocks of vaccines to be given to more people, which is important while need for the vaccine is greater than the number of doses available. This study is to evaluate the safety and side effect profile of giving lower doses of the Pfizer or Moderna COVID-19 vaccine to young adults, as well as assessing its impact on the immune response to the virus which causes COVID-19, called SARS-CoV-2.

We will be enrolling adults aged 18 to 30 years old who have had 2 doses of BNT162b2 (Pfizer) or mRNA-1273 (Moderna), are at least 3 months (84 days) after their second dose and have not yet had a booster.

What are the vaccines against?




covid19 breakdown


Am I suitable to take part?

Adults who are aged 18 – 30 years who have had 2 doses of BNT162b2 (Pfizer) or mRNA-1273 (Moderna) and are at least 3 months (84 days) since their second dose are eligible to take part, providing that they have not yet received a booster. In order to participate in the study:

  • You must agree not to donate blood during the trial


Summary of the study

Participants will be randomised to receive either a single dose of the BNT162b2 (Pfizer) vaccine (30mcg), one third of the normal dose of BNT162b2 (10mcg, the same dose being used in other countries for children aged 5 – 11 years), a half dose of the mRNA-1273 (Moderna) vaccine (50mcg, the dose being used by the NHS as a 3rd dose booster) or one quarter of a dose of the mRNA-1273 vaccine (25mcg). They will then complete a diary of their symptoms over the next 7 days, and any other adverse events which occur up to 3 months after having had the vaccine. There are several follow up visits to have blood tests to check for immune markers, and to check on the participants health.

Participants will initially be “blinded”, meaning they will not be told which vaccine they have been given until after they have attended a follow up visit 84 days after being vaccinated. If a participant is travelling prior to their day 84 follow up visit it may be possible to unblind them before the day 84 follow up visit if there is an urgent need and they have attended the follow up visit that is 28 days after vaccination.

What vaccines are being used in this study?


BNT162b2 (Pfizer/BioNTech) 30mcg
BNT162b2 (Pfizer/BioNTech) 10mcg
mRNA-1273 (Moderna) 50mcg
mRNA-1273 (Moderna) 25mcg


BNT162b2 (Pfizer/BioNTech): Common use under emergency provision

This is the vaccine commonly known as ‘The Pfizer vaccine.’ This is a messenger RNA (mRNA) vaccine.  This vaccine uses a small amount of the genetic coding material (mRNA) of the SARS-CoV-2 spike (S) protein packaged inside very small fatty particles (lipid nanoparticles). When these are injected into your body, your cells take up these fatty particles, and then start producing the SARS-CoV-2 spike protein.  Your immune system then “sees” these spike proteins and makes a protective immune reaction against them.   The original mRNA that has been taken into your cells is broken down within a few days and cannot be incorporated into your own genetic code.

This vaccine has been tested in more than 40,000 people worldwide in clinical trials and subsequently given to tens of millions of people, and has been shown to be both safe, and effective.

The vaccine does not contain a live version of the SARS-CoV-2 coronavirus and therefore cannot give you COVID-19. The potential side effects of these vaccines are discussed in more detail in the section ‘What are the risks of taking part in this trial’.


mRNA-1273 (Moderna): Common use under emergency provision

This is the vaccine commonly known as “The Moderna vaccine”. This is also a messenger RNA (mRNA) vaccine which works in a similar way to BNT162b2 (The Pfizer vaccine).

This vaccine has been tested in more than 30,000 people worldwide in clinical trials and subsequently given to tens of millions of people, has been shown to be both safe, and effective.

Participants will not know which vaccine or dose they have received until 84 days after they have received the vaccine.

What are the side effects of these vaccines?

Common side effects


Other common systemic side effects

-    Fatigue
-    Headaches
-    Muscle aches
-    Joint aches
-    Feeling unwell (malaise)
-    Feeling sick or nauseated or vomiting

Other less common side effects:

-    Abdominal pain
-    Decreased appetite
-    Feeling dizzy
-    Swollen lymph nodes (glands)
-    Excessive sweating, itching skin or rash



What are the advantages of taking part?

We anticipate that participating in the trial will mean that you gain some additional protection against the coronavirus. Most importantly, the information gained from the trial will make a valuable contribution to the pandemic response.

Are there any risks from taking part in the study?




What’s next?



Download the Participant Information Sheet (PDF)


Complete the Pre-screening Questionnaire


For further details contact us on :

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