COV-Boost Young Adult Fractional Dose Sub-study (COV-BOOST YAFD)

A randomised, phase II UK multi-centre study to determine reactogenicity and immunogenicity of booster vaccination against ancestral and novel variants of SARS-CoV-2.

Status: Closed to Recruitment 

Population: Adults (18-30 years old)

Disease group: COVID-19

Funder: National Institute for Health Research (NIHR) 

Sponsor: University Hospital Southampton NHS Foundation Trust

Principal Investigator: Prof. Paul Heath

Study Aims

We are studying the use of lower doses of 2 approved vaccines for COVID-19: BNT162b2 – (Pfizer) and mRNA-1273 (Moderna) when given to young adults as a 3rd or 4th dose booster vaccine. 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 equivalent immune response in young adults as higher doses. They may also be associated with fewer side effects. 

This study is trying to determine the side effects, safety and immune response of giving different 3rd or 4th COVID-19 vaccine doses of BNT162b2 (Pfizer) and mRNA-1272 (Moderna) to people aged 18-30 years old who have previously received 2 or 3 doses of the BNT162b2 (Pfizer) or mRNA-1272 (Moderna).  

What does participating in the study involve?

The study lasts for 242 days (about 8 months) and involves 5 study visits with blood tests taken at each visit: 

  • Screening and vaccination visit: At the first visit, study staff talk to potential participants about the study and what to expect, and about their health and details of previous covid vaccines to check whether they can take part in the study.  If someone is eligible to take part and would like to do so, they are asked to sign a consent form. Participants then have vital signs checked and may have a physical examination by a doctor. They will have baseline antibody blood tests and tests of liver and kidney function for safety. Then, they are randomly allocated to receive either a low dose or normal dose of BNT162b2 (Pfizer), or a low dose or normal dose of mRNA1273.214 (Moderna bivalent), as an injection into the arm.  
  • Follow-up after vaccination an “e-Diary”, thermometer and tape measure is given to participants to record their temperature and any symptoms they have for 7 days after they have the vaccine, and any symptoms for 21 days after the vaccine.  
  • Follow-up visits: participants return to the clinic for four short visits to check they are fine, any changes to their health or medicines and to have blood tests. 
  • Participants are told which vaccine they received after the day 84 visit.  

For more information about the study, visit the study website here: