A single-blind, randomised, phase II UK multi-centre study to determine reactogenicity and immunogenicity of heterologous prime/boost COVID-19 vaccine schedules – Stage 2 (COM-COV2)

Project summary:

This is a single-blind trial to determine whether having doses of two different types of COVID-19 vaccines gives as good an immune system response as having two doses of the same vaccine. Adults aged 50 years and above who have already had one dose of COVID-19 vaccine (AstraZeneca or Pfizer) through the NHS rollout programme are recruited to the trial. Participants received their second dose through the trial. Participants were allocated at random and received either:

  • The same vaccine as their first dose (AstraZeneca or Pfizer)
  • Moderna vaccine
  • Novavax vaccine

Participants were asked to complete an electronic diary for up to three months after vaccination in order to record any symptoms or medical conditions.

The last visit (D294) of the trial has been brought forward to accommodate the third dose currently offered by NHS. All study visits were completed in January 2022.



UK Vaccine Task Force (VTF), National Institute Health Research (NIHR), Collaboration for Epidemic Preparedness (CEPI), Novavax (Vaccine supply)

Leader researcher:

Professor Matthew Snape

Lead institution:

University of Oxford

Vaccine type:


Recruitment for clinical trial open: